LTC4215 datasheet(12/20 Pages) LINER

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LTC4234

4234fa

For more information www.linear.com/LTC4234

APPLICATIONS INFORMATION

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 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:

CT = tCB • 0.083[µF/ms]

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 internal

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

LTC4234 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 5.7A as the TIMER pin ramps up.

Current Limit Adjustment

The default value of the active current limit is 22.5A. The

current limit threshold can be adjusted lower by placing

a resistor between the ISET pin and ground. As shown in

the Functional Block 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 22.5A 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 voltage at

the ISETpinandthereforelowerthecurrentlimitthreshold.

The overall current limit threshold precision is reduced to

±15% when using a 20k resistor to halve the threshold.

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

reduced running current while the maximum available

current limit is used at start-up.

Monitor MOSFET Temperature

The voltage at the ISET pin increases linearly with increas-

ing temperature. The temperature profile of the ISET pin is

shown in the Typical Performance Characteristics section.

Figure 4. Short-Circuit Waveform

∆VGATE

10V/DIV

IOUT

5A/DIV

VOUT

10V/DIV

TIMER

2V/DIV

1ms/DIV

4234 F04

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