LTC4261 datasheet(24/74 Pages) AD

24 / 74 page

LTC4283

Rev. B

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APPLICATIONS INFORMATION

5. OV is higher than 1.406V (overvoltage fault).

6. UVL is lower than 1.833V and UVH is lower than

2.048V (undervoltage fault).

7. TMR reaches its 2.048V threshold (overcurrent fault).

8. DRAIN rises above 2.048V or GATE dips below VIN

– 1.8V and this condition lasts longer than a pre-

configured delay (FET bad fault).

9. PGIO3, when configured as PGI#/PGI input, is high/

low when the PGI check delay of 512ms expires

(PGI fault).

10. PGIO4 pin, when configured as EXT_FAULT#/EXT_

FAULT, is low/high (external fault).

11. The RBT_EN bit in REBOOT register 0xA2 is set.

For condition 8, if the FET_BAD_TURN_OFF bit in

CONTROL_1 register 0x0A is cleared, the MOSFET

remains on following a FET bad fault. For condition 10,

if the EXT_FAULT_TURN_OFF bit in CONFIG_3 register

0x0F is cleared, the MOSFET remains on following an

external fault. For condition 11, the LTC4283 will automat-

ically reboot after a programmable delay. See Reboot on

I2C Command. For each independent GATE turn-off fault,

the LTC4283 can be configured to latch off the MOSFET

or go into an auto-retry sequence after the fault occurs.

Overcurrent Protection

The LTC4283 features two levels of protection from short-

circuit and overcurrent conditions. Load current is moni-

tored by SENSE+ and SENSE− pins and sense resistor.

There are two distinct thresholds for the sense voltages:

VILIM and VILIM(FAST). VILIM is configurable from 15mV to

30mV in 1mV steps and VILIM(FAST) is always twice VILIM.

See Current Limit Adjustment for details.

If the sense voltage reaches VILIM, GATE is pulled down

by 50mA current until the active current limit loop is

engaged. In the event of a catastrophic short-circuit or

a sudden input step, where the sense voltage reaches

VILIM(FAST), the GATE is immediately pulled down by a

2.3A current to limit peak current through the MOSFET.

When the sense voltage drops to VILIM, the active current

limit loop is engaged.

SOA Timer

During active current limit, the power dissipation in the

MOSFET is large. If this power dissipation persists, the

MOSFET can reach temperatures that cause damage.

MOSFET manufacturers specify the safe limits on operat-

ing voltage, current and time as a curve referred to as the

Safe Operating Area (SOA). Commonly, a circuit breaker

timer sets a maximum time for the MOSFET to operate in a

current limit mode. When this timer expires, the MOSFET

is turned off to protect it from overheating. Traditional

circuit breakers often employ a fixed current charging an

external capacitor. The minimum timer timeout must be

set to allow the worst-case operating condition, such as

completely charging a large bypass capacitor at output

during startup or riding through a large input step. Then

upon fault conditions such as an output short-circuit at

full supply voltage, the MOSFET must withstand the large

power throughout the entire timer duration. Therefore,

the MOSFET must be selected to withstand the worst-

case SOA condition that occurs during any possible nor-

mal operating condition or fault condition. This not only

substantially increases the cost of the MOSFET, but also

complicates the design procedure and MOSFET selection.

The LTC4283 features a circuit breaker timer that better

fits the SOA of the MOSFET. When active current limit is

engaged, the OC_STATUS bit in FAULT_STATUS register

0x03 is set, and the TMR pin is activated and charged up

by a current proportional to the power dissipation in the

MOSFET. A proper electric model (RC network) is selected

to represent the thermal behavior of the MOSFET. When

this RC network is connected between TMR and VEE, the

TMR voltage is proportional to the rise in the MOSFET junc-

tion temperature. The LTC4283 compares the TMR voltage

to a fixed threshold voltage of 2.048V, which represents

the maximum allowable junction temperature rise in the

MOSFET. When the TMR voltage crosses this threshold,

the LTC4283 turns off the MOSFET. Therefore, to provide

an appropriate protection of a MOSFET, one simply selects

the MOSFET that meets the SOA requirement for allowable

operating conditions such as startup and input step. The

MOSFET is automatically turned off before it is subject to

any condition that would exceed its SOA rating.

LTC4261 Datasheet(PDF) 24 Page - Analog Devices

LTC4261 Datasheet(HTML) 24 Page - Analog Devices

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