LTC1702A View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC1702A

Dual 550kHz Synchronous 2-Phase Switching Regulator Controller

Linear Technology 

Prev 21 22 23 24 25 26 27 28 29 30 Next

LTC1702A

APPLICATIONS INFORMATION

5V

+

10µF

VCOMP

TO

ANALYZER 0.1µF

AC

SOURCE

FROM

ANALYZER

NC

RB 10k

10Ω MBR0530T

VCC PVCC

BOOST2

TG

1/2 LTC1702A

COMP SW

FB

BG

RUN/SS FCB

FAULT

SGND PGND

+

CIN

1µF

QT

LEXT

VOUT

TO

ANALYZER

QB

+

COUT

1702A F12

Figure 12. Modulator Gain/Phase Measurement Set-Up

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

*©2001 Linear Technology

*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

rl out1 out 0.005

;inductor value

;inductor series R

*output cap

cout out out2 1000u

resr out2 0 0.01

;capacitor value

;capacitor ESR

*1702A internals

emod mod 0 comp 0 5

vstim comp 0 0 ac 1

.ac dec 100 1k 1meg

.probe

.end

;3.3 for 3.3V supply

;ac stimulus

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:

K

=

⎛

Tan⎝⎜

BOOST

2

+

⎞

45°⎠⎟

C2 = 1

2πƒGKR1

( ) C1= C2 K2 – 1

R2 = K

2πƒC1

( ) VREF R1

RB = VOUT – VREF

1702afa

23

Share Link: