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3DQDVRQLF
3.5. Oscillator and Constant Voltage
Control Circuit
The oscillator within IC801 makes use of the charging
and discharging of internal capacitor C1 (4700pF)
and generates pulse signals which turn the internal
power MOSFET On and Off. The constant voltage
control of a switch mode power supply is performed
by fixing the OFF time of the MOSFET (around 50uS)
and changing the ON time in the pulse width control
operation.
and discharging of internal capacitor C1 (4700pF)
and generates pulse signals which turn the internal
power MOSFET On and Off. The constant voltage
control of a switch mode power supply is performed
by fixing the OFF time of the MOSFET (around 50uS)
and changing the ON time in the pulse width control
operation.
3.5.1. ’ON’ Condition and Time
When the switching power MOSFET is ON, C1
begins to charge.
begins to charge.
3.5.2. From ’ON’ to ’OFF’
When the voltage on C1 reaches (approx.) 6.5V, the
output from the oscillator is reversed, and the internal
switching power MOSFET switches OFF.
output from the oscillator is reversed, and the internal
switching power MOSFET switches OFF.
3.5.3. ’OFF’ Condition and Time
With the power MOSFET now OFF, Capacitor C1
starts discharging through R1, at the fixed time
determined by the time constant C1, R1.
starts discharging through R1, at the fixed time
determined by the time constant C1, R1.
3.5.4. From ’OFF’ to ’ON’
When C1 voltage has dropped to around 3.7V, the
output from the oscillator is reversed again and the
power MOSFET again turns ON, thus repeating the
cycle.
output from the oscillator is reversed again and the
power MOSFET again turns ON, thus repeating the
cycle.
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