MB3785A Datasheet, PDF (6/29 Pages) Fujitsu Component Limited. – Switching Regulator Controller (4 Channels plus High-Precision, High-Frequency Capabilities)

MB3785A
s FUNCTIONAL DESCRIPTION
1. Switching Regulator Function
(1) Reference voltage circuit
The reference voltage circuit generates a temperature-compensated reference voltage (≅ 2.50 V) using the voltage
supplied from the power supply terminal (pin 18). This voltage is used as the operating power supply for the internal
circuits of the IC. The reference voltage can also be supplied to an external device from the VREF terminal (pin 19).
(2) Triangular-wave oscillator circuit
By connecting a timing capacitor and a resistor to the CT (pin 17) and the RT (pin 16) terminals, it is possible to
generate any desired triangular oscillation waveform. The oscillation can also be obtained by using a ceramic
resonator connected to pins 14 and 15.
This waveform has an amplitude of 1.3 V to 1.9 V and is input to the internal PWM comparator of the IC. At the
same time, it can also be supplied to an external device from the CT terminal (pin 17).
(3) Error amplifier
This amplifier detects the output voltage of the switching regulator and outputs a PWM control signal accordingly.
It has a wide common-mode input voltage range from –0.2 V to VCC –1.8 V and allows easy setting from an external
power supply, making the system suitable for DC motor speed control.
By connecting a feedback resistor and capacitor from the error amplifier output pin to the inverted input pin, you
can form any desired loop gain, for stable phase compensation.
(4) PWM comparator
• CH1 & CH2
The PWM comparators in these channels are a voltage comparator with two inverted input and one non-inverted
input, that is, a voltage-pulse width converter to control the output pulse on-time according to the input voltage. It
turns on the output transistor when the triangular wave from the oscillator is higher than both the error amplifier
output and the DTC-pin voltages.
• CH3 & CH4
The PWM comparators in these channels are a voltage comparator with one inverted input and two non-inverted
inputs, that is, a voltage-pulse width converter to control the output pulse on-time according to the input voltage. It
turns on the output transistor when the triangular wave from the oscillator is lower than both the error amplifier
output and the DTC-pin voltages.
These four channels can be provided with a soft start function by using the DTC pin.
(5) Output circuit
The output circuit is comprised of a totem-pole configuration and can drive a PNP transistor (30 mA max.)
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