M63151FP

Rev.1.0, Sep.16.2003, page 7 of 13

M63151FP PWM control method

A triangular waveform is output on the CFC pin under the control of the values of the external capacitor connected to

Cfc and the external resistor connected to Rfc. The PWM duty cycle is determined through comparison of this voltage

with the control voltage determined by the current output from CPout and the external filter connected to the CPout and

PREF pins, i.e. the voltage on PREF.

That is, PWM-ON is satisfied when the triangular waveform is at a level below the control voltage and PWM-OFF is

satisfied (regenerative) when the triangular waveform rises above the control voltage. How the relation between the

voltages on Cfc and Pref determines the output voltage is depicted below.

Min.0.85 V less to max.4.00 V or more

(a) When VPREF < 1.25 V,

PWM duty cycle = 0% (always regennerative)

(b) When1.25 V < VPREF < 3.75 V, PWM control

(c) When3.75 V < VPREF, PWM duty cycle =100%

Cfc output waveform rage

Peak = 3.75 V

Trough = 1.25 V

(a) Minimum voltage: 1.25 V

0.25 V

(b) Maximum voltage: 3.75 V

0.25 V

VPREFthrehold voltage (VtPREF)

1.25V 0.25 V

(a) When VPREF< VtVPREF, motor ison

(b) When VPREF< VtVPREF, motor is always off.

PREF threshold voltage

High:PWM - ON

PWM - Duty cycle

Cfc output voltage

VPREF (control voltage)

VPREF Operating range

Low:PWM - OFF

→

→

→

±

±

±

While the motor is rotating, the Cfc output voltage is driven low with the same timing as switching of phase for the

motor in order to improve changes in the rotation speed.

Therefore, the voltage waveform output on Cfc during actual motor operation is as shown in the following figure.

0 V

1.25 V

3.75 V

Phase U

The dashed lines indicate

the timing of phase switching.

The hold line shows the PWM

output voltage wavform.

Phase V

Phase W

Cfc output voltage

VPREF

→

→

→