LTC4219 LINER [Linear Technology], LTC4219 Datasheet - Page 12

• Manufacturer Part Number: LTC4219
• Description: 5A Integrated Hot Swap Controller
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LTC4219
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
the Functional Diagram the voltage at the I
the clamp circuit) sets the CS amplifi er’s built-in offset
voltage. This offset voltage directly determines the active
current limit value. With the I
the I
effi cient reference. This voltage is set to 0.618V at room
Tying the TIMER pin to INTV
the internally generated (circuit breaker) delay of 2ms.
In either case the FLT pin is pulled low to indicate an
overcurrent fault has turned off the pass MOSFET. For a
given circuit breaker time delay, the equation for setting
the timing capacitor’s value is as follows:
After the switch is turned off, the TIMER pin begins dis-
charging the timing capacitor with a 2μA pull-down current.
When the TIMER pin reaches its 0.2V threshold, an internal
100ms timer is started. After the 100ms delay, the switch
is allowed to turn on again if the overcurrent fault has been
cleared. Bringing the EN1 pin above 1.235V and then low
will clear the fault. If the TIMER pin is tied to INTV
the switch is allowed to turn on again (after an internal
100ms delay) if the overcurrent fault is cleared.
The waveform in Figure 4 shows how the output latches
off following a short-circuit. The current in the MOSFET
is 1.4A as the timer ramps up.
12
C
T
SET
10V/DIV
10V/DIV
AV
= t
2A/DIV
2V/DIV
TIMER
V
I
GATE
OUT
OUT
pin is determined by a positive temperature co-
CB
• 0.083(μF/ms)
Figure 4. Short-Circuit Waveform
SET
1ms/DIV
pin and ground. As shown in
CC
SET
will force the part to use
pin open, the voltage at
4219 F04
SET
pin (via
CC
then
temperature which corresponds to a 5.6A current limit at
room temperature.
An external resistor placed between the I
forms a resistive divider with the internal 20k sourcing
resistor. The divider acts to lower the voltage at the I
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
ing temperature. The temperature profi le of the I
shown in the Typical Performance Characteristics section.
Using a comparator or ADC to measure the I
provides an indicator of the MOSFET temperature.
There is an over-temp circuit in the LTC4219 that monitors
an internal voltage similar to the I
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 an internal sense
resistor. The voltage on the sense resistor is converted to
a current that is sourced out of the I
I
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
INTV
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.
SENSE
CC
amplifi er is 20μA/A referenced from the MOSFET
– 0.7V.
SET
pin increases linearly with increas-
MON
SET
MON
pin voltage. When
SET
pin is from 0V to
pin. The gain of
pin and ground
SET
SET
voltage
pin is
4219f
SET