LTC1705 データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

メーカー

LTC1705

Dual 550kHz Synchronous Switching Regulator Controller with 5-Bit VID and 150mA LDO

Linear Technology 

LTC1705

APPLICATIO S I FOR ATIO

and the actual MOSFETs, inductor and input and output

capacitors that the final design will use. This breadboard

should use appropriate construction techniques for high

speed analog circuitry: bypass capacitors located close to

the LTC1705, no long wires connecting components,

appropriately sized ground returns, etc. Wire the feedback

amplifier as a simple Type 1 loop, with a 10k resistor from

VOUT to FB and a 0.1µF feedback capacitor from COMP to

FB. Choose the bias resistor (RB) as required to set the

desired output voltage. Disconnect RB from ground and

connect it to a signal generator or to the source output of

a network analyzer (Figure 9) to inject a test signal into the

loop. Measure the gain and phase from the COMP pin to

the output node at the positive terminal of the output

capacitor. Make sure the analyzer’s input is AC coupled so

that the DC voltages present at both the COMP and VOUT

nodes don’t corrupt the measurements or damage the

analyzer.

+

10µF

10Ω

5V

MBR0530T

VCOMP TO

ANALYZER

0.1µF

RB

10k

AC SOURCE

FROM

ANALYZER

VCC

COMP

PVCC

TG

BOOST

FB

LTC1705

SW

BG

RUN/SS

GND PGND

QT

1µF

L

QB

+

COUT

NC

VOUT TO

ANALYZER

1705 F09

Figure 9. 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

VOUT in degrees. Refer to your SPICE manual for details of

how to generate this plot.

*1705 modulator gain/phase

*2000 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 ;capacitor value

resr out2 0 0.01 ;capacitor ESR

*1705 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 5. 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.

20

Share Link: