LT1376 Просмотр технического описания (PDF) - Linear Technology

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LT1376

1.5A, 500kHz Step-Down Switching Regulators

Linear Technology 

LT1375/LT1376

APPLICATIONS INFORMATION

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

100µF output capacitor. Phase drop is limited to about

85°. Phase recovers and gain levels off at the zero fre-

quency (≈16kHz) set by capacitor ESR (0.1Ω).

Error amplifier transconductance phase and gain are shown

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

transconductance of 2000µMho, with an output imped-

ance of 200kΩ 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 0.0033µF, giving

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

to 90° and staying there. The overall loop has a gain of

LT1375

LT1376 CURRENT MODE

POWER STAGE

VSW

gm = 2A/V

ERROR

AMPLIFIER

FB

GND

VC

2.42V

OUTPUT

R1

ESR

+

C1

R2

RC

CF

CC

Figure 12. Model for Loop Response

1375/76 F12

3000

200

PHASE

2500

150

2000

GAIN

( ) 1500 VFB 2 • 10–3

ROUT

200k

100

VC

COUT

12pF

50

1000 ERROR AMPLIFIER EQUIVALENT CIRCUIT 0

RLOAD = 50Ω

500

100

1k

10k 100k

FREQUENCY (Hz)

–50

1M 10M

1375/76 F14

Figure 14. Error Amplifier Gain and Phase

40

20

GAIN

40

VIN = 10V

VOUT = 5V

IOUT = 500mA

0

0

–40

PHASE

–20

–80

–40

10

100

1k

10k 100k

FREQUENCY (Hz)

–120

1M

1375/76 F13

Figure 13. Response from VC Pin to Output

80

200

GAIN

60

150

40

100

20

PHASE

50

0

VIN = 10V

VOUT = 5V, IOUT = 500mA

COUT = 100µF, 10V, AVX TPS

CC = 3.3nF, RC = 0, L = 10µH

–20

10

100

1k

10k

FREQUENCY (Hz)

0

–50

100k 1M

1375/76 F15

Figure 15. Overall Loop Characteristics

20

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