MAX5083ATE datasheet(13/18 Pages) MAXIM

13 / 18 page

Pick a value for the feedback resistor R5 in Figure 3

(values between 1kΩ and 10kΩ are adequate).

C7 is then calculated as:

0.8 f

π×

fC occurs between fZ2 and fP2. The error-amplifier gain

(GEA) at fC is due primarily to C6 and R5. Therefore,

GEA(fC) = 2π x fC x C6 x R5 and the modulator gain at

fC is:

MOD(DC)

MOD(fC)

OUT

(2 )

L C

π ××

Since GEA(fC) x GMOD(fC) = 1, C6 is calculated by:

OUT

MOD(DC)

R5 G

× ×

×π

fP2 is set at one-half the switching frequency (fSW). R6 is

then calculated by:

C6 0.5 f

π×

Since R3 >> R6, R3 + R6 can be approximated as R3. R3

is then calculated as:

≈

π×

fP3 is set at 5xfC. Therefore, C8 is calculated as:

C7 R5 f

−

π×

Compensation When fC > fZESR

For larger ESR capacitors such as tantalum and aluminum

electrolytic ones, fZESR can occur before fC. If fZESR < fC,

then fC occurs between fP2 and fP3. fZ1 and fZ2 remain

the same as before however, fP2 is now set equal to fZESR.

The output capacitor’s ESR zero frequency is higher than

fLC but lower than the closedloop crossover frequency.

The equations that define the error amplifier’s poles and

zeroes (fZ1, fZ2, fP1, fP2, and fP3) are the same as before.

However, fP2 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 capacitors (tantalum or aluminum electrolytic).

Pick a value for the feedback resistor R5 in Figure 4 (val-

ues between 1kΩ and 10kΩ are adequate).

C7 is then calculated as:

0.8 f

π×

The error amplifier gain between fP2 and fP3 is approxi-

mately equal to R5/R6 (given that R6 << R3). R6 can then

be calculated as:

R5 10 f

××

≈

C6 is then calculated as:

OUT

ESR

Figure 3. Error Amplifier Compensation Circuit (Closed-Loop

and Error-Amplifier Gain Plot) for Ceramic Capacitors

GAIN

(dB)

VOUT

REF

COMP

FREQUENCY

CLOSED-LOOP

GAIN

fZ1 fZ2

fP2 fP3

MAX5082/MAX5083

1.5A, 40V, MAXPower Step-Down

DC-DC Converters

www.maximintegrated.com

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