LM3753 Datasheet, PDF (27/38 Pages) National Semiconductor (TI) – Scalable 2-Phase Synchronous Buck Controllers with Integrated FET Drivers and Linear Regulator Controller

For output capacitors with very low ESR, if the target
crossover frequency is more than 10 times the filter pole fre-
quency, bandwidth limiting of the error amplifier may occur.
See the Comprehensive Equations section to incorporate the
error amplifier bandwidth into the design procedure.
For reference, the parallel equivalent C_O and R_C at any fre-
quency can be calculated from:
At the target crossover frequency X1 = 0.00603, X2 = 0.0603,
Z = 0.00592 and A = 1.213. The parallel equivalent C_O = 478
μF and R_C = 2.1 mΩ.
Calculate the error amplifier gain coefficient and the compen-
sation component values. The (1 − ω_P/ω_C) term is the cor-
rection factor for the modulator damping.
30091976
FIGURE 16. Error Amplifier Gain
For the design example, the calculated values are G_C = 1.71,
C_HF = 103 pF, C_COMP = 2236 pF, R_COMP = 6527Ω, R_FF = 245
and C_FF = 4483 pF.
Using standard values of C_HF = 100 pF, C_COMP = 2200 pF,
R_COMP = 6.2 kΩ, R_FF = 240Ω and C_FF = 4700 pF, the error
amplifier plots of gain and phase are shown in Figure 16 and
Figure 17.
30091977
FIGURE 17. Error Amplifier Phase
The complete control loop transfer function is equal to the
product of the power stage transfer function and error ampli-
fier transfer function. For the Bode plots, the overall loop gain
is the equal to the sum in dB and the overall phase is equal
to the sum in degrees. Results are shown in Figure 18 and
Figure 19. The crossover frequency is 57 kHz with a phase
margin of 73°.
27
www.national.com