Fujitsu MB39A104 manual About TIMER-LATCH Protection Circuit, Overcurrent detection circuit

MB39A104

■ABOUT TIMER-LATCH PROTECTION CIRCUIT

1. Setting Timer-Latch Overcurrent Protection Detection Current

The overcurrent protection circuit is actuated upon completion of the soft-start period. When an overcurrent flows, the circuit detects the increase in the voltage between the FET’s drain and source using the external FET ON resistor (RON), actuates the timer circuit, and starts charging the capacitor CSCP connected to the CSCP terminal (pin 8). If the overcurrent remains flowing beyond the predetermined period of time, the circuit sets the latch to fix OUT terminals (pin 3, 22) at “H” level and turn off the external FET. The detection current value can be set by the resistors (RLIM1 and RLIM2) connected between the FET’s drain and the ILIM1 terminal (pin 5) and between the drain and the ILIM2 terminal (pin 20), respectively.

The internal current (ILIM) can be set by the timing resistor (RT) connected to the RT terminal (pin 12).

Time until activating timer circuit and setting latch is equal to short-circuit detection time in "2. Setting Time Constant for Timer-Latch Short-Circuit Protection Circuit".

Internal current value: ILIM

ILIM (∝A) =:	2700
		RT (kΩ)
Detection current value: IOCP
IOCP (A) =:	ILIM(A) ⋅ RLIM(Ω)		−		(VIN(V) − VO(V)) ⋅ VO(V)
		RON (Ω)		2	⋅ VIN(V) ⋅ fOSC(Hz) ⋅ L(H)

RLIM : Overcurrent detection resistor

RON : External FET ON resistor

VIN : Input voltage

VO : DC/DC converter output voltage

fOSC : Oscillation frequency

L : Coil inductance

To reset the actuated protection circuit, either the power supply turn off and on again or set the CTL terminal (pin 24) to the "L" level to lower the VREF terminal (pin 17) voltage to 2.4 V (Min) or less.

• Overcurrent detection circuit

VIN

		Current
		Protection
		Logic
(1 ∝A)
CSCP
8		VREF
S	R
Latch		UVLO

− +

Each

Channel

Drive

Q1 L

(VS2)

21VS1

4

VO

(ILIM2)

20ILIM1 5 (RLIM)

17