LTC2913 datasheet(12/24 Pages) LINER

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LT4363

4363fa

tFLT

= 15.5ms/µF

TOTAL FAULT TIMER = tFLT + tWARNING

tWARNING

= 16.67ms/µF

tFLT = 96.9ms/µF

tWARNING

= 16.67ms/µF

VTMR(V)

ITMR = 6µA

VDS = 75V

(ITMR = 50µA)

VDS = 10V

(ITMR = 8µA)

1.375

1.275

TIME

4363 F01

0.50

Fault Timer Operation in Overcurrent

TMR pin behavior in overcurrent is substantially the same

as in overvoltage. In the presence of an overcurrent con-

dition when the LT4363 regulates the output current, the

timer charges from 0.5V to 1.275V with a current that

varies as a function of VDS (see Figure 2). The current is

about 5 times the value produced in overvoltage, under

similar conditions VDS, increasing linearly from 8µA with

VDS < 0.5V to 260µA with VDS = 80V. VDS is inferred from

the drop across VCC and OUT. Because VDS is measured

indirectly, clamping or filtering at the VCC pin affects the

timer current response. A graph of Overcurrent TMR Cur-

rent vs (VCC – VOUT) is shown in the Typical Performance

Characteristics.

When TMR reaches 1.275V, the FLT pin is latched low as

an early warning of impending shutdown. But unlike the

overvoltage case, the timer current is not reduced and

instead continues unabated until TMR reaches 1.375V,

producing an early warning period given by:

CTMR = tWARNING •

ITMR

100mV

When TMR reaches 1.375V, the MOSFET is turned off and

allowed to cool for an extended period. The total elapsed

time between the onset of current limiting and turn-off

is given by:

tLIM =CTMR •

0.875V

ITMR

Because ITMR is a function of VCC – VOUT, the exact time

in current limit depends upon the input waveform and the

timerequiredfortheoutputcurrenttocomeintoregulation.

Cool Down Phase

Cool Down behavior is the same whether initiated by

overvoltage or overcurrent. During the cool down phase,

the timer continues to charge from 1.375V to 4.3V with

2µA, and then discharges back down to 0.5V with 2µA,

for a total equivalent voltage swing of 6.725V. The cool

down time is given by:

tCOOL =CTMR •

2.925V

+3.8V

2µA

UptothispointtheoperationoftheLT4363-1andLT4363-2

is the same. Behavior at the end of the cool down phase

and in response to the SHDN pin is entirely different.

At the end of the cool down phase the LT4363-1 remains

latched off and FLT remains low. It may be restarted by

pulling the SHDN pin low for at least 100µs or by cycling

power. The cool down phase may be interrupted at any-

time by pulling SHDN low for at least 1s/µF of CTMR; the

LT4363-1 will restart when SHDN goes high.

The LT4363-2 will automatically retry at the end of the

cool down phase. Retry is inhibited if the OV pin is above

1.275V; this prevents repetitive retries while the input is

held in a sustained overvoltage condition. Retry is auto-

APPLICATIONS INFORMATION

Figure 1. Overvoltage Fault Timer Current

Figure 2. Overcurrent Fault Timer Current

tFLT

= 2.98ms/µF

TOTAL FAULT TIMER = tFLT + tWARNING

tFLT = 22.14ms/µF

tWARNING

= 2.86ms/µF

tWARNING

= 0.38ms/µF

VTMR(V)

VDS = 10V

(ITMR = 35µA)

1.375

1.275

0.50

TIME

4363 F02

VDS = 80V

(ITMR = 260µA)

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