Fujitsu MB3788 manual Latch-Type Short-Circuit Protection Circuit

MB3788

■HOW TO SET TIME CONSTANT FOR TIMER & LATCH-TYPE SHORT-CIRCUIT PROTECTION CIRCUIT

If the load conditions of the switching regulator are stable, the outputs of comparators 1 and 2 do not change, so the SP comparator outputs a High level. At this time, the SCP pin (pin 15) is held at about 50 mV.

If the load conditions change suddenly due to a load short-circuit, for example, the output voltage of the com- parator of the channel becomes a High-level signal (more than 2.1 V). Then, the SVP comparator outputs a Low level and transistor Q1 is turned off. The short-circuit protection capacitor CPE externally connected to the SCP pin starts to charge.

VPE = 50 mV + tPE ⋅ 10-6/CPE

0.65 = 50 mV + tPE ⋅ 10-6/CPE

CPE = tPE /0.6 (s)

Once the capacitor CPE is charged to about 0.65 V, the SR latch is set and the output drive transistor is turned off. At this time, the duty cycle is made low and the output voltage of the SCP pin (pin 15) is held at Low level. This closes the SR latch input to discharge CPE.

							2.5 V
			1 ∝A
			15	R
			S
					Low	PWM	OUT
Comparator 1	-				input
			SR latch-type		voltage	comparator
Comparator 2	-	Q2	CPE	circuit	protection
	+	Q1			circuit
2.1 V

Fig. 5 Latch-Type Short-Circuit Protection Circuit

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