Fujitsu MB39A104 manual Overcurrent Protection Circuit Range of Operation

MB39A104

Overcurrent Protection Circuit: Range of Operation

When an overcurrent flow occurs, if the increased voltage between the drain and source of the FET is detected by means of the external FET (Q1) resistor, operational stability is lost when the external FET (Q1) ON interval determined by the oscillation frequency, input voltage, and output voltage falls below 450 ns.

Therefore, the circuit should be used within a range that ensures that the ON interval does not fall below 450ns, according to the following formula.

ON interval 450 (ns) ≥	VO (V)
	VIN (V)	⋅
		fOSC (Hz)

If the ON interval of the external FET (Q1) is below 450ns, we recommend the use of an overcurrent detection resistor RS to detect overcurrent, as shown below.

This example shows the range of operation of the overcurrent detection function with a setting of Vo = 3.3V.

• Method to detect by current when external FET(Q1) is turned on
		VIN		Overcurrent Detection Function Operating Range
			(Rs)
					1600
			ErrAmp
			Q1		1400
									VO = set to 3.3 V
	21 (VS2)
					1200
	4	VS1		(kHz)	1000
				fOSC	800
	(ILIM2)				400		Operation Range
−
	20	ILIM1	Connect to RS		200
+	5		when
					0
			using RS
					6	8	10	12	14	16	18	20
								VCC (V)

• Method to detect by mean current
	VIN		Overcurrent Detection Function Operating Range
		ErrAmp		1600
	Q1	RS		1400
	21 (VS2)			1200			VO = set to 3.3 V
	4 VS1		(kHz)	1000
				800
			fOSC
				600		Operation Range
−	(ILIM2)			400
	20 ILIM1			200
+
	5
				0	8	10	12	14	16	18	20
				6
							VCC (V)

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