MAX1745AUB+T - Maxim Integrated

General Description

The MAX1744/MAX1745 are step-down DC-DC con-

trollers capable of handling up to 36V inputs. These

parts use a proprietary current-limited control scheme

for excellent light- and full-load efficiency, while their

330kHz (max) switching frequency permits small exter-

nal components for space-critical applications.

Operation to 100% duty cycle permits the lowest possi-

ble dropout voltage.

The MAX1744 contains an internal feedback network

that provides a pin-selectable output voltage of either

3.3V or 5V. The MAX1745 uses an external feedback

network to generate an adjustable output voltage

between 1.25V and 18V.

The MAX1744/MAX1745 are available in a space-sav-

ing 10-pin μMAX®package.

________________________Applications

Automotive Electronics

Telecom Systems

Wall-Cube-Powered Devices

Industrial Control Systems

Firewire®/IEEE®1394

____________________________Features

♦High-Voltage Operation (Up to 36V IN)

♦Efficiency > 90%

♦Output Power Capability Exceeds 50W

♦10-Pin µMax Package

♦Low-Dropout Voltage

♦100% (max) Duty Cycle

♦90µA Quiescent Current

♦4µA Shutdown Current

♦Up to 330kHz Switching Frequency

♦Output Voltage

5V or 3.3V (MAX1744)

Adjustable 1.25V to 18V (MAX1745)

♦Current-Limited Control Scheme

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

________________________________________________________________

Maxim Integrated Products

19-1776; Rev 4; 8/09

Ordering Information

PART TEMP RANGE PIN-PACKAGE

MAX1744EUB+ -40°C to +85°C 10 μMAX

MAX1744AUB+ -40°C to +125°C 10 μMAX

MAX1744EUB/V+ -40°C to +85°C 10 μMAX

MAX1745EUB+ -40°C to +85°C 10 μMAX

MAX1745AUB+ -40°C to +125°C 10 μMAX

MAX1745EUB/V+ -40°C to +85°C 10 μMAX

GND OUT

REF

MAX1744

3/5

3.3V

OFF SHDN

IN VH EXT P

4.5V TO 36V

OUT

3.3V

OR 5V

Typical Operating Circuit

EXT

3/5 (FB)

( ) ARE FOR MAX1745 ONLY.

REF

GND

MAX1744

MAX1745

μMAX

TOP VIEW

CSOUT

SHDN

Pin Configuration

For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,

or visit Maxim's website at www.maxim-ic.com.

EVALUATION KIT

AVAILABLE

μMAX is a registered trademark of Maxim Integrated Products, Inc.

Firewire is a registered trademark of Apple, Inc.

IEEE is a registered service mark of the Institute of Electrical

and Electronics Engineers, Inc.

Denotes a lead(Pb)-free/RoHS-compliant package.

/V Denotes an automotive qualified part.

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

2 _______________________________________________________________________________________

ABSOLUTE MAXIMUM RATINGS

ELECTRICAL CHARACTERISTICS

(VIN = VSHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA= 0°C to +85°C, unless otherwise noted. Typical values at VIN = VSHDN =

36V, TA= +25°C.)

Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional

operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to

absolute maximum rating conditions for extended periods may affect device reliability.

IN, EXT, SHDN to GND...........................................-0.3V to +38V

VH to GND..............................................................-0.3V to +34V

VH, EXT to IN............................................................-7V to +0.3V

CS, OUT to GND ....................................................-0.3V to +20V

FB, 3/5, REF to GND .....................................-0.3V to (VL + 0.3V)

VL to GND...................................................................-0.3V to 6V

Continuous Power Dissipation (TA= +70°C)

10-Pin μMAX (derate 5.6mW/°C above 70°C) .............444mW

Operating Temperature Range

MAX174_EUB ..................................................-40°C to +85°C

MAX174_AUB ................................................-40°C to +125°C

Junction Temperature......................................................+150°C

Storage Temperature Range .............................-65°C to +150°C

Lead Temperature (soldering, 10s) ................................+300°C

PARAMETER CONDITIONS MIN TYP MAX UNITS

Input Voltage Range 4.5 36 V

Supply Current into IN VSHDN = VIN = 5.5V to 36V 90 140 μA

Shutdown Supply Current SHDN = GND 4 12 μA

3/5 = VL 4.85 5.00 5.15

Output Voltage (MAX1744) 3/5 = GND 3.20 3.30 3.40 V

OUT Input Current (MAX1744) 3/5 = VL, VOUT = 5V 28 44 μA

FB Threshold Voltage (MAX1745) Falling edge, hysteresis = 8mV 1.22 1.25 1.28 V

FB Input Current (MAX1745) -50 50 nA

VH Output Voltage with Respect to IN VIN = 5.5V to 36V, IVH = 100μA to 20mA -6.0 -5.3 -4.3 V

VL Output Voltage VIN = 5.5V to 36V, IVL = 100μA to 2mA 4.5 5.0 5.5 V

VL Undervoltage Lockout 2.0 3.0 4.1 V

VCS = VOUT = 2.5V to 18V 85 100 115

CS Threshold Voltage VCS = VOUT = VGND 80 110 150 mV

VCS = VOUT = 2.5V to 18V 0 15 25

CS Input Current VCS = VOUT = VGND -25 0 μA

SHDN, 3/5 Logic-High Threshold VIN = 4.5V to 36V 2.4 V

SHDN, 3/5 Logic-Low Threshold VIN = 4.5V to 36V 0.4 V

3/5 Input Current SHDN = GND ±1μA

3/5 = GND ±1

SHDN Input Current VSHDN = 36V 12 μA

EXT Resistance 820Ω

Minimum EXT Off-Time 1.5 2.0 2.5 μs

Minimum EXT On-Time 0.7 1.0 1.5 μs

Output Line Regulation Figure 1, 5.5V < VIN < 36V, ILOAD = 1A 5 mV/V

Output Load Regulation Figure 1, VIN = 12V, 30mA < ILOAD < 2A 15 mV/A

Reference Voltage IREF = 0 1.22 1.25 1.28 V

REF Load Regulation 0 ≤ IREF ≤ 100μA 4 10 mV

REF Line Regulation VIN = 4.5V to 36V, IREF = 0 30 60 μV/V

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

_______________________________________________________________________________________ 3

ELECTRICAL CHARACTERISTICS

(VIN = VSHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA= -40°C to +85°C, unless otherwise noted.) (Note 1)

PARAMETER CONDITIONS MIN MAX UNITS

Input Voltage Range 4.5 36 V

Supply Current into IN VSHDN = VIN = 5.5V to 36V 140 μA

Shutdown Supply Current SHDN = GND 12 μA

3/5 = VL 4.85 5.15

Output Voltage (MAX1744) 3/5 = GND 3.20 3.40 V

OUT Input Current (MAX1744) 3/5 = VL, VOUT = 5V 44 μA

FB Threshold Voltage (MAX1745) Falling edge, hysteresis = 8mV 1.22 1.28 V

FB Input Current (MAX1745) -50 50 nA

VH Output Voltage with Respect to IN VIN = 5.5V to 36V, IVH = 100μA to 20mA -6.0V -4.3V V

VL Output Voltage VIN = 5.5V to 36V, IVL = 100μA to 2mA 4.5 5.5 V

VL Undervoltage Lockout 2.0 4.1 V

VCS = VOUT = 2.5V to 18V 85 115

CS Threshold Voltage VCS = VOUT = VGND 80 150 mV

VCS = VOUT = 2.5V to 18V 0 25

CS Input Current VCS = VOUT = VGND -25 0 μA

SHDN, 3/5 Logic-High Threshold VIN = 4.5V to 36V 2.4 V

SHDN, 3/5 Logic-Low Threshold VIN = 4.5V to 36V 0.4 V

3/5 Input Current SHDN = GND ±1μA

3/5 = GND ±1

SHDN Input Current VSHDN = 36V 12 μA

EXT Resistance 20 Ω

Minimum EXT Off-Time 1.5 2.5 μs

Minimum EXT On-Time 0.7 1.5 μs

Reference Voltage IREF = 0 1.22 1.28 V

REF Load Regulation 0 ≤ IREF ≤ 100μA 10 mV

REF Line Regulation VIN = 4.5V to 36V, IREF = 0 60 μV/V

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

4 _______________________________________________________________________________________

ELECTRICAL CHARACTERISTICS

(VIN = VSHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA= -40°C to +125°C, unless otherwise noted.) (Note 1)

PARAMETER CONDITIONS MIN MAX UNITS

Input Voltage Range 4.5 36 V

Supply Current into IN VSHDN = VIN = 5.5V to 36V 140 μA

Shutdown Supply Current SHDN = GND 15 μA

3/5 = VL 4.85 5.15

Output Voltage (MAX1744) 3/5 = GND 3.20 3.40 V

OUT Input Current (MAX1744) 3/5 = VL, VOUT = 5V 44 μA

FB Threshold Voltage (MAX1745) Falling edge, hysteresis = 8mV 1.22 1.28 V

FB Input Current (MAX1745) -50 50 nA

VH Output Voltage with Respect to IN VIN = 5.5V to 36V, IVH = 100μA to 20mA -6.0V -4.3V V

VL Output Voltage VIN = 5.5V to 36V, IVL = 100μA to 2mA 4.5 5.5 V

VL Undervoltage Lockout 1.6 4.1 V

VCS = VOUT = 2.5V to 18V 85 115

CS Threshold Voltage VCS = VOUT = VGND 80 150 mV

VCS = VOUT = 2.5V to 18V 0 25

CS Input Current VCS = VOUT = VGND -25 0 μA

SHDN, 3/5 Logic-High Threshold VIN = 4.5V to 36V 2.4 V

SHDN, 3/5 Logic-Low Threshold VIN = 4.5V to 36V 0.4 V

3/5 Input Current SHDN = GND ±1μA

3/5 = GND ±1

SHDN Input Current VSHDN = 36V 12 μA

EXT Resistance 20 Ω

Minimum EXT Off-Time 1.5 2.5 μs

Minimum EXT On-Time 0.7 1.5 μs

Reference Voltage IREF = 0 1.22 1.28 V

REF Load Regulation 0 ≤ IREF ≤ 100μA 10 mV

REF Line Regulation VIN = 4.5V to 36V, IREF = 0 80 μV/V

Note 1: Specifications to -40°C are guaranteed by design, not production tested.

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

_______________________________________________________________________________________

100

0.0001 0.001 0.01 0.1 1 10

EFFICIENCY vs. LOAD CURRENT

(VOUT = +3.3V)

MAX1744/5toc01

LOAD CURRENT (A)

EFFICIENCY (%)

A: VIN = +5.5V

B: VIN = +12.0V

C: VIN = +24.0V

D: VIN = +36.0V

100

0.0001 0.001 0.01 0.1 1 10

EFFICIENCY vs. LOAD CURRENT

(VOUT = +5.0V)

MAX1744/5toc02

LOAD CURRENT (A)

EFFICIENCY

(%)

A: VIN = +7.2V

B: VIN = +12.0V

C: VIN = +24.0V

D: VIN = +36.0V

100

105

110

0 10203040

IN PIN QUIESCENT CURRENT

vs. INPUT VOLTAGE (5.5V TO 36V)

MAX1744/5toc03

INPUT VOLTAGE (V)

QUIESCENT CURRENT (μA)

3.5 4.5 5.5

IN PIN QUIESCENT CURRENT

vs. INPUT VOLTAGE (3.5V TO 5.5V)

MAX1744/5toc04

INPUT VOLTAGE (V)

QUIESCENT CURRENT (mA)

VOUT = 3.3V

120

100

140

0 10203040

SWITCHING FREQUENCY

vs. INPUT VOLTAGE

MAX1744/5toc05

INPUT VOLTAGE (V)

SWITCHING FREQUENCY (kHz)

VOUT = 3.3V

IOUT = 2.0A

IN PIN QUIESCENT CURRENT

vs. TEMPERATURE

MAX1744/5toc06

TEMPERATURE (°C)

QUIESCENT CURRENT (μA)

1007525 500-25

-50 125

100

120

0 1000 2000 3000 4000 5000

EXT RISE AND FALL TIMES

vs. CAPACITANCE

MAX1744/5toc07

CAPACITANCE (pF)

tRISE AND tFALL (ns)

VIN = +5V

tFALL

tRISE

EXT RISE AND FALL TIMES

vs. TEMPERATURE

MAX1744/5toc08

TEMPERATURE (°C)

tRISE AND tFALL (ns)

1007525 500-25

-50 125

tRISE

tFALL

VIN = +5V

CL = 1000pF

CURRENT-SENSE TRIP LEVEL

vs. TEMPERATURE

MAX1744/5toc09

TEMPERATURE (°C)

CURRENT-SENSE TRIP LEVEL (mV)

1007550250-25

100

105

110

115

-50 125

Typical Operating Characteristics

(Circuit of Figure 1, TA = +25°C, unless otherwise specified.)

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

6 _______________________________________________________________________________________

Typical Operating Characteristics (continued)

(Circuit of Figure 1, TA = +25°C, unless otherwise specified.)

REFERENCE OUTPUT VOLTAGE CHANGE

vs. TEMPERATURE

MAX1744/5toc10

TEMPERATURE (°C)

REFERENCE OUTPUT VOLTAGE CHANGE (%)

1007525 500-25

-4

-3

-2

-1

-5

-50 125

VOUT

2V/div

SHUTDOWN

PULSE

5V/div

MAX1744

ENTERING/EXITING SHUTDOWN

MAX1744/5toc11

2ms/div

RL = 3.3Ω

VIN = 7.2V, VOUT = 3.3V, LOAD CURRENT = 0.1A TO 2A

A: VOUT, 50mV/div, 3.3V AC-COUPLED

B: LOAD CURRENT, 1A/div

LOAD-TRANSIENT RESPONSE

MAX1744/5toc12

50μs/div

VOUT = 5V, LOAD CURRENT = 1A

A: VOUT, 100mV/div, AC-COUPLED

B: VIN, 6V TO 12V, 5V/div

LINE-TRANSIENT RESPONSE

MAX1744/5toc13

4ms/div

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

_______________________________________________________________________________________ 7

Detailed Description

The MAX1744/MAX1745 are high-voltage step-down

DC-DC converter controllers. These devices offer high

efficiency over a wide range of input/output voltages

and currents, making them optimal for use in applica-

tions such as telecom, automotive, and industrial con-

trol. Using an external P-channel MOSFET and

current-sense resistor allows design flexibility and

improved efficiency. The MAX1744/MAX1745 automati-

cally switch from PWM operation at medium and heavy

loads to pulse-skipping operation at light loads to

improve light-load efficiency. The low 90μA quiescent

current further optimizes these parts for applications

where low input current is critical. Operation to 100%

duty cycle allows the lowest possible dropout voltage,

which allows a wider input voltage variation. The small

size, high switching frequency, and low parts count

minimize the required circuit board area and compo-

nent cost. Figure 1 shows the MAX1744 typical applica-

tion circuit.

Operating Modes

When delivering low output currents, the MAX1744/

MAX1745 operate in discontinuous-conduction mode.

Current through the inductor starts at zero, rises as

high as the current limit, then ramps down to zero dur-

ing each cycle (Figure 3). The switch waveform exhibits

ringing, which occurs at the resonant frequency of the

inductor and stray capacitance, due to residual energy

trapped in the core when the commutation diode (D1 in

Figure 1) turns off.

When delivering medium-to-high output currents, the

MAX1744/MAX1745 operate in PWM continuous-con-

duction mode (Figure 4). In this mode, current always

flows through the inductor and never ramps to zero.

The control circuit adjusts the switch duty cycle to

maintain regulation without exceeding the peak switch-

ing current set by the current-sense resistor.

Pin Description

NAME

MAX1744 MAX1745

FUNCTION

1 GND GND Ground

2VL VL

5V Linear Regulator Output. VL provides power to the internal circuitry and can supply up

to 1mA to an external load. Bypass VL to GND with 4.7μF or greater capacitor.

3 REF REF 1.25V Reference Output. REF can supply up to 100μA to an external load. Bypass REF to

GND with a 0.1μF or greater ceramic capacitor.

4 3/5 — 3.3V or 5V Selection. Connect 3/5 to GND to set the output voltage to 3.3V. Connect 3/5 to

VL to set the output voltage to 5V.

4— FB

Feedback Input for Adjustable Output Operation. Connect to an external voltage-divider

between the output and FB to set the output voltage. The regulation voltage threshold is

1.25V.

5 OUT OUT

Sense Input for Fixed 5V or 3.3V Output Operation (MAX1744) and Negative Current-Sense

Input (MAX1744/5). OUT is connected to an internal voltage-divider (MAX1744). OUT does

not supply current.

6CS CS

Current-Sense Input. Connect the current-sense resistor between CS and OUT. External

MOSFET is turned off when the voltage across the resistor is equal to or greater than the

current limit trip level (100mV).

7SHDN SHDN

Active-Low Shutdown Input. Connect SHDN to IN for normal operation. Drive SHDN to low

to shut the part off. In shutdown mode, the reference, output, external MOSFET, and

internal regulators are turned off.

8VH VH

High-Side Linear Regulator Output. VH provides a regulated output voltage that is 5V below

IN. The external P-channel MOSFET gate is driven between IN and VH. Bypass VH to IN

with a 4.7μF or greater capacitor (see the Capacitor Selection section).

9 EXT EXT Gate Drive for External P-Channel MOSFET. EXT swings between IN and VH.

10 IN IN Positive Supply Input. Bypass IN to GND with a 0.47μF or greater ceramic capacitor.

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

8 _______________________________________________________________________________________

100% Duty Cycle and Dropout

The MAX1744/MAX1745 operate with a duty cycle up to

100%. This feature extends the input voltage range by

turning the MOSFET on continuously when the supply

voltage approaches the output voltage. This services

the load when conventional switching regulators with

less than 100% duty cycle would fail. Dropout voltage is

defined as the difference between the input and output

voltages when the input is low enough for the output to

drop out of regulation. Dropout depends on the

MOSFET drain-to-source on-resistance, current-sense

resistor, and inductor series resistance, and is propor-

tional to the load current:

Regulation Control Scheme

The MAX1744/MAX1755 have a unique operating

scheme that allows PWM operation at medium and high

current, with automatic switching to pulse-skipping

mode at lower currents to improve light-load efficiency.

Figure 2 shows the simplified block diagram.

Under medium- and heavy-load operation, the inductor

current is continuous and the part operates in PWM

mode. In this mode, the switching frequency is set by

either the 1μs minimum on-time or the 2μs minimum off-

time, depending on the duty cycle. The duty cycle is

approximately the output voltage divided by the input

voltage. If the duty cycle is less than 33%, the minimum

on-time controls the frequency; and the frequency is

approximately f ≈1MHz ✕D, where D is the duty cycle.

If the duty cycle is greater than 33%, the off-time sets the

frequency; and the frequency is approximately f ≈500kHz

✕(1 - D).

In both cases, the voltage is regulated by the error

comparator. For low duty cycles (<33%), the MOSFET

is turned on for the minimum on-time, causing fixed-on-

time operation. During the MOSFET on-time, the output

voltage rises. Once the MOSFET is turned off, the volt-

age drops to the regulation threshold (set by the inter-

nal voltage-divider for the MAX1745 and by the external

voltage-divider for the MAX1744), at which time another

cycle is initiated. For high duty cycles (>33%), the

MOSFET remains off for the minimum off-time, causing

fixed-off-time operation. In this case, the MOSFET

remains on until the output voltage rises to the regula-

tion threshold. Then the MOSFET turns off for the mini-

mum off-time, initiating another cycle.

By switching between fixed-on-time and fixed-off-time

operation, the MAX1744/MAX1745 can operate at high

input-output ratios, yet still operate up to 100% duty

cycle for low dropout. Note that when transitioning from

fixed-on-time to fixed-off-time operation, the output volt-

age drops slightly due to the output ripple voltage. In

fixed-on-time operation, the minimum output voltage is

regulated, but in fixed-off-time operation, the maximum

output voltage is regulated. Thus, as the input voltage

drops below approximately three times the output volt-

age, a decrease in line regulation can be expected.

The drop in voltage is approximately VDROP ≈VRIPPLE / 2.

At light output loads, the inductor current is discontinu-

ous, causing the MAX1744/MAX1745 to operate at

Figure 1. Typical Application Circuit

GND

REF

MAX1744

3/5

3.3V

OFF SHDN

EXT

INPUT

4.5V TO 36V

NIHON

EC2IQ506

FAIRCHILD

NDS9407

OUT

3.3V OR 5V

220μF

22μH

RSENSE

40mΩ

4.7μF

LOW ESR

0.47μF

Dropout voltage=

IxR +R

OUT DS(ON) SENNSE INDUCTOR

⎡

⎣⎤

⎦

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

_______________________________________________________________________________________ 9

lower frequencies, reducing the MOSFET gate drive

and switching losses. In discontinuous mode, under

most circumstances, the on-time will be the fixed mini-

mum on-time of 1μs. If the inductor value is small, or

the current-sense resistor large, the current limit will be

tripped before the minimum on-time, terminating the

on-time and thus setting the fixed on-time.

If the inductance is too large, or the output capacitance

high and equivalent series resistance (ESR) low, then

the MOSFET remains on longer than the minimum on-

time, until the output capacitor charges beyond the

error comparator’s (VOUT / 1.25V) ✕8mV hysteresis,

causing the part to operate in hysteretic mode.

Operating in hysteretic mode results in lower frequency

operation. The transition to hysteretic mode occurs at

the critical output capacitor ESR:

ESRCRITICAL = (VOUT / 1.25V) ✕8mV / IRIPPLE

where IRIPPLE is the inductor ripple current, and can be

determined by:

IRIPPLE = (VIN - VOUT) ✕tON(MIN) / L

where tON(MIN) is the minimum on-time (1μs) for mini-

mum on-time-control, or:

IRIPPLE = (VOUT) ✕tOFF(MIN) / L

where tOFF(MIN) is the minimum off-time (2μs) for mini-

mum off-time-control.

REF

MINIMUM

OFF-TIME

ONE SHOT

Q TRIG

LINEAR

REGULATOR

LINEAR

REGULATOR

1.25

REFERENCE

(FB)

ERROR

COMPARATOR

OUT

3/5

SHDN

100mV

SHDN

EXT IN

MINIMUM

ON-TIME

ONE SHOT

Q TRIG

( ) MAX1745 ONLY

- - - MAX1744 ONLY

Figure 2. Simplified Functional Diagram

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

10 ______________________________________________________________________________________

VL Linear Regulator

The MAX1744/MAX1745 contain a 5V low-side linear reg-

ulator (VL) that powers the internal circuit and can supply

up to 1mA to an external load. This allows the

MAX1744/MAX1745 to operate up to 36V input, while

maintaining low quiescent current and high switching fre-

quency. When the input voltage goes below 5.5V, this

regulator goes into dropout and the IN pin quiescent cur-

rent will rise. See the

Typical Operating Characteristics

Bypass VL with a 4.7μF or greater capacitor.

VH Linear Regulator

The MAX1744/MAX1745 contain a high-side linear regu-

lator (VH) that regulates its output to 5V below IN (the

positive supply input voltage). This regulator limits the

external P-channel MOSFET gate swing (EXT), allowing

high input voltage operation without exceeding the

MOSFET gate-source breakdown. Bypass VH with a

4.7μF or greater capacitor between IN and VH. Fast line

transients may drive the voltage on VH negative. The

clamp diode (D2) prevents damage to the IC during

such a condition. A Schottky diode with a minimum 40V

reverse rating such as the Nihon EP05Q04 is sufficient

for most applications.

Quiescent Current

The devices’ typical quiescent current is 90μA.

However, actual applications draw additional current to

supply MOSFET switching currents, OUT pin current,

external feedback resistors (if used), and both the diode

and capacitor leakage currents. For example, in the cir-

cuit of Figure 1, with IN at 30V and VOUT at 5V, typical

no-load supply current for the entire circuit is 100μA.

Shutdown Mode

When SHDN is low, the device enters shutdown mode. In

this mode, the internal circuitry is turned off. EXT is pulled

to IN, turning off the external MOSFET. The shutdown

supply current drops to less than 10μA. SHDN is a logic-

level input. Connect SHDN to IN for normal operation.

Reference

The 1.25V reference is suitable for driving small external

loads. It has a guaranteed 10mV maximum load regula-

tion while sourcing load currents up to 100μA. The refer-

ence is turned off during shutdown. Bypass the

reference with 0.1μF for normal operation. Place the

bypass capacitor within 0.2in (5mm) of REF, with a direct

trace to GND.

Design Information

Setting the Output Voltage

The MAX1744’s output voltage can be selected to 3.3V

or 5V under logic control by using the 3/5 pin. Connect

the 3/5 pin to GND to ensure a 3.3V output, or connect

the 3/5 pin to VLto ensure a 5V output.

The MAX1745’s output voltage is set using two resis-

tors, R2 and R3 (Figure 5), which form a voltage-divider

between the output and FB. R2 is given by:

where VREF = 1.25V. Since the input bias current at FB

has a maximum value of 50nA, large values (10kΩto

200kΩ) can be used for R3 with no significant accuracy

R2= R3 x V

OUT

REF

−

⎛

⎝

⎜⎞

⎠

⎟

CIRCUIT OF FIGURE 1, VIN = 18V, VOUT = 3.3V, ILOAD = 100mA

A: MOSFET DRAIN, 10V/div

B: OUT, 50mV/div, 3.3V DC OFFSET

C: INDUCTOR CURRENT, 1A/div

10μs/div

Figure 3. Discontinuous-Conduction Mode, Light-Load-Current

Waveform

CIRCUIT OF FIGURE 1, VIN = 18V, VOUT = 3.3V, ILOAD = 1.5A

A: MOSFET DRAIN, 10V/div

B: OUT, 50mV/div, 3.3V DC OFFSET

C: INDUCTOR CURRENT, 1A/div

10μs/div

Figure 4. Continuous-Conduction Mode, Heavy-Load-Current

Waveform

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

______________________________________________________________________________________ 11

loss. For 1% error, the current through R2 should be at

least 100 times FB’s input bias current.

Current-Sense-Resistor Selection

The current-sense comparator limits the peak switching

current to VCS/RSENSE, where RSENSE is the value of

the current-sense resistor and VCS is the current-sense

threshold. VCS is typically 100mV. Minimizing the peak

switching current will increase efficiency and reduce

the size and cost of external components. However,

since available output current is a function of the peak

switching current, the peak current limit must not be set

too low.

Set the peak current limit to 1.3 times the maximum

load current by setting the current-sense resistor to:

Inductor Selection

The essential parameters for inductor selection are induc-

tance and current rating. The MAX1744/MAX1745 ope-

rate with a wide range of inductance values. In many

applications, values between 4.7μH and 100μH take best

advantage of the controller’s high switching frequency.

Calculate the minimum inductance value as follows:

where 1μs is the minimum on-time. Inductor values

between 2 and 10 times L(MIN) are recommended. With

high inductor values, the MAX1744/MAX1745 begin

continuous-conduction operation at a lower fraction of

the full load (see the

Detailed Description

section).

The inductor’s saturation and heating current ratings

must be greater than the peak switching current to pre-

vent overheating and core saturation. Saturation occurs

when the inductor’s magnetic flux density reaches the

maximum level the core can support, and inductance

starts to fall. The heating current rating is the maximum

DC current the inductor can sustain without overheating.

For optimum efficiency, the inductor windings’ resis-

tance should be less than the current-sense resistance.

If necessary, use a toroid, pot-core, or shielded-core

inductor to minimize radiated noise. Table 1 lists induc-

tor types and suppliers for various applications.

External Switching Transistor

The MAX1744/MAX1745 drive a P-channel enhance-

ment-mode MOSFET. The EXT output swings from VH

to IN. Be sure that the MOSFET’s on-resistance is spec-

ified for 5V gate drive or less. Table 1 recommends

MOSFET suppliers.

Four important parameters for selecting a P-channel

MOSFET are drain-to-source breakdown voltage, cur-

rent rating, total gate charge (Qg), and RDS(ON). The

drain-to-source breakdown voltage rating should be at

least a few volts higher than VIN. Choose a MOSFET

with a maximum continuous drain-current rating higher

than the peak current limit:

The Qg specification should be 80nC or less to ensure

fast drain voltage rise and fall times, and reduce power

losses during transition through the linear region. Qg

specifies all of the capacitances associated with charging

the MOSFET gate. EXT pin rise and fall times vary with dif-

ferent capacitive loads, as shown in the

Typical Operating

Characteristics

. RDS(ON) should be as low as practical to

reduce power losses while the MOSFET is on. It should

be equal to or less than the current-sense resistor.

ID(MAX LIM MAX CS MAX

SENSE

)()

()

≥=

V-V

(MIN) IN OUT

()

CS MIN

1μ

R= V

1.3 x I

CS CS(MIN)

OUT(MAX)

FROM

OUTPUT

TO FB

Figure 5. Adjustable-Output Operation Using the MAX1745

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

12 ______________________________________________________________________________________

Diode Selection

The MAX1744/MAX1745’s high switching frequency

demands a high-speed rectifier. Schottky diodes, such

as the 1N5817–1N5822 family or surface-mount equiva-

lents, are recommended. Ultra-high-speed rectifiers

with reverse recovery times around 50ns or faster

should be used for high output voltages, where the

increased forward drop causes less efficiency degra-

dation. Make sure that the diode’s peak current rating

exceeds the peak current limit set by RSENSE, and that

its breakdown voltage exceeds VIN. Schottky diodes

are preferred for heavy loads due to their low forward

voltage, especially in low-voltage applications. For

high-temperature applications, some Schottky diodes

may be inadequate due to their high leakage currents.

In such cases, ultra-high-speed rectifiers are recom-

mended, although a Schottky diode with a higher

reverse voltage rating can often provide acceptable

performance.

Capacitor Selection

Choose filter capacitors to service input and output

peak currents with acceptable voltage ripple. ESR in

the output capacitor is a major contributor to output rip-

ple, so low-ESR capacitors are recommended. Low-

ESR tantalum, polymer, or ceramic capacitors are best.

Low-ESR aluminum electrolytic capacitors are tolera-

ble, but standard aluminum electrolytic capacitors are

not recommended.

Voltage ripple is the sum of contributions from ESR and

the capacitor value:

For tantalum capacitors, the ripple is determined by the

ESR, but for ceramic capacitors, the ripple is mostly

due to the capacitance. Voltage ripple as a conse-

quence of ESR is approximated by:

The ripple due to the capacitance is approximately:

Estimate input and output capacitor values for given

voltage ripple as follows:

where IΔLis the change in inductor current.

These equations are suitable for initial capacitor selec-

tion; final values should be set by testing a prototype or

evaluation kit. When using tantalum capacitors, use

good soldering practices to prevent excessive heat

from damaging the devices and increasing their ESR.

Also, ensure that the tantalum capacitors’ surge-current

ratings exceed the startup inrush and peak switching

currents.

Pursuing output ripple lower than the error compara-

tor’s hysteresis (0.6% of the output voltage) is not prac-

tical, since the MAX1744/MAX1745 will switch at slower

frequencies, increasing inductor ripple current thresh-

old. Choose an output capacitor with a working voltage

rating higher than the output voltage.

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

CLI

IN L

RIPPLE CIN IN

OUT L

RIPPL

EECOUT OUT

IN OUT

,−

⎛

⎝

⎜⎞

⎠

⎟

VLI

2CV

RIPPLE,C PEAK

≈2

VRIPPLE,ESR ≈−

()RESR Ipp

VRIPPLE ≈+

RIPPLE ESR RIPPLE C

Table 1. Component Suppliers

COMPANY COUNTRY PHONE FAX

803-946-0690

AVX USA

800-282-4975

803-626-3123

Coilcraft USA 847-639-6400 847-639-1469

Coiltronics USA 516-241-7876 516-241-9339

Dale/Vishay USA 402-564-3131 402-563-6418

Kemet USA 408-986-0424 408-986-1442

Inter nati onal

Recti fi er USA 310-322-3331 310-322-3332

IRC USA 512-992-7900 512-992-3377

Motorola USA 602-303-5454 602-994-6430

Nichicon USA

Japan

847- 843- 7500

81- 7- 5231- 8461

847-843-2798

81-7-5256-4158

Nihon USA

Japan

805- 867- 2555

81- 3- 3494- 7411

805-867-2698

81-3-3494-7414

Sanyo USA

Japan

619- 661- 6835

81- 7- 2070- 6306

619-661-1055

81-7-2070-1174

408-988-8000

orSiliconix USA

800-554-5565

408-970-3950

Sprague USA 603-224-1961 603-224-1430

Sumida USA

Japan

847- 956- 0666

81- 3- 3607- 5111

847-956-0702

81-3-3607-5144

United

Chemi-Con USA 714-255-9500 714-255-9400

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

______________________________________________________________________________________ 13

ripple at IN, caused by the circuit’s switching action.

Use a low-ESR capacitor. Two smaller-value low-ESR

capacitors can be connected in parallel if necessary.

Choose input capacitors with working voltage ratings

higher than the maximum input voltage.

Place a surface-mount ceramic capacitor very close to

IN and GND. This capacitor bypasses the MAX1744/

MAX1745, minimizing the effects of spikes and ringing

on the power source (IN).

Bypass REF with 0.1μF. This capacitor should be

placed within 0.2 inches (5mm) of the IC, next to REF,

with a direct trace to GND.

Layout Considerations

High-frequency switching regulators are sensitive to PC

board layout. Poor layout introduces switching noise into

the current and voltage feedback signals and may

degrade performance. The current-sense resistor must

be placed within 0.2 inches (5mm) of the controller IC,

directly between OUT and CS. Place voltage feedback

resistors (MAX1745) next to the FB pin (no more than

0.2in) rather than near the output. Place the 0.47μF input

bypass capacitor within 0.2in (5mm) of IN.

Refer to the MAX1744 Evaluation Kit manual for a two-

layer PC board example.

Chip Information

PROCESS: BiCMOS

MAX1744/MAX1745

High-Voltage, Step-Down DC-DC

Controllers in µMAX

14 ______________________________________________________________________________________

10LUMAX.EPS

Note: MAX1744/MAX1745 do not feature exposed pads.

PACKAGE TYPE PACKAGE CODE DOCUMENT NO.

10 μMAX U10CN+1 21-0061

Package Information

For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.

High-Voltage, Step-Down DC-DC

Controllers in µMAX

MAX1744/MAX1745

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are

implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________

Revision History

REVISION

NUMBER

REVISION

DATE DESCRIPTION PAGES

CHANGED

0 7/00 Initial release. —

2 8/06 — —

3 4/09 Added lead-free and automotive qualified packages to Ordering Information. 1–4, 10, 13

4 8/09 Added MAX1744 automotive package to Ordering Information. 1

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