MB3870 Datasheet, PDF (14/25 Pages) Fujitsu Component Limited. – Pentium II DC/DC Converter IC

MB3870
s FUNCTION DESCRIPTION
1. Switching Regulator Function
(1) Reference voltage circuit (Ref)
The reference voltage circuit uses the voltage supply from the VCC pin (pin 23) to generate a temperature
compensated reference voltage (≅ 3.5 V) for use as the reference voltage for the internal circuits of the IC chip.
It is also possible to supply a reference voltage output of up to 1 mA to external circuits through the VREF pin
(pin 24).
(2) Triangular wave oscillator (OSC)
The triangular wave form is generated using an on-chip frequency selection capacitor, plus the frequency
selection resistance connected to the RT pin (pin 1).
The triangular wave is input to the PWM comparator circuits on the IC.
(3) Error amplifier (Error Amp.)
The error amplifier circuit is used to detect the output voltage from the DC/DC converter for output as the PWM
control signal. The in-phase input range covers the full range from 0 V to VCC – 0.9 V. By connecting a feedback
resistance and capacitor between the FB pin (pin 6) and –IN pin (pin 5), it is possible to create any desired
level of loop gain, thereby providing stable phase compensation to the system.
Also, it is possible to prevent current spikes at power supply start-up by connecting a soft start capacitor to the
CS pin (pin 4), the non-inverting input pin for Error Amp. The use of Error Amp. for soft start detection makes
it possible for a system to operate on a fixed soft start time that is independent of the output load on the DC/
DC converter.
(4) PWM comparators (PWM Comp.1, PWM Comp.2)
PWM Comp.1 and PWM Comp.2 are voltage-pulse width converters that control output voltage according to
input voltage.
PWM Comp.1 controls the pulse width on the main-side output circuit, and PWM Comp.2 controls the pulse
width on the synchronous rectifier side output circuit.
The triangular wave generated by the triangular wave oscillator is compared with the output voltage from Error
Amp., and during intervals when Error Amp. output is higher than the triangular wave, the main-side output
transistor is switched on and the synchronous rectifier side output transistor is turned off.
PWM Comp.1 is set to a maximum duty cycle of approximately 90%.
(5) Output circuits (Drive1, Drive2)
The output circuits on both the main-side and synchronous rectifier-side have a totem-pole configuration, and
are capable of driving an external N-ch. MOS FET.
(6) Power supply control circuit (CTL)
This circuit is able to control power supply ON/OFF switching from the CTL pin (pin 19). (During standby mode,
supply current is 0 µA TYP.)
(7) DAC circuit (D/A)
This circuit controls the output voltage to the CPU between 1.3 V and 3.5 V, as selected by the 5-bit (32-step)
input signal information.
The output voltage can be set in 100 mV steps between 3.5 V and 2.1 V, and in 50 mV steps between 2.05 V
and 1.3 V.
When all D/A input pins VD4 through VD0 (pin 18 through pin 14) are set to “H” level, the DC/DC converter
output voltage is 0 V.
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