MIC2169 Micrel Semiconductor, MIC2169 Datasheet - Page 11

• Manufacturer Part Number: MIC2169
• Description: 500 KHZ PWM SYNCHRONOUS BUCK CONTROL IC
• Manufacturer: Micrel Semiconductor

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:
Therefore, f
By looking at the phase curve, it can be seen that the output
capacitor ESR (0.050 ) cancels one of the two poles (LC
system by introducing a zero at:
Therefore, F
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 compari-
son purposes, Figure 6, shows the same phase curve with an
ESR value of 0.002 .
November 2003
MIC2169
f
ZERO
f
LC
Figure 4. The Gain Curve for G(s)
LC
ZERO
Figure 5. Phase Curve for G(s)
2
= 3.6kHz
2
= 6.36kHz.
L C
ESR C
1
1
OUT
OUT
OUT
)
11
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 MIC2169 voltage control loop by using high ESR value
output capacitors.
g
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.
The transfer function with R1, C1, and C2 for the internal g
error amplifier can be approximated by the following equa-
tion:
The above equation can be simplified by assuming C2<<C1,
From the above transfer function, one can see that R1 and C1
introduce a zero and R1 and C2 a pole at the following
frequencies:
Error Amplifier(z) g
m
Error Amplifier
Figure 6. The Phase Curve with ESR = 0.002
Fzero=
Fpole =
Fpole@origin =
Error Amplifier(z) g
1
1
/
/
2
2
m
R1
C2
1
/
2
s
C1
R1
m
C1 C2 1 R1
C1
s
1 R1 S C1
C1 1 R1 C2 S
1 R1 S C1
C1 C2 S
C1 C2
M9999-111803
Micrel
m