LTC1702AIGN datasheet(23/36 Pages) LINER

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LTC1702A

1702afa

With the gain/phase plot in hand, a loop crossover fre-

quency can be chosen. Usually the curves look something

like Figure 8. Choose the crossover frequency in the rising

or flat parts of the phase curve, beyond the external LC

poles. Frequencies between 10kHz and 50kHz usually

work well. Note the gain (GAIN, in dB) and phase (PHASE,

in degrees) at this point. The desired feedback amplifier

gain will be – GAIN to make the loop gain 0dB at this

frequency. Now calculate the needed phase boost, assum-

ing 60

° as a target phase margin:

BOOST = – (PHASE + 30

°)

If the required BOOST is less than 60

°, a type 2 loop can

be used successfully, saving two external components.

BOOST values greater than 60

° usually require type 3

loops for satisfactory performance.

Finally, choose a convenient resistor value for R1 (10k is

usually a good value). Now calculate the remaining values:

(K is a constant used in the calculations)

ƒ = chosen crossover frequency

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

gain)

Type 2 Loop:

Tan

BOOST

GKR

CC K

REF

OUT

REF

=+ °

⎛

⎝⎜

⎞

⎠⎟

πƒ

()

πƒ

()

2 –

–

APPLICATIONS INFORMATION

If breadboard measurement is not practical, a SPICE

simulation can be used to generate approximate gain/

phase curves. Plug the expected capacitor, inductor and

MOSFET values into the following SPICE deck and gener-

ate an AC plot of V(VOUT)/V(COMP) in dB and phase of

V(OUT) in degrees. Refer to your SPICE manual for details

of how to generate this plot.

*1702A modulator gain/phase

*this file written to run with PSpice 8.0

*may require modifications for other SPICE

simulators

*MOSFETs

rfet mod sw 0.02

;MOSFET rdson

*inductor

lext sw out1 1u

;inductor value

rl out1 out 0.005

;inductor series R

*output cap

cout out out2 1000u

;capacitor value

resr out2 0 0.01

;capacitor ESR

*1702A internals

emod mod 0 comp 0 5

;3.3 for 3.3V supply

vstim comp 0 0 ac 1

;ac stimulus

.ac dec 100 1k 1meg

.probe

.end

BOOST2

FCB

FAULT

COMP

RUN/SS

1/2 LTC1702A

VCC

Ω MBR0530T

CIN

µF

VOUT

ANALYZER

VCOMP

ANALYZER

SOURCE

FROM

ANALYZER

LEXT

µF

0.1

µF

PVCC

SGND PGND

10k

COUT

1702A F12

Figure 12. Modulator Gain/Phase Measurement Set-Up

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