LM3655 datasheet(16/21 Pages) NSC

16 / 21 page

13.0 Safety Functions (Continued)

These specs apply to the entire Normal Temperature Range.

Specification

Test Conditions

Min

Typ

Max

Units

Short Circuit Discharge Current “I

SHUTOFF-THRESH”

The voltage here divided by M4 resistance = I

SHUTOFF.

CELL = 3.6V

Voltage across M4

−20˚C to +70˚C

−20˚C to +125˚C

425

350

1000

Short Circuit Discharge Current Recovery “

SHUTOFF”IRADIO_B+ =ISHUTOFF

0.01

SHUTOFF-THRESH refers to a voltage threshold between the Cell pins and the Radio B+ pins. When the voltage between these pins

exceeds this voltage threshold, M4 is to open. This circuit reacts very quickly as it is intended to be immediate. This voltage

threshold is then referred to current by dividing it by the resistance of M4 (the resistance between the Cell and Radio B+ pins).

For example the threshold current (I

SHUTOFF) is 7 amps for a threshold voltage of 700 mV and a resistance of 100 m

Ω.Itis

specified in this manner to reflect the circuit implementation.

If the short circuit current is less than the lower limit, there will be no interruption of I

RADIOB+ short-circuit current. IREG will reduce

RADIOB+ to 4A in approximately 400 µs. If the current is greater than the upper limit, the short-circuit event will be guaranteed to

activate I

SC. The sequence is: PMOS M4 is turned off for

SHUTOFF and allowed to turn on with the current regulating loop limiting

current to 4 Amps in 400 µs. Very large currents and associated energy are avoided for this 400 µs settling period.

20111509

FIGURE 6. Short-Circuit Protection Waveform

LM3655 bảng dữ liệu(PDF) 16 Page - National Semiconductor (TI)