MB3882

13

s

s

s

s FUNCTION DESCRIPTION

1.

DC/DC Converter Function

(1) Reference Voltage Block

The reference voltage circuit takes the voltage feed from the power supply terminal (pin 23) and generates a

temperature compensated reference voltage (2.5 V typ.) , for use as the reference voltage for the power supply

control unit.

Also, an external load current can be obtained from the power supply at the VREF terminal (pin 24) , up to a

maximum of 1 mA.

(2) Triangular Wave Oscillator Block

A triangular waveform with amplitude 1.3 V to 1.9 V can be generated by connecting a timing capacitor and

resistor to the CT terminal (pin 1) and RT terminal (pin 2) , respectively.

The triangular oscillator waveform can be input to the IC’s internal PWM comparator, as well as supplied externally

from the CT terminal.

(3) Error Amp Block (Error Amp.)

The error Amp. is an amplifier that detects the output voltage from the DC/DC converter and outputs a PWM

control signal. The error Amp. has a broad in-phase input voltage range of 0 to Vcc

−1.8 V that can be easily set

by the external power supply.

In addition, an arbitrary loop gain can be set up by connecting a feedback resistor and capacitor between the

error Amp. output terminal and inverter input terminal, providing stable phase compensation to the system.

Also, power-on rush current can be prevented by connecting a soft start capacitor between the error Amp. non-

inverted input terminals CS1 terminal (pin 4) and CS2 terminal (pin 21) . The soft start function operates with a

stable soft start time that is not dependent on the output load of the DC/DC converter.

(4) PWM Comparator Block (PWM Comp.)

This is a voltage - pulse width modulator that controls the output duty according to the input voltage.

(5) Output Block

The output block has totem pole configuration on both the main side and synchronous rectifier side, and can

drive an external N-ch MOSFET.

Also, the high output drive capability (700 mA max : duty

≤ 5%) provides high gate-source capacitor, enabling

the use of low on-resistor FET devices.

Main side :

Turns the output transistor on in the intervals in which the error Amp. output

voltage is higher than the triangular wave voltage.

Synchronous rectifier side : Turns the output transistor on in the intervals in which the triangular wave volt-

age the is lower than error Amp. voltage.