L7985 datasheet(13/42 Pages) STMICROELECTRONICS

13 / 42 page

DocID022446 Rev 7

13/42

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

L7985 数据表(PDF) 13 Page - STMicroelectronics

L7985 数据表(HTML) 13 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

数据表下载