MAX15020 Datasheet, PDF (15/19 Pages) Maxim Integrated Products – 2A, 40V Step-Down DC-DC Converter with Dynamic Output-Voltage Programming

2A, 40V Step-Down DC-DC Converter with
Dynamic Output-Voltage Programming
C11
R6
V_OUT
R7
R8
C13
R9
C12
FB
ERROR
AMPLIFIER
SS
COMP
C11
R6
V_OUT
R7
R8
C13
R9
C12
FB
ERROR
AMPLIFIER
SS
COMP
CLOSED-LOOP
GAIN
GAIN
(dB)
ERROR-
AMPLIFIER
GAIN
GAIN
(dB)
CLOSED-LOOP
GAIN
ERROR-
AMPLIFIER
GAIN
f_Z1 f_Z2
f_C
f_P2 f_P3
FREQUENCY (Hz)
Figure 3. Error-Amplifier Compensation Circuit (Closed-Loop
and Error-Amplifier Gain Plot) for Ceramic Capacitors
Compensation when f_C > f_ZESR
For larger ESR capacitors such as tantalum and alu-
minum electrolytics, 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 zeros (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 R9 in Figure 4
(values between 1kΩ and 10kΩ are adequate).
C12 is then calculated as:
C12 =
1
2π × 0.8 × f_LC × R9
The error-amplifier gain between f_P2 and f_P3 is approxi-
mately equal to R9 / R6 (given that R6 << R7). R6 can
then be calculated as:
f_Z1 f_Z2
f_P2
f_{C fP3} FREQUENCY (Hz)
Figure 4. Error-Amplifier Compensation Circuit (Closed-Loop
and Error-Amplifier Gain Plot) for Higher ESR Output Capacitors
R6
=
R9
× 10 ×
f_C2
f_LC2
C11 is then calculated as:
C11 = ^{COUT × ESR}
R6
Since R7 >> R6, R7 + R6 can be approximated as R7.
R7 is then calculated as:
R7 =
1
2π × f_LC × C11
f_P3 is set at 5 x f_C. Therefore, C13 is calculated as:
C13 =
C12
2π × C12 × R9 × f_P3 − 1
Based on the calculations above, the following com-
pensation values are recommended when the switch-
ing frequency of DC-DC converter ranges from 100kHz
to 500kHz. (Note: The compensation parameters in
Figure 2 are strongly recommended if the switching
frequency is from 300kHz to 500kHz.)
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