LTC3613 datasheet(26/44 Pages) AD

26 / 44 page

LTC7131-1

Rev. 0

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APPLICATIONS INFORMATION

50Ω resistor between the output capacitor and the

VSENSE+ pin (this is R7 on the DC2824 demo board).

Then use a network analyzer to inject an AC signal

across this resistor and plot the Gain and Phase. If a

network analyzer is not available, a close approxima-

tion can obtained with a PSPICE simulator using the

LTC7131-1 power supply model shown in Figure 13.

The error amplifier EA can be modeled as an ideal op

amp (for crossover frequencies < 200kHz) and the

inductor as a voltage controlled current source. The

gain/phase plot should look similar to Figure 14.

4. Add the calculated components to the LTC7131-1 cir-

cuit and check the load step response. If not adequate,

the components values may need to be tweaked fur-

ther or recalculated with a lower crossover frequency

until the desired response is obtained.

RC1 = A convenient resistor value ~1kΩ.

f = chosen crossover frequency

G = 10(GAIN/20) (this converts GAIN in dB to G in abso-

lute gain)

BOOST = –(PHASE + 30°)

Type 2 Loop:

= tan

BOOST

+ 45°

⎛

⎝⎜

⎞

⎠⎟

CC1 =

π • f • G •K •RC1

CC2 = CC1 K2 − 1

( )

RC2 =

π • f • CC2

(11)

Type 3 Loop:

= tan

BOOST

+ 45°

⎛

⎝⎜

⎞

⎠⎟

CC1 =

π • f • G •RC1

CC2 = CC1 K − 1

(

)

RC2 =

π • f • CC2

RC3 =

RC1

− 1

πf K •RC3

(12)

FREQUENCY (Hz)

7131-1 F14

–90

–180

GAIN

PHASE

Figure 14. Transfer Function of Buck Modulator

3. Calculate the component values. Once the gain and

phase are known, note the gain (GAIN, in dB) and

phase (PHASE, in degrees) at the crossover fre-

quency. The compensation components can then be

calculated to make the loop gain (VSENSE+ to VOUT)

equal to 0dB and the phase margin equal to 60° at

this frequency. Use Equation 11 and Equation 12 to

calculate the component values. Normally Equation

11 (the Type 2 Loop) will provide the required BOOST

for 60° phase margin. If not, use Equation 12 (the

Type 3 Loop).

LTC3613 Datasheet(PDF) 26 Page - Analog Devices

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