Datasheet

LTC4232
11
4232fa
applicaTions inForMaTion
Current Limit Adjustment
The default value of the active current limit is 5.6A. The
current limit threshold can be adjusted lower by placing
a resistor between the I
SET
pin and ground. As shown in
the Functional Diagram the voltage at the I
SET
pin (via
the clamp circuit) sets the CS amplifier’s built-in offset
voltage. This offset voltage directly determines the active
current limit value. With the I
SET
pin open, the voltage at
the I
SET
pin is determined by a positive temperature co-
efficient reference. This voltage is set to 0.618V at room
temperature which corresponds to a 5.6A current limit at
room temperature.
An external resistor placed between the I
SET
pin and ground
forms a resistive divider with the internal 20k sourcing
resistor. The divider acts to lower the voltage at the I
SET
pin and therefore lower the current limit threshold. The
overall current limit threshold precision is reduced to
±12% when using a 20k resistor to halve the threshold.
Using a switch (connected to ground) in series with this
external resistor allows the active current limit to change
only when the switch is closed. This feature can be used
when the start-up
current exceeds
the typical maximum
load current.
Monitor MOSFET Temperature
The voltage at the I
SET
pin increases linearly with increas-
ing temperature. The temperature profile of the I
SET
pin is
shown in the Typical Performance Characteristics section.
Using a comparator or ADC to measure the I
SET
voltage
provides an indicator of the MOSFET temperature.
There is an overtemperature circuit in the LTC4232 that
monitors an internal voltage similar to the I
SET
pin voltage.
When the die temperature exceeds 145°C the circuit turns
off the MOSFET until the temperature drops to 125°C.
Monitor MOSFET Current
The current in the MOSFET passes through a sense resis-
tor. The voltage on the sense resistor is converted to a
current that is sourced out of the I
MON
pin. The gain of
I
SENSE
amplifier is 20µA/A referenced from the MOSFET
current. This output current can be converted to a voltage
using an external resistor to drive a comparator or ADC.
The voltage compliance for the I
MON
pin is from 0V to
INTV
CC
– 0.7V.
A microcontroller with a built-in comparator can build a
simple integrating single-slope ADC by resetting a capaci-
tor that is charged with this current. When the
capacitor
voltage trips the comparator and the capacitor is reset, a
timer is started. The time between resets will indicate the
MOSFET current.
Monitor OV and UV Faults
Protecting the load from an overvoltage condition is the
main function of the OV pin. In Figure 1 an external resis-
tive divider (driving the OV pin) connects to a comparator
to turn off the MOSFET when the V
DD
voltage exceeds
15.2V. If the V
DD
pin subsequently falls back below 14.9V,
the switch will be allowed to turn on immediately. In the
LTC4232 the OV pin threshold is 1.235V when rising, and
1.215V when falling out of overvoltage.
The UV pin functions as an undervoltage protection pin
or as an “ON” pin. In the Figure 1 application the MOSFET
turns off when V
DD
falls below 9.23V. If the V
DD
pin sub-
sequently rises above 9.88V for 100ms, the switch will
be allowed to turn on again. The LTC4232 UV turn-on/off
thresholds are 1.235V (rising) and 1.155V (falling).
In the cases of an undervoltage or overvoltage the MOSFET
turns off and there is indication on the PG status pin. When
the overvoltage is removed the MOSFET’s gate ramps
up
immediately at the rate determined by the INRUSH block.
Figure 4. Short-Circuit Waveform
∆V
GATE
10V/DIV
I
OUT
2A/DIV
V
OUT
10V/DIV
TIMER
2V/DIV
1ms/DIV
4232 F04