MC7731N6 Datasheet, PDF (6/10 Pages) Cystech Electonics Corp. – Current Mode PWM Controller

CYStech Electronics Corp.
Spec. No. : C494N6
Issued Date : 2012.04.02
Revised Date :
Page No. : 6/10
When VFB decreases further, the power supply will enter into burst mode operation to decrease the power
consumed at no load condition. Besides there is no audible noise in any load condition.
Oscillator Operation
A resistor from RI pin to ground will generate a constant current source for MC7731N6. This current is
used to charge/discharge an internal capacitor and hence the internal clock and switching frequency are
determined. Increase the resistance will decrease the current source and reduce the switching frequency.
The relation between Ri and switching frequency is:
Built-in Slope Compensation
The sensed voltage across the sense resistor is used for pwm control, and pulse by pulse current limit.
Built-in slope compensation circuit adds a voltage ramp onto the current sense input voltage. This greatly
improves the close loop stability and prevents the sub-harmonic oscillation of peak current mode pwm
control scheme.
Leading Edge Blanking
Each time when the power MOSFET is switched on, a turn-on spike will inevitably occur on the
sense-resistor. To avoid premature termination of the switching pulse, a 270 nsec leading-edge blanking
time is built in. Conventional RC filtering can therefore be omitted. During this blanking period, the
current-limit comparator is disabled and it cannot switch off the gate driver.
Gate Driver
The output stage of MC7731N6 is a fast totem pole gate driver. Cross conduction has been avoided to
minimize heat dissipation, increases efficiency and enhances reliability. The output driver is clamped by an
internal 18V Zener diode in order to protect power MOSFET transistors against undesirable gate over
voltage. A soft driving waveform is implemented to minimize EMI.
Frequency Jitter
The frequency jitter function is integrated in the controller, the jitter is modulated by a periodic signal,
the modulate signal frequency is much smaller than the oscillator frequency, By this way，the EMI noise
has a wider spectrum with lower amplitudes.
MC7731N6
CYStek Product Specification