MB3775 Datasheet, PDF (15/25 Pages) Fujitsu Component Limited. – SWITCHING REGULATOR CONTROLLER

MB3775
s APPLICATION
1. How to set the error amplifier frequency characteristic
Figure 22 shows the equivalent circuit of the error amplifier.
The frequency characteristic of the error amplifier is set by R1, R2, and CP . The high-frequency gain is set by the ratio of resistors
R1 and R2 in the IC (set value 0 dB).
When CP = 0.1 µF, the gain at 20 kHz ≤ f ≤ 5 MHz is about 0 dB. The roll-off frequency is adjusted by changing external phase
compensating capacitor CP (see Fig. 24).
When high frequency gain is needed or the phase must be advanced at a low frequency, connect a resistor RP between the FB
terminal and CP as shown in Figure 23 (see Fig. 25).
Fig. 22 - Error Amp. Equivalent Circuit
Error Amp.
[- IN]
[+ IN]
-
x 120
+
R1 38 kΩ
PWM COMP
R2 470 Ω
[FB]
CP
[- IN]
[+ IN]
Fig. 23 - Error Amp. Equivalent Circuit (Insert RP)
Error Amp.
-
x 120
+
R1 38 kΩ
PWM COMP
R2 470 Ω
[FB]
RP
CP
NOTE:
As shown above, the frequency characteristic of the error amplifier is set by the external phase compensating capacitor
CP.
Wchhaerancatecreisrtaicm. i(c-3c0hi_°pCc.=a.1p/a5caitnodr
m80u_°sCt b.=.e1/u3sfeodr
to meet the requirements of
the frequency characteristic,
a small system, be careful of its temperature
so a sufficient phase margin must be allowed
for at room temperature.) Ceramic chip capacitors with a low temperature characteristic (B characteristic) or film
capacitors are recommended (see Fig. 26 to 28).
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