A7985ATR datasheet(15/44 Pages) STMICROELECTRONICS

15 / 44 page

DocID023128 Rev 5

15/44

A7985A

Functional description

This kind of overcurrent protection is effective if the output current is limited. To prevent the

current from diverging, the current ripple in the inductor during the ON-time must not be

higher than the current ripple during the OFF-time. That is:

Equation 4

If the output voltage is shorted, VOUT 0, IOUT = ILIM, D/FSW = TON_MIN, (1-D)/FSW 1/FSW.

So from the above equation the maximum switching frequency that guarantees to limit the

current results:

Equation 5

With RDS(on) = 300 m, DRC = 0.08 , the worst condition is with VIN = 38 V, ILIM = 2.5 A;

the maximum frequency to keep the output current limited during the short-circuit results

74 kHz.

Based on the pulse-by-pulse mechanism, that reduces the switching frequency down to one

eighth, the maximum FSW, adjusted by the FSW pin, that assures a full effective output

current limitation is 74 kHz * 8 = 592 kHz.

If, with VIN = 38 V, the switching frequency is set higher than 592 kHz, during short-circuit

condition the system finds a different equilibrium with higher current. For example, with

FSW = 700 kHz and the output shorted to ground, the output current is limited around:

Equation 6

where FSW* is 700 kHz divided by eight.

VIN V

–

OUT

RDSON IOUT



DCR IOUT



–

LFSW



------------------------------------------------------------------------------------------------------------ D



VOUT VF RDSON IOUT



DCR IOUT



LFSW



-----------------------------------------------------------------------------------------------------------

–





FSW

VF DCR I



LIM



VIN

RDSON DCR

 I

LIM



–



-------------------------------------------------------------------------------

TON_MIN

----------------------



IOUT

VIN FSW



VF TON_MIN



–

DRC TON_MIN





RDSON DCR

 F



----------------------------------------------------------------------------------------------------------------

3.68A

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