MAX15014_07 Datasheet, PDF(19/26 Page) Maxim Integrated Products – 1A, 4.5V to 40V Input Buck Converters with 50mA Auxiliary LDO Regulators

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MAX15014_07 Datasheet, PDF (19/26 Pages) Maxim Integrated Products – 1A, 4.5V to 40V Input Buck Converters with 50mA Auxiliary LDO Regulators

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

50mA Auxiliary LDO Regulators

The following equations define the power modulator:

GMOD _ DC

VIN

VRAMP

= 10

fLC =

2 Ã Ï Ã L Ã COUT

fZESR

COUT

Ã ESR

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

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

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

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

(fP1) near the origin so that tight voltage regulation at DC

can be achieved.

The two zeroes are at:

fZI

2Ï

and

fZ2

2Ï

(R3

+ R6)

and the higher frequency poles are at:

fP2

2Ï

Ã R6

and

fP3

2Ï

Ã R5

Ã C7

C7 + C8

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

C6 R6

VOUT

REF

COMP

GAIN

(dB)

CLOSED-LOOP

GAIN

fZ1 fZ2 fC fP2 fP3

FREQUENCY

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 fC < fZESR

This is usually the case when a ceramic capacitor is

selected. In this case, fZESR occurs after fC. Figure 3

shows the error amplifier feedback as well as its gain

response.

fZ1 is set to 0.5 to 0.8 x fLC and fZ2 is set to fLC to com-

pensate for the gain and phase loss due to the double

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

slope, poles fP2 and fP3 are set above the crossover fre-

quency fC.

The values for R3 and R4 are already determined in the

Setting the Output Voltage section. The value of R3 is

also used in the following calculations.

Since fZ2 < fC < fP2, then R3 >> R6, and R3 + R6 can

be approximated as R3.

Now we can calculate C6 for zero fZ2 :

C6 =

2Ï Ã fLC Ã R3

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