Power Supply User's Manual

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.

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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 : V

VO (V) = (R3 + R4) / R4 × 4.2 (V)

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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) × R

S (Ω)

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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 : f

OSC

fOSC (kHz) := 13630 / RT (kΩ)

−

4.2 V

−INE3

OUTD