Operational Description

<Error Amp 1, 2>

<OSC Generator>

<Ramp Wave Generator 1, 2>

The Ramp Wave Generator generates a saw-tooth waveform based on outputs from the Phase Shift Generator.

<PWM Comparator 1, 2>

<PWM/PFM Controller 1, 2>

<Vref with Soft Start 1, 2>

<Chip Enable Function>

XC9503 Series

2ch. Step-down DC/DC Controller ICs

This circuit generates PFM pulses.

Control can be switched between PWM control and PWM/PFM automatic switching control using external signals.

The PWM/PFM automatic switching mode is selected when the voltage of the PWM1 (2) pin is less than 0.2V, and the control

switches between PWM and PFM automatically depending on the load. As the PFM circuit generates pulses based on outputs

from the PWM comparator, shifting between modes occurs smoothly. PWM control mode is selected when the voltage of the

PWM1 (2) pin is more than 0.65V. Noise is easily reduced with PWM control since the switching frequency is fixed.

Control suited to the application can easily be selected which is useful in audio applications, for example, where traditionally,

efficiencies have been sacrificed during stand-by as a result of using PWM control (due to the noise problems associated with

the PFM mode in stand-by).

This circuit generates the Switching Frequency which in turn generates the reference clock.

The PWM Comparator compares outputs from the Error Amp. and saw-tooth waveform. When the voltage from the Error Amp's

output is low, the external switch will be set to ON.

This function controls the operation and shutdown of the IC. When the voltage of the EN1 or EN2 pins is 0.2V or less, the mode

will be chip disable, the channel's operations will stop and the EXT pins will be kept at a highlevel (the external P-type MOSFET

will be OFF). When both EN1 and EN2 are in a state of chip disable, current consumption will be no more than 3.0

µA.

When the EN1 or EN2 pin's voltage is 0.65V or more, the mode will be chip enable and operations will recommence. With soft-

start, 95% of the set output voltage will be reached within 10mS (TYP) from the moment of chip enable.

The reference voltage, Vref (FB pin voltage)=0.9V, is adjusted and fixed by laser trimming (for output voltage settings, please

refer to page 8). To protect against inrush current, when the power is switched on, and also to protect against voltage overshoot,

soft-start time is set internally to 10ms. It should be noted, however, that this circuit does not protect the load capacitor (CL) from

inrush current. With the Vref voltage limited and depending upon the input to the error amps, the operation maintains a balance

between the two inputs of the error amps and controls the EXT pin's ON time so that it doesn't increase more than is necessary.

The XC9503 series are multi-functional, 2 channel step-down DC/DC converter controller ICs with built-in high speed, low ON

resistance drivers.

The Error Amplifier is designed to monitor the output voltage and it compares the feedback voltage (FB) with the reference

voltage. In response to feedback of a voltage lower than the reference voltage, the output voltage of the error amp. decreases.

7