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LT1578CS8-2.5

1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1578/LT1578-2.5

APPLICATIONS INFORMATION

the output. Gain is set by the 1.5A/V transconductance of

the LT1578 power section and the effective complex

impedance from output to ground. Gain rolls off smoothly

above the 160Hz pole frequency set by the 100µF output

capacitor. Phase drop is limited to about 85°. Phase

recovers and gain levels off at the zero frequency (≈16kHz)

set by capacitor ESR (0.1Ω).

Error amplifier transconductance phase and gain are shown

in Figure 11. The error amplifier can be modeled as a

transconductance of 1000µMho, with an output imped-

ance of 570kΩ in parallel with 2.4pF. In all practical

applications, the compensation network from the VC pin to

ground has a much lower impedance than the output

impedance of the amplifier at frequencies above 200Hz.

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 12, full loop phase/gain characteristics are

shown with a compensation capacitor of 100pF, giving the

error amplifier a pole at 2.8kHz, with phase rolling off to

90° and staying there. The overall loop has a gain of 66dB

at low frequency, rolling off to unity-gain at 58kHz. The

phase plot shows a two-pole characteristic until the ESR

of the output capacitor brings it back to single pole above

16kHz. Phase margin is about 77° at unity-gain.

LT1578

CURRENT MODE

POWER STAGE

gm = 1.5A/V

VSW

ERROR

AMPLIFIER

FB

GND

VC

1.21V

RC

CF

CC

OUTPUT

R1

ESR

+

C1

R2

1578 F09

Figure 9. Model for Loop Response

40

40

VIN = 10V

VOUT = 5V

IOUT = 500mA

20

0

GAIN

PHASE

0

–40

–20

–80

–40

10

100

1k

10k

FREQUENCY (Hz)

–120

100k

1578 F07

Figure 10. Response from VC Pin to Output

20

2000

200

PHASE

1500

150

1000

GAIN

( ) 500 VFB 1 × 10–3

ROUT

570k

100

VC

COUT

2.4pF

50

0 ERROR AMPLIFIER EQUIVALENT CIRCUIT 0

RLOAD = 50Ω

–500

10

100

1k

10k

FREQUENCY (Hz)

–50

100k 1M

1578 F11

Figure 11. Error Amplifier Gain and Phase

80

180

60

135

PHASE

40

90

20

VIN = 10V

VOUT = 5V

IOUT = 500mA

0

COUT = 100µF

10V, AVX TPS

CC = 100pF

L = 30µH

–20

10

100

1k

10k

FREQUENCY (Hz)

GAIN

100k

45

0

–45

1M

1578 F12

Figure 12. Overall Loop Characteristics

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