LM2735XMFSLASHNOPB datasheet(12/59 Pages) TI

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10 kHz

5 kHz o

(

)

R2 xCf

2 S

ZERO -

REF

OUT

1¸¸

100

10k

100k

FREQUENCY

-80

-60

-40

-20

-180

-90

180

RHP-Zero

Ext (Cf)

gm-Pole

RC-Pole

Vi = 5V

Vo = 12V

Io = 500 mA

Co = 10 mF

Lo = 5 mH

Ext (Cf)-Pole

gm-zero

-Zero

D = 0.625

Cf = 220 pF

Fz-cf = 8 kHz

RHP-Zero = 107 kHz

Fp-rc = 660 Hz

Fp-cf = 77 kHz

100

10k

100k

FREQUENCY

-80

-60

-40

-20

-180

-90

180

RHP-Zero

gm-Zero

gm-Pole

RC-Pole

Vi = 5V

Vo = 12V

Io = 500 mA

Co = 10 2F

Lo = 5 2H

LM2735, LM2735-Q1

SNVS485G – JUNE 2007 – REVISED AUGUST 2015

www.ti.com

Feature Description (continued)

Figure 15. LM2735 Without External Compensation

One can see that the crossover frequency is fine, but the phase margin at 0 dB is very low (22°). A zero can be

placed just above the crossover frequency so that the phase margin will be bumped up to a minimum of 45°.

Below is the same application with a zero added at 8 kHz.

Figure 16. LM2735 With External Compensation

The simplest method to determine the compensation component value is as follows.

Set the output voltage with the following equation.

where

•

R1 is the bottom resistor and R2 is the resistor tied to the output voltage.

(1)

The next step is to calculate the value of C3. The internal compensation has been designed so that when a zero

is added from 5 kHz to 10 kHz, the converter will have good transient response with plenty of phase margin for

all input and output voltage combinations.

(2)

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Product Folder Links: LM2735 LM2735-Q1

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