LTC4232 datasheet(12/18 Pages) AD

12 / 18 page

LT4200

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

140mA GATE-to-OUT current. The voltage on the GATE is

regulated in order to limit the current to 57A. At this point,

a circuit breaker time delay starts by charging the external

timing capacitor with a 100µA pull-up current from the

TIMER pin. If the TIMER pin reaches its 1.235V threshold,

the internal switch turns off (with a 250µA current from

GATE to ground). Included in the Typical Performance

Characteristics curves is a graph of the safe operating

area for the MOSFET. From this graph, one can determine

the MOSFET’s maximum time in current limit for a given

output power.

Tying the TIMER pin to INTVCC will force the part to use

the internally generated (circuit breaker) delay of 0.5ms.

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 given by Equation 3.

CT = tCB • 0.083[µF/ms]

(3)

After the switch is turned off, the TIMER pin begins dis-

charging the timing capacitor with a 2µA pull-down cur-

rent. When the TIMER pin reaches its 0.21V threshold, it

completes one timer cycle. After eight TIMER pin cycles

(ramping to 1.235V and then below 0.21V) plus the 48ms

debounce time, the switch is allowed to turn on again if

the overcurrent fault latch has been cleared. Bringing the

UV pin below 0.6V for a minimum of 1µs and then high

will clear the fault latch. If the TIMER pin is tied to INTVCC

then the switch is allowed to turn on again (after an inter-

nal 900ms cool down time plus the 48ms debounce time),

if the overcurrent fault latch is cleared.

Tying the FLT# pin to the UV pin allows the part to self-

clear the fault and turn the MOSFET on as soon as TIMER

pin has ramped below 0.21V for the eighth time followed

by the 48ms debounce time. In this auto-retry mode, the

LT4200 repeatedly tries to turn on after an overcurrent at

a period determined by the capacitor on the TIMER pin.

The auto retry mode also functions when the TIMER pin

is tied to INTVCC.

The waveform in Figure 4 shows how the output latches

off following a short-circuit. The current in the MOSFET

is 6.8A as the TIMER pin ramps up.

∆VGATE

10V/DIV

IOUT

5A/DIV

VOUT

10V/DIV

TIMER

2V/DIV

1ms/DIV

4200 F04

Figure 4. Short-Circuit Waveform

Current Limit Adjustment

The default value of the active current limit is 57A. The

current limit threshold can be adjusted lower by placing

a resistor between the ISET pin and ground. As shown

in the Functional Diagram the voltage at the ISET 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 ISET pin open, the voltage

at the ISET pin is determined by a positive temperature

coefficient reference. This voltage is set to 0.618V which

corresponds to a 57A current limit at room temperature.

An external resistor RSET placed between the ISET pin

and ground forms a resistive divider with the internal 20k

RISET sourcing resistor. The divider acts to lower the volt-

age at the ISET pin and therefore lower the current limit

threshold. The overall current limit threshold precision

is reduced to ±15% when using a 20k resistor to halve

the threshold.

The RSET resistor for a desired current limit level can be

calculated using the Equation 4.

RSET =

20kΩ

ILIM

− 1

⎛

⎝⎜

⎞

⎠⎟

(4)

where:

RSET = value of the resistor in Ω

ILIM = Current Limit in A, 5.2 < ILIM < 57

Using a switch (connected to ground) in series with RSET

allows the active current limit to change only when the

switch is closed. This feature can be used to program a

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