Detailed Description
The MAX1614 uses an internal, monolithic charge pump
and low-dropout linear regulator to supply the required
8V VGS voltage to fully enhance an n-channel MOSFET
high-side switch (Figure 1). The charge pump typically
supplies 30μA, charging 800pF of gate capacitance in
400μs (VBATT = 15V). For slower turn-on times, simply
add a small capacitor between the GATE and SRC pins.
When turned off, GATE and SRC pull low and typically
discharge an 800pF gate capacitance in 80μs.
The MAX1614 provides separate on/off control inputs (ON
and OFF). ON and OFF connect, respectively, to the SET
and RESET inputs of an internal flip-flop. When ON is
pulsed low (with OFF = high), the internal charge pump
turns on, and GATE is pumped to 8V above SRC, turning
on the external MOSFETs. The charge pump maintains
gate drive to the external MOSFETs until OFF is pulsed low.
When this happens, the internal charge pump turns off, and
GATE discharges to ground through an internal switch. For
slower turn-on times, simply add a small capacitor.
Applications Information
Connecting ON/OFF to 3V or 5V Logic
ON and OFF internally connect to 2μA max pullup current
sources (Figure 1). The open-circuit voltage for ON and
OFF ranges from 7V to 10.5V (nominally 8.5V). Since the
current sources are relatively weak, connecting ON and
OFF directly to logic powered from lower voltages (e.g.,
3V or 5V) poses no problem if the gate outputs driving
these pins can sink at least 2μA while high.
Although the MAX1614 shutdown function was designed
to operate with a single pushbutton on/off switch, it can
also be driven by a single gate. Connect ON to GND and
drive OFF directly (Figure 2).
Maximum Switching Rate
The MAX1614 is not intended for fast switching applica-
tions. In fact, it is specifically designed to limit the rate of
change of the load current, ΔI/Δt. The maximum switching
rate is limited by the turn-on time, which is a function of
the charge-pump output current and the total capacitance
on GATE (CGATE). Calculate the turn-on time as a func-
tion of external MOSFET gate capacitance using the
Gate Charging Current vs. VBATT graph in the Typical
Operating Characteristics. Since turn-off time is small
compared to turn-on time, the maximum switching rate is
approximately 1/tON.
Adding Gate Capacitance
The charge pump uses an internal monolithic trans-
fer capacitor to charge the external MOSFET gates.
Normally, the external MOSFET’s gate capacitance is suf-
ficient to serve as a reservoir capacitor. If the MOSFETs
are located at a significant distance from the MAX1614,
place a local bypass capacitor (100pF typ) across the
GATE and SRC pins. For slower turn-on times, simply
add a small capacitor between GATE and SRC.
PIN
NAME
FUNCTION
1
ON SET
Input to the On/Off Latch. Pulse
ON
low with
OFF
high to turn on the external MOSFET switch.
When
both
ON
and
OFF
are low, the part is off.
2OFF RESET
Input to the On/Off Latch. Pulse
OFF
low with
ON
high to turn off the external MOSFET switch.
When
both
ON
and
OFF
are low, the part is off.
3LBO
Open-Drain, Low-Battery Comparator Output.
LBO
is low when VLBI is below the trip
point.
4 LBI
Low-Battery
Comparator Input.
LBO
goes low when VLBI falls below 1.20V (typ). Connect a voltage-
divider
between BATT, LBI, and GND to set the battery
undervoltage
trip threshold (see Typical Operating Circuit).
5
GND System Ground
6
GATE Gate-Drive Output. Connect to the gates of external, n-channel
MOSFETs.
When the
MAX1614
is off, GATE
actively pulls to GND.
7
SRC Source Input. Connect to the sources of external, n-channel
MOSFETs.
When the
MAX1614
is off, SRC
actively pulls to
GND.
8
BATT Battery Input. Connect to a battery voltage between 5V and
26V.
MAX1614 High-Side, n-Channel MOSFET
Switch Driver
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Pin Description