MAX15015EVKIT【PDF 19/26 页】EVAL KIT FOR MAX15015

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1A, 4.5V to 40V Input Buck Converters with

50mA Auxiliary LDO Regulators

The following equations define the power modulator:

C8

G MOD _ DC =

V IN

V RAMP

= 10

R5

C7

f LC =

1

2 × π × L × C OUT

C6

R3

R6

f ZESR =

1

2 × π × C OUT × ESR

V OUT

R4

EA

COMP

The switching frequency is internally set at 500kHz for

MAX15015/MAX15017 and can vary from 400kHz to

600kHz when driven with an external SYNC signal. The

REF

switching frequency is internally set at 135kHz for

MAX15014/MAX15016 and can vary from 100kHz to

200kHz when driven with an external sync signal. The

crossover frequency (fC), which is the frequency when

GAIN

(dB)

CLOSED-LOOP

GAIN

EA

GAIN

the closed-loop gain is equal to unity, should be set to

around 1/10 of the switching frequency or below.

The crossover frequency occurs above the LC double-

pole frequency, and the error amplifier must provide a

gain and phase bump to compensate for the rapid gain

f Z1 f Z2

f C

f P2 f P3

FREQUENCY

and phase loss from the LC double pole, which exhibits

little damping.

This is accomplished by utilizing a Type 3 compensator

that introduces two zeroes and three poles into the con-

trol loop. The error amplifier has a low-frequency pole

(f P1 ) near the origin so that tight voltage regulation at DC

can be achieved.

The two zeroes are at:

Figure 3. Error Amplifier Compensation Circuit (Closed-Loop

and Error-Amplifier Gain Plot) for Ceramic Capacitors

value and tend to be more expensive. Aluminum elec-

trolytic capacitors have much larger ESR but can reach

much larger capacitance values.

Compensation when f C < f ZESR

and

f ZI =

1

2 π × R 5 × C 7

This is usually the case when a ceramic capacitor is

selected. In this case, f ZESR occurs after f C . Figure 3

shows the error amplifier feedback as well as its gain

response.

f Z2 =

1

2 π × ( R 3 + R 6 ) × C 6

f Z1 is set to 0.5 to 0.8 x f LC and f Z2 is set to f LC to com-

pensate for the gain and phase loss due to the double

pole. To achieve a 0dB crossover with -20dB/decade

and the higher frequency poles are at:

slope, poles f P2 and f P3 are set above the crossover fre-

f P2 =

1

2 π × R 6 × C 6

quency f C .

The values for R3 and R4 are already determined in the

Setting the Output Voltage section. The value of R3 is

and

f P3 =

1

C7 × C 8

2 π × R 5 ×

C 7 + C 8

also used in the following calculations.

Since f Z2 < f C < f P2 , then R3 >> R6, and R3 + R6 can

be approximated as R3.

Now we can calculate C6 for zero f Z2 :

The compensation design primarily depends on the

type of output capacitor. Ceramic capacitors exhibit

very low ESR, and are well suited for high-switching-

frequency applications, but are limited in capacitance

C 6 =

1

2 π × f LC × R 3

______________________________________________________________________________________

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