Rainbow-electronics MAX8514 Manual do Utilizador descarregar pdf (Página 24)

MAX8513/MAX8514

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

with Voltage Monitor and Power-On Reset

24 ______________________________________________________________________________________

This gain is also set by the ratio of R3/R1 where R1 is

calculated in the OUT1 Voltage Setting section. Thus:

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 error-amplifier

first zero, f

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

the second zero, f

, at the LC double-pole frequency.

Hence:

Set the error-amplifier f

at f

ZESR

, and f

to 1/2 the

switching frequency, if f

ZESR

< 1/2 f

. If f

ZESR

> 1/2 f

then set f

at 1/2 f

and f

at f

ZESR

The gain of the error amplifier between f

and f

set by the ratio of R3/R

and is equal to:

where R

is the parallel combination of R1 and R4 and

is equal to:

Therefore:

C11 can then be calculated as:

and C12 as:

Below is a numerical example to calculate the error-

amplifier compensation values used in the Typical

Applications Circuit of Figure 5:

= 12V (nomimal input voltage)

RAMP

= 1V

OUT1

= 3.3V

FB1

= 1.25V

L1A = 1.8µH

C4 = 47µF/ 6.3V ceramic, with R

ESR

= 0.008Ω

= 1.4MHz

The LC double-pole frequency is calculated as:

Pick R2 = 8.06kΩ.

The modulator gain at DC is:

Pick f

= 100kHz.

kHz

RRG

MOD fc

EA fZ fZ

PMOD

C MOD fC

EA fZ fZ

()

. . .

=×













=×

=×=

−

17 4

100

0 363

17 4

100 0 363

0 479

13 3 0 479 6 37

ΩΩ

MOD DC

RAMP

()

==12

k1806

125

1133 .

.=×













=ΩΩ-

LA C

kHz

PMOD

ZESR

ESR

×××

××

×××

21 4

21810 4710

17 3

20008 47 10

423

CRf

253

×××

()

π -1

××π

and

PMOD

PEAfZfZ

()

212-

EA fZ fZ

PMOD

()

PMOD

××π

C MOD fc

()

1 2 ... 19 20 21 22 23 24 25 26 27 28 29 ... 34 35

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