LTC1704B datasheet(21/28 Pages) LINER

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LTC1704/LTC1704B

1704bfa

APPLICATIO S I FOR ATIO

*1704 modulator gain/phase

*2001 Linear Technology

*this file written to run with PSpice 9.0

*may

require

modifications

for

other

SPICE

simulators

*MOSFETs

rfet mod sw 0.02

;MOSFET rdson

*inductor

lext sw out1 1u

;inductor value

rl out1 outsw 0.005

;inductor series R

*output cap

cout outsw out2 1000u

;capacitor value

resr out2 0 0.01

;capacitor ESR

*1704 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

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

quency can be chosen. Usually the curves look something

like Figure 7. 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 at 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)

f = chosen crossover frequency

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

absolute gain)

TYPE 2 Loop:

Tan

BOOST

fGKR

CC K

REF

OUTSW

REF

=+ °









=−

()

−

()

TYPE 3 Loop:

Tan

BOOST

fGR

CC K

fKR

REF

OUTSW

REF

=+ °









=−

()

−

()

SWITCHING SUPPLY CURRENT LIMIT

PROGRAMMING

Programming the current limit on the LTC1704 switcher

supply is straightforward. The IMAX pin sets the current

limit by setting the maximum allowable voltage drop

across QB (the bottom MOSFET) before the current limit

circuit engages. The voltage across QB is set by its on-

resistance and the current flowing in the inductor, which

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