analysis LED Flexi
30/09/2013 09:10
Where , C is the output terminal of the capacitor size , in the ON state of the main transistor parasitic capacitance.
2.4 PWM comparator
The PWM comparator compares the current sample after the slope compensation signal and the error amplifier compares the output control signal when the current sampling circuit output signal reaches a peak value of the control signal , PWM signal is toggled to produce a very narrow pulse signal , trigger off power tube . This design of the PWM comparator shown in Figure 5 , compared to ordinary comparator, differential pair left more than a transistor MP47.
Figure 5 PWM comparator. T8 Tube
Circuit starts when the output voltage is low because the system will make the error amplifier output signal is high , the gate drive signal causes the duty cycle reaches 100% , causing the DC / DC output generates a high pulse. After increasing SS pin , at startup , SS terminal voltage signal will rise slowly , masked Vea high control signal to the gate driving signal duty cycle gradually increased , to achieve a soft start circuit . When the soft-start after a successful , SS terminal voltage also rises above Vea, thereby turning off MP47, the circuit enters the normal operating condition.
3 Simulation results and analysis LED Flexi
3.1 The error amplifier gain and phase curves
Figure 6 shows the error amplifier gain and phase curves , which , Gain and Phase represent the open-loop gain of the error amplifier and phase . As can be seen from the figure , in the ON state , the static gain of the error amplifier can be achieved 70dB, and the 3dB bandwidth of up to 10kHz or more; also be seen , the circuit has only one main pole , so you can have a 90 ° phase margin .
