L7986TR datasheet(13/43 Pages) STMICROELECTRONICS

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L7986

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 3

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 = 3.7 A;

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

88 kHz.

The pulse-by-pulse mechanism, which 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 88 kHz * 8 = 706 kHz.

If, with VIN = 38 V, the switching frequency is set higher than 706 kHz, during short-circuit

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

FSW = 800 kHz and the output shorted to ground, the output current is limited around:

Equation 5

where FSW* is 800 kHz divided by eight.

VIN VOUT 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



–

DCR TON_MIN





RDSON DCR

 F



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

4.2A

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