L7985 datasheet(13/42 Pages) STMICROELECTRONICS

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L7985

Functional description

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

So, from Equation 3, the maximum switching frequency that guarantees to limit the current

results:

Equation 4

With RDSON = 300 m, DCR = 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.

The pulse-by-pulse mechanism, which reduces the switching frequency down to one eighth

of the maximum FSW, adjusted by the FSW pin, assures that 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 5

where FSW* is 700 kHz divided by eight.

Figure 8. Overcurrent protection

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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