Si9750

Vishay Siliconix

www.vishay.com

4

Document Number: 70028

S-40754—Rev. D, 19-Apr-04

FUNCTIONAL DESCRIPTION

The Si9750 together with an n-channel MOSFET provides the

following functions:

D limits di/dt current for hot insertion applications

D provides complete short circuit protection

D high-side drive allows n-channel MOSFET to be

used, for lower power dissipation

D industry-standard microprocessor reset function

D logic control input and outputs

Setting the Current Limit

The current limit point is determined by the voltage across

current limit circuit is shown in Figure 1

The steady state current is set by the equation:

(1)

Due to the highly capacitive nature of some loads, the Si9750

has an option to increase the current limit point to a much

If the HI/LO pin is tied low the current limit is 20% higher during

turn-on than the steady state current limit point.

(2)

(with pin HI/LO=Low)

If a higher current limit is needed at start-up, the HI/LO pin can

be tied high. The equation becomes:

(3)

(HI/LO = High)

Notice that any current limit can be set at turn-on using an

Relaxation Mode Current Limit

In an overload condition, the Si9750 will go into a relaxation

mode current limit operation that not only protects the source

and load, but also reduces the power dissipated in the

MOSFET. When an overload is detected, the circuit quickly

turns off, then goes into a retry mode whereby the current is

ramped up slowly. If the fault still exists, the current will ramp

down again. This sequence will repeat indefinitely at a period

but the period should be >50 ms.

FIGURE 1.

−

+

Overcurrent

LIMSET

1 kW, "20%

SENSE

(Optional)

HI/LO

HI/LO

GATE

Si9750

330 W

33 nF

330 W