QUICK_SWITCHING_PNP_REGULATOR

Published:2009/7/13 1:36:00 Author:May | From:SeekIC

The MIC5205 is a low-dropout PNP regulator that incorporates a noise bypass pin for additional noise reduction. A single 10-nF capacitor, connected from the bypass pin to ground, reduces output noise by Vout/1.24 V (12 dB for the 5-V part) and creates a noise pole below 100 Hz. Switch-on time is increased from 80 μs to 15 ms. With the addition of a few components, the following circuit preserves a low switch-on time. A low-to-high signal on EN switches the output on quickly. This allows R1 and C1 to hold Q1 off while R2 and C2 hold Q2 on. Cn is also quickly charged to the bypass pin voltage through Q2 by the voltage divider R3, R4. C1 and C2 then charge to the output voltage,turntng Q1 on and Q2 off. Cn is now switched from the voltage divider to the bypass pin. The regulator is now in the low-noise configuratipn, which takes about 100 μs. When EN goes low, C1 and C2 discharge through R1, R2, R3, R4, and Rload. This resets the circuit for the next turn-on cycle. C3 helps prevent overshoot on the output. Ratio R3/R4 can be found empirically: First, set the ratio close to the ratio 1.24 V/(Vout-1.24 V), then adjust the value so the output turns on quickly without overshoot. The final tolerance needs to be 1 percent. Switch-off time is determined only by output capacitor size and the load.



Reprinted Url Of This Article:
http://www.seekic.com/circuit_diagram/Basic_Circuit/QUICK_SWITCHING_PNP_REGULATOR.html

Print this Page | Comments | Reading(3)