MIC2168A Datasheet, PDF (14/18 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck Control IC

Micrel
MIC2168A
From the point of view of compensating the voltage
loop, it is recommended to use higher ESR output
capacitors since they provide a 90° phase gain in the
power path. For comparison purposes, Figure 6, shows
the same phase curve with an ESR value of 0.002Ω.
Figure 4. The Gain Curve for G(s)
Figure 6. The Phase Curve with ESR = 0.002Ω
Figure 5. Phase Curve for G(s)
It can be seen from the transfer function G(s) and the
gain curve that the output inductor and capacitor create
a two pole system with a break frequency at:
f_C = _{2 × π}
1
L × C_OUT
Therefore, f_LC = 3.6kHz.
By looking at the phase curve, it can be seen that the
output capacitor ESR (0.050Ω) cancels one of the two
poles (LCOUT) system by introducing a zero at:
It can be seen from Figure 5 that at 50kHz, the phase is
approximately –90° versus Figure 6 where the number
is –150°. This means that the transconductance error
amplifier has to provide a phase boost of about 45° to
achieve a closed loop phase margin of 45° at a
crossover frequency of 50kHz for Figure 4, versus 105°
for Figure 6. The simple RC and C2 compensation
scheme allows a maximum error amplifier phase boost
of about 90°. Therefore, it is easier to stabilize the
MIC2168A voltage control loop by using high ESR value
output capacitors.
g_m Error Amplifier
It is undesirable to have high error amplifier gain at high
frequencies because high-frequency noise spikes would
be picked up and transmitted at large amplitude to the
output, thus, gain should be permitted to fall off at high
frequencies. At low frequency, it is desired to have high
open-loop gain to attenuate the power line ripple. Thus,
the error amplifier gain should be allowed to increase
rapidly at low frequencies.
f_ZERO
=
2×π
1
× ESR × C_OUT
Therefore, F_ZERO = 6.36kHz.
January 2010
14
M9999-011510