LM3743_08 Datasheet, PDF (21/28 Pages) National Semiconductor (TI) – High-Performance Synchronous Buck Controller with Comprehensive Fault Protection Features

The gain and phase of the error amplifier are shown in Figure
10.
20177423
20177424
FIGURE 10. Error Amp. Gain and Phase
In VM regulators, the top feedback resistor R2 forms a part of
the compensation. Setting R2 to 10 kΩ±1%, usually gives
values for the other compensation resistors and capacitors
that fall within a reasonable range. (Capacitances > 1 pF, re-
sistances <1 MΩ) C7, C8, C9, R4, and R5 are selected to
provide the poles and zeroes at the desired frequencies, us-
ing the following equations:
In practice, a good trade off between phase margin and band-
width can be obtained by selecting the closest ±10% capac-
itor values above what are suggested for C7 and C8, the
closest ±10% capacitor value below the suggestion for C9,
and the closest ±1% resistor values below the suggestions
for R4, R5. Note that if the suggested value for R5 is less than
100Ω, it should be replaced by a short circuit. Following this
guideline, the compensation components will be:
C7 = 47 pF±10%, C9 = 1.5 nF±10%
C8 = 2.2 nF±10%, R4 = 22.6 kΩ±1%
R5 = 2.1 kΩ±1%
The transfer function of the compensation block can be de-
rived by considering the compensation components as
impedance blocks Z_F and Z_I around an inverting op-amp:
As with the generic equation, G_EA-ACTUAL must be modified to
take into account the limited bandwidth of the error amplifier.
The result is:
21
www.national.com