LT1376CS datasheet(20/28 Pages) LINER

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LT1375/LT1376

APPLICATIONS INFORMATION

Error amplifier transconductance phase and gain are shown

in Figure 14. The error amplifier can be modeled as a

transconductance of 2000

µMho, with an output imped-

ance of 200k

Ω in parallel with 12pF. In all practical

applications, the compensation network from VC pin to

ground has a much lower impedance than the output

impedance of the amplifier at frequencies above 500Hz.

This means that the error amplifier characteristics them-

selves do not contribute excess phase shift to the loop, and

the phase/gain characteristics of the error amplifier sec-

tion are completely controlled by the external compensa-

tion network.

In Figure 15, full loop phase/gain characteristics are

shown with a compensation capacitor of 0.0033

µF, giving

the error amplifier a pole at 240Hz, with phase rolling off

to 90

° and staying there. The overall loop has a gain of

introduce multiple poles into the feedback loop. The

inductor and output capacitor on a conventional step-

down converter actually form a resonant tank circuit that

can exhibit peaking and a rapid 180

° phase shift at the

resonant frequency. By contrast, the LT1376 uses a “cur-

rent mode” architecture to help alleviate phase shift cre-

ated by the inductor. The basic connections are shown in

Figure 12. Figure 13 shows a Bode plot of the phase and

gain of the power section of the LT1376, measured from

the VC pin to the output. Gain is set by the 2A/V transcon-

ductance of the LT1376 power section and the effective

complex impedance from output to ground. Gain rolls off

smoothly above the 100Hz pole frequency set by the

100

µF output capacitor. Phase drop is limited to about

°. Phase recovers and gain levels off at the zero fre-

quency (

≈16kHz) set by capacitor ESR (0.1Ω).

Figure 13. Response from VC Pin to Output

Figure 15. Overall Loop Characteristics

FREQUENCY (Hz)

–20

–40

–80

–120

10k

1375/76 F13

100

100k

GAIN

PHASE

VIN = 10V

VOUT = 5V

IOUT = 500mA

FREQUENCY (Hz)

–20

200

150

100

–50

10k

1375/76 F15

100

100k

GAIN

PHASE

VIN = 10V

VOUT = 5V, IOUT = 500mA

COUT = 100µF, 10V, AVX TPS

CC = 3.3nF, RC = 0, L = 10µH

Figure 12. Model for Loop Response

Figure 14. Error Amplifier Gain and Phase

2.42V

VSW

LT1375

LT1376

GND

1375/76 F12

OUTPUT

ESR

ERROR

AMPLIFIER

CURRENT MODE

POWER STAGE

gm = 2A/V

FREQUENCY (Hz)

3000

2500

2000

1500

1000

500

200

150

100

–50

100

10k

100k

10M

1375/76 F14

GAIN

PHASE

ROUT

200k

COUT

12pF

ERROR AMPLIFIER EQUIVALENT CIRCUIT

RLOAD = 50Ω

VFB 2 • 10

–3

)

(

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