Fujitsu MB3887 manual Setting The Charging Voltage, Method Of Setting The Charging Current

MB3887

3. Soft-Start Function

Soft-start block (SOFT)

Connecting a capacitor to the CS terminal (pin 22) prevents rush currents when the IC is turned on. Using an error amplifier for soft-start detection makes the soft-start time constant, being independent of the output load of the DC/DC converter.

■SETTING THE CHARGING VOLTAGE

The charging voltage (DC/DC output voltage) can be set by connecting external voltage setting resistors (R3, R4) to the −INE3 terminal (pin 16) . Be sure to select a resistor value that allows you to ignore the on-resistor (35 Ω, 1mA) of the internal FET connected to the OUTD terminal (pin 11) . In standby mode, the charging voltage is applied to OUTD termial. Therefore, output voltage must be adjusted so that voltage applied to OUTD terminal is 17 V or less.

Battery charging voltage : VO

VO (V) =	(R3 + R4) / R4 ⋅ 4.2 (V)
	B	VO
	R3		<Error Amp3>
		−INE3	−
		16
			+
	R4		+
		11	4.2 V
		OUTD
		22
		CS

■METHOD OF SETTING THE CHARGING CURRENT

The charge current (output limit current) value can be set with the voltage at the +INE1 terminal (pin 9) .

If a current exceeding the set value attempts to flow, the charge voltage drops according to the set current value.

Battery charge current setting voltage : +INE1 +INE1 (V) = 20 ⋅ I1 (A) ⋅ RS (Ω)

■METHOD OF SETTING THE TRIANGULAR WAVE OSCILLATION FREQUENCY

The triangular wave oscillation frequency can be set by the timing resistor (RT) connected the RT terminal (pin 17) .

Triangular wave oscillation frequency : fOSC fOSC (kHz) =: 13630 / RT (kΩ)

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