LTP8800-1A

8

Rev. B

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APPLICATIONS INFORMATION

COMPENSATION

The LTP8800-1A offers programmable loop compen-

sation to optimize the transient response without any

hardware change. A Type 3 filter architecture has been

implemented. To tailor the loop response to the specific

application, the low frequency gain, zero location, pole

location and high frequency gain can all be set individually

(see the Digital Filter Programming Registers section).

From the sensed voltage to the duty cycle, the transfer

function of the filter in z-domain is resolved by Equation 1.

H(z)

=

D

LFG

•

1

(1

− z−1)

+

C

HFG

1

−

B

256

z−1

⎛

⎝⎜

⎞

⎠⎟

1

−

A

256

z−1

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝

⎜

⎜

⎜

⎞

⎠

⎟

⎟

⎟

(1)

Where:

A = filter pole register value (in decimal), 0xFE03.

B = filter zero register value (in decimal), 0xFE02.

C = high frequency gain register value (in decimal),

0xFE04.

D = low frequency gain register value (in decimal),

0xFE01.

LFG = 9.5488 × 107/fSW.

HFG = 5.968 × 106/fSW.

As shown in Figure 1, adjusting low frequency gain reg-

ister value will change the gain of the compensation over

the low frequency range without moving the pole and zero

locations. Adjusting high frequency gain register value

will change the gain of the compensation over the high

frequency range without moving the pole and zero loca-

tions. As shown in Figure 2, adjusting the pole and zero

register values will move the double poles and double

zeroes of the compensation. Increasing the filter zero and

pole register values will separate the double zeroes and

double poles. It is recommended that LTpowerPlay® be

used to program the filter.

It is recommended that the user determines the appro-

priate value for the compensation registers using the

LTpowerCAD® tool. An example of the bode plot of

the typical application circuit with the recommended

compensation settings is shown in Figure 3. Measured

bode plot of the LTP8800-1A in circuit Figure 10 with

register setting (in decimal): 0xFE02 = 226, 0xFE03 =

160, 0xFE04 = 50, 0xFE01 = 8. (Crossover frequency:

28.84kHz, phase margin 64.5deg, gain margin 17.31dB).

FREQUENCY (Hz)

10

100

1k

10k

100k

1M

5

10

15

20

25

30

35

40

45

50

55

88001a F01

LG INCREASE

HG INCREASE

Figure 1. Compensation Gain Adjustment

FREQUENCY (Hz)

10

100

1k

10k

100k

1M

88001a F02

5

10

15

20

25

30

35

40

45

50

55

ZERO

INCREASE

POLE

INCREASE

Figure 2. Compensation Poles and Zeroes Adjustment

Figure 3. Measured Bode Plot of the LTP8800-1A