LTC4232 datasheet(10/16 Pages) LINER

10 / 16 page

LTC4232

4232fb

For more information www.linear.com/LTC4232

APPLICATIONS INFORMATION

Anovercurrentfaultoccurswhenthecurrentlimitcircuitry

has been engaged for longer than the timeout delay set

by the TIMER. Current limiting begins when the MOSFET

current reaches 1.5A to 5.6A (depending on the foldback).

The GATE pin is then brought down with a 140mA GATE-

to-OUT current. The voltage on the GATE is regulated in

order to limit the current to less than 5.6A. At this point,

a circuit breaker time delay starts by charging the external

timing capacitor from the TIMER pin with a 100µA pull-up

current. 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 over-

current fault has turned off the pass MOSFET. For a given

the 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

discharging the timing capacitor with a 2µA pull-down

current. When the TIMER pin reaches its 0.21V 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 UV pin below 0.6V

and then high will clear the fault. If the TIMER pin is tied

to INTVCC then the switch is allowed to turn on again (after

an internal 100ms delay) if the overcurrent fault 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. In this auto-retry mode the

LTC4232 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 1.4A as the timer ramps up.

10µF, especially if the wiring inductance from the supply

to the VDD pin is greater than 3µH. The possibility of oscil-

lation will increase as the load current (during power-up)

increases. There are two ways to prevent this type of

oscillation. The simplest way is to avoid load capacitances

below 10µF. For wiring inductance larger than 20µH, the

minimumloadcapacitancemayextendto100µF.Asecond

choice is to connect an external gate capacitor CP >1.5nF

as shown in Figure 3.

Figure 3. Compensation for Small CLOAD

4232 F03

LTC4232

*OPTIONAL

RC TO LOWER

INRUSH CURRENT

GATE

2.2nF

Turn-Off Sequence

The switch can be turned off by a variety of conditions. A

normal turn-off is initiated by the UV pin going below its

1.235V threshold. Additionally, several fault conditions

will turn off the switch. These include an input overvolt-

age (OV pin), overcurrent circuit breaker (SENSE pin) or

overtemperature. Normally the switch is turned off with

a 250µA current pulling down the GATE pin to ground.

With the switch turned off, the OUT voltage drops which

pulls the FB pin below its threshold. PG then pulls low to

indicate output power is no longer good.

If VDD drops below 2.65V for greater than 5µs or INTVCC

drops below 2.5V for greater than 1µs, a fast shutdown

of the switch is initiated. The GATE is pulled down with a

140mA current to the OUT pin.

Overcurrent Fault

The LTC4232 features an adjustable current limit with

foldback that protects against short-circuits or excessive

load current. To prevent excessive power dissipation in the

switch during active current limit, the available current is

reduced as a function of the output voltage sensed by the

FB pin. A graph in the Typical Performance Characteristics

curves shows the current limit versus FB voltage.

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