Rainbow-electronics MAX8514 Manual do Utilizador descarregar pdf (Página 25)

MAX8513/MAX8514

Wide-Input, High-Frequency, Triple-Output Supplies

with Voltage Monitor and Power-On Reset

______________________________________________________________________________________ 25

Use 6.8kΩ.

Use 4.7nF.

Use 620Ω.

Use 680pF.

Pick f

= 700kHz, which is the midpoint between f

ZESR

and 1/2 the switching frequency.

Use 33pF.

Case 2: Electrolytic Output Capacitor (operating at

lower switching frequencies, f

ZESR

< f

)

The modulator gain at f

is:

The output capacitor’s ESR zero frequency is higher

than the LC double-pole frequency but lower than the

closed-loop crossover frequency. Here the modulator

already has a -20dB/decade slope; therefore, the error-

amplifier gain must have a 0dB/decade slope at f

, so

the loop crosses over at the desired -20dB/decade

slope. The error-amplifier circuit configuration is the

same as Case 1; however, the closed-loop crossover

frequency is now between f

and f

, as illustrated in

Figure 4.

The equations that define the error amplifier’s poles

and zeroes (f

, f

, and f

) are the same as for

Case 1. However, f

is now lower than the closed-loop

crossover frequency.

The error-amplifier gain at f

is:

And the gain of the error amplifier between f

and

is:

Due to the underdamped (Q > 1) nature of the output LC

double pole, the error-amplifier zero frequencies must be

set less than the LC double-pole frequency to provide

adequate phase boost. Set the first zero of the error

amplifier, f

, at 1/4th the LC double-pole frequency. Set

the second zero, f

, at the LC double-pole frequency.

Set the second pole, f

, at f

ZESR

EA fZ fZ EA fc

P MOD fc

() ()

()

−

EA fc

MOD fc

()

MOD fc MOD DC

PMOD

ZESR C

() ( )

CRf

nF k kHz

253

247 68 700

33 7

×××

×× ×

()

(. . )

-1

-1Ω

Rf kHz

2 620 423

607

××

ππΩ

k kHz

kHz

PMOD

PEAfZfZ

. .

()

368174

423 0 479

583

13 3 583

609

212-

Ω

ΩΩ

Ω

Rf k kHz

PMOD

68 174

538

××

ππΩ

Figure 4. Case 2: Error-Amplifier Compensation Circuit (Closed-

Loop and Error-Amplifier Gain Plot)

REF

C12

C11

COMP

OUT1

GAIN

(dB)

FREQUENCY

CLOSED-LOOP GAIN

EA GAIN

1 2 ... 20 21 22 23 24 25 26 27 28 29 30 ... 34 35

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