Rainbow Electronics MAX5081 - Manual (Page 14)

MAX5080/MAX5081
1A, 40V, MAXPower Step-Down
DC-DC Converters
14 ______________________________________________________________________________________
Pick a value for the feedback resistor R5 in Figure 3
(values between 1k
Ω and 10kΩ are adequate).
C7 is then calculated as:
f C occurs between f Z2 and f P2 . The error-amplifier gain
(G EA ) at f C is due primarily to C6 and R5. Therefore,
G EA(fC) = 2
π x f
C x C6 x R5 and the modulator gain at
f C is:
Since G
EA(fC)
x G
MOD(fC)
= 1, C6 is calculated by:
f P2 is set at 1/2 the switching frequency (f SW ). R6 is
then calculated by:
Since R3 >> R6, R3 + R6 can be approximated as R3.
R3 is then calculated as:
f P3 is set at 5xf C . Therefore C8 is calculated as:
Compensation When f C > f ZESR
For larger ESR capacitors such as tantalum and alu-
minum electrolytic ones, f
ZESR
can occur before f
C
. If
f ZESR < f C , then f C occurs between f P2 and f P3 . f Z1 and
f Z2 remain the same as before however, f P2 is now set
equal to f ZESR . The output capacitor’s ESR zero fre-
quency is higher than f LC but lower than the closed-
loop crossover frequency. The equations that define
the error amplifier’s poles and zeroes (f
Z1
, f
Z2
, f
P1
, f
P2
,
and f P3 ) are the same as before. However, f P2 is now
lower than the closed-loop crossover frequency. Figure
4 shows the error amplifier feedback as well as its gain
response for circuits that use higher-ESR output capac-
itors (tantalum or aluminum electrolytic).
Pick a value for the feedback resistor R5 in Figure 4 (val-
ues between 1k
Ω and 10kΩ are adequate).
C7 is then calculated as:
The error amplifier gain between f P2 and f P3 is approxi-
mately equal to R5/R6 (given that R6 << R3). R6 can
then be calculated as:
C6 is then calculated as:
C
C ESR
R
6
6
=
×
OUT
R
R f
f
LC
C
6
5 10 2
2
≈
Ч Ч
C
1
f
LC
7
2 0 8 5
=
Ч Ч Ч π . R
( )
C
C
C R f
8
7
2 7 5 1
=
Ч Ч Ч − π
P3
R
f C
3
1
2 6
≈
Ч Ч π
LC
.
R
C f
6
1
2 6 0 5
=
Ч Ч Ч π
SW
C
f L C
R G
C
6
2
5
=
Ч Ч Ч
Ч
OUT
MOD(DC)
π
( )
G
G
L C f
C
MOD(fC)
MOD(DC)
OUT
=
Ч Ч Ч 2 2 2 π
C
1
f
LC
7
2 0 8 5
=
Ч Ч Ч π
. R
GAIN
(dB)
V OUT
REF
R3
COMP
R6
R5
C6
R4
FREQUENCY
CLOSED-LOOP
GAIN
EA
GAIN
f Z1 f Z2 f C f P2 f P3
C8
EA
C7
Figure 3. Error Amplifier Compensation Circuit (Closed-Loop
and Error-Amplifier Gain Plot) for Ceramic Capacitors