Fujitsu MB3878 manual Setting the Charging Voltage, Method of Setting the Charging Current

MB3878

■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. Be sure to select a resistor value that allows you to ignore the on resistor (70 Ω, 1mA) of the internal FET connected to the OUTD terminal (pin 11).

Battery charging voltage: VO
VO (V) =	(R3 + R4) / R4 ⋅ 4.2 (V)
	B	VO
	R3		< Error Amp.3 >
		−INE3	−
		16
			+
	R4		+
		11	4.2 V
		OUTD
		22
		CS

■METHOD OF SETTING THE CHARGING CURRENT

The charge current (output control 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) = 25 ⋅ I1 (A) ⋅ RS (Ω)

■METHOD OF SETTING THE SOFT-START TIME

Upon activation, the IC starts charging the capacitor (Cs) connected to the CS terminal (pin 22).

The error amplifier causes soft-start operation to be performed with the output voltage in proportion to the CS terminal voltage regardless of the load current of the DC/DC converter.

Soft-start time: ts (Time taken for the output voltage to reach 100 %)

ts (s) =: 4.2 ⋅ CS (∝F)

■METHOD OF SETTING THE TRIANGULAR WAVE OSCILLATOR FREQUENCY

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

Triangular wave oscillator frequency: fOSC

fOSC (kHz) =: 13630 / RT (kΩ)

14