M54679FP
5. Oscillation circuit.
External capacitor is charged and discharged by the constant current and a triangular waveform appears to Vref
terminal.
The waveform voltage level is shown in right figure.
This triangular waveform is a carrier frequency of PWM circuit.
The carrier frequency change if this external capacitor value is changed.
M54679FP is designed that the oscillation frequency is 30kHz
if the external capacitor value is 390pF.
The oscillation frequency is in inverse proportion to the value of a external capacitor.
2.5V
0.5V
6. Spike current cancellation.
Output power transistors go to ON, then the spike current appears on the RS (current sensing) in a short time and this is
caused by the internal delay time.
M54679FP has the cancellation circuit of the spike current as the current sensing comparators do not cause error
functions.
So, the function of current sensing comparators are shut off during 2ms since the output power transistors go to ON.
7. Ph signal delay circuit.
M54679FP has a delay time of 3.0µs until output H-bridge power stage go to ON since Ph signal change Low to high.
This delay time is enough short time for the frequency (plus rate) of Ph signal and there is no problem in the normal
function.
8. Rs and S1 or S2 terminal.
If S1 or S2 terminal (non-inverted input of the current sensing comparators) is connected the nearest position of current
sensing resistor, the error of the current sensing by means of wire resistance on the board will be decreased.
9. Voltage stabilizer.
M54679FP has a voltage stabilizer of 3.5V.
The reference voltage (Vref) can connect the output (Regout)
of voltage stabilizer directly.
In this case, the current capability of the output of voltage stabilizer is 1.0mA (source current), 0.1mA (sink current).
10. Setting output current.
As the output circuit of M54679FP is designed by the bipolar type NPN transistors, the current that go through the
motor coil is smaller about 15mA (typical) than the current that go through the current sensing resistor.
This is caused by the base current of the power transistors.
Therefore, be aware this base current when the output current is set.
Rev.1.0, Sep.19.2003, page 7 of 12
RENESAS [ RENESAS TECHNOLOGY CORP ]
