Rainbow-electronics MAX15020 Manual do Utilizador descarregar pdf (Página 15)

MAX15020

2A, 40V Step-Down DC-DC Converter with

Dynamic Output-Voltage Programming

______________________________________________________________________________________ 15

Compensation when f

> f

ZESR

For larger ESR capacitors such as tantalum and alu-

minum electrolytics, f

ZESR

can occur before f

. If f

ZESR

< f

, then f

occurs between f

and f

. f

and f

remain the same as before, however, f

is now set

equal to f

ZESR

. The output capacitor’s ESR zero fre-

quency is higher than f

but lower than the closed-

loop crossover frequency. The equations that define

the error amplifier’s poles and zeros (f

, f

and f

) are the same as before. However, f

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:

The error-amplifier gain between f

and f

is approxi-

mately equal to R9 / R6 (given that R6 << R7). R6 can

then be calculated as:

C11 is then calculated as:

Since R7 >> R6, R7 + R6 can be approximated as R7.

R7 is then calculated as:

is set at 5 x f

. Therefore, C13 is calculated as:

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.)

CRf

2129 1

×××

−π

211

××π

C ESR

OUT

910

××

208 9

×××π .

C12

CLOSED-LOOP

GAIN

FREQUENCY (Hz)

ERROR-

AMPLIFIER

GAIN

(dB)

C13

C11

OUT

COMP

ERROR

AMPLIFIER

Figure 3. Error-Amplifier Compensation Circuit (Closed-Loop

and Error-Amplifier Gain Plot) for Ceramic Capacitors

C12

CLOSED-LOOP

GAIN

ERROR-

AMPLIFIER

GAIN

C13

C11

OUT

COMP

ERROR

AMPLIFIER

FREQUENCY (Hz)

GAIN

(dB)

Figure 4. Error-Amplifier Compensation Circuit (Closed-Loop

and Error-Amplifier Gain Plot) for Higher ESR Output Capacitors

1 2 ... 10 11 12 13 14 15 16 17 18 19

MAX15020 1

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