MB3871 Datasheet, PDF (16/25 Pages) Fujitsu Component Limited. – Mobile Pentium II DC/DC Converter IC

MB3871
s METHOD OF SETTING THE SOFT START TIME
At startup of the MB3871, the capacitor (CS) connected to the CS pin begins charging. This produces a soft
start, by providing output voltage from Error Amp. that is proportional to the CS pin voltage regardless of the
DC/DC converter load current.
Soft start time (time to output setting voltage VD)
tS (sec) ^{~ VD} × CS (µF)
2 (µA)
s TIME SETTING BY SHORT DETECTION
When load conditions change rapidly with the reduced output voltage, as when a load fault occurs, the Capacitor
Cscp connected to the CSCP pin (pin 22) is charged to threshold voltage (VTH:=0.68V) and sets a latch, the
external FET is turned off (inactive interval 100%).
Short detection time
tPE (sec) ^~ 0.68 × CSCP (µF) / 2 (µA)
s OSCILLATOR FREQUENCY SETTING
The oscillator frequency can be set by connecting resistance to the RT pin (pin 1).
Oscillator frequency
fOSC (kHz) ^~ 26250 / RT (kΩ)
s METHOD OF SETTING THE DISCHARGE TIME
• An “L” level CTL signal while the ENB pin is set to “H” level causes the resistance (RS) connected to the RS
pin to discharge electrical charge the capacitor (CS) connected to the CS pin, causing the output voltage to
fall gradually regardless of the DC/DC converter load current.
Discharge time (time to 0.05 V output voltage)
toff (msec) ^~ RS (kΩ) × CS (µF) × ln ( ^VD )
VTH (CS COMP)
• As long as the ENB pin is set to “L” level, the discharge control function is switched OFF.
s D/A BLOCK VD3 to VD0 SWITCHING
• Switching of the VD3 to VD0 pin signal during the MB3871 operation may cause transient fluctuation in output
voltage from the DC/DC converter. The resulting voltage instability may cause an “L” level from the PWRGOOD
block, activating the OVP protection and shutting off the output from the DC/DC converter.
To switch VD3 to VD0 pin settings, first input an “L” level control signal to the CTL pin to place the MB3871
in standby status.
• When all VD3 to VD0 pin signals are set to “H” level, the DC/DC converter output is switched OFF.
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