Max1802 digital camera step-down power supply – Rainbow Electronics MAX1802 User Manual
Page 24
MAX1802
Digital Camera Step-Down
Power Supply
24
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3) Place the compensation zero at the same frequency
as the maximum output pole frequency (in Hz):
Solving for CC:
Use values of C
C
<10nF. If the above calculation deter-
mines that the capacitor should be >10nF, use C
C
=
10nF, skip step 4, and go to step 5.
4) Determine the crossover frequency (in Hz):
and to maintain at least 10dB gain margin, make sure
that the crossover frequency is
≤1/3 of the ESR zero
frequency, or 3f
C
≤ Z
O
, or ESR
≤ D / 6 V
REF
.
If this is not the case, go to step 5 to reduce the error-
amplifier high-frequency gain to decrease the
crossover frequency.
5) The high-frequency gain may be reduced, thus
reducing the crossover frequency, as long as the
zero due to the compensation network remains at or
below the crossover frequency. In this case:
and
Choose C
OUT
, R
C
, and C
C
to satisfy both equations
simultaneously.
Continuous Inductor Current
For continuous inductor current, there are two condi-
tions that change, requiring different compensation.
The response of the control loop includes a right-half-
plane zero and a complex pole pair due to the inductor
and output capacitor. For stable operation, the con-
troller-loop gain must drop below unity (0dB) at a much
lower frequency than the right-half-plane zero frequen-
cy. The zero arising from the ESR of the output capaci-
tor is typically used to compensate the control circuit
by increasing the phase near the crossover frequency,
increasing the phase margin. If a low-value, low-ESR
output capacitor (such as a ceramic capacitor) is used,
the ESR-related zero occurs at too high a frequency
and does not increase the phase margin. In this case,
use a lower value inductor so that it operates with dis-
continuous current (see the Discontinuous Inductor
Current section).
For continuous inductor current, the gain of the voltage
divider is A
VDV
= V
REF
/ V
OUT,
and the DC gain of the
error amplifier is A
VEA
= 2000. The gain through the
PWM controller in continuous current is:
Thus, the total DC loop gain is: A
VDC
= 2000 V
OUT
/ V
IN
.
The complex pole pair due to the inductor and output
capacitor occurs at the frequency (in Hz):
The pole and zero due to the compensation network at
COMP occur at the frequencies (in Hz):
Z
1
2 R C
C
C
C
=
π
P
G
C
C
C
EA
C
C
=
(
)
=
×
4000
1
4 10
7
π
π
P
V
V
LC
O
OUT
IN
OUT
=
2
π
A
VO
OUT
IN REF
V
V V
=
2
f
R V
C
C
REF
=
≥
G
D
R C
EA C
OUT
C
C
π
π
1
2
ESR
D
G
EA C
≤
R
V
REF
6
f
V
C
C
REF
=
π D
OUT
C
C
V
OUT OUT
C OUT MAX
C
OUT
IN
OUT
IN
V
V
R I
2V
V
=
−
−
(
)
(
)
Z
R C
V
C
C
C
OUT
LOAD MIN
OUT
C
OUT
IN
IN
2V
V
2
V
R
=
=
−
−
(
)
1
2
π
π
(
)
FREQUENCY
A
VDC
GAIN
(dB)
PHASE
180
°
90
°
0
°
O
-20
20
40
60
80
PHASE
Z
C
= P
O
Z
0
P
C
GAIN
Figure 7. Discontinuous-Current, Voltage-Mode, Step-Up
Controller Bode Plot