LT1374CFE 데이터 시트보기 (PDF) - Linear Technology
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LT1374CFE Datasheet PDF : 32 Pages
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LT1374
APPLICATIONS INFORMATION
Error amplifier transconductance phase and gain are
shown in Figure 11. The error amplifier can be modeled
as a transconductance of 2000µMho, with an output
impedance 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 12, full loop phase/gain characteristics are
shown with a compensation capacitor of 1.5nF, giving the
error amplifier a pole at 530Hz, with phase rolling off to 90°
LT1374
CURRENT MODE
POWER STAGE
gm = 5.3A/V
VSW
ERROR
AMPLIFIER
FB
GND
VC
2.42V
RC
CF
CC
OUTPUT
R1
ESR
+
C1
R2
1374 F09
Figure 9. Model for Loop Response
and staying there. The overall loop has a gain of 74dB at
low frequency, rolling off to unity-gain at 100kHz. Phase
shows a two-pole characteristic until the ESR of the output
capacitor brings it back above 10kHz. Phase margin is
about 75° at unity-gain.
Analog experts will note that around 4.4kHz, phase dips
very close to the zero phase margin line. This is typical of
switching regulators, especially those that operate over a
wide range of loads. This region of low phase is not a
problem as long as it does not occur near unity-gain. In
practice, the variability of output capacitor ESR tends to
dominate all other effects with respect to loop response.
Variations in ESR will cause unity-gain to move around,
but at the same time phase moves with it so that adequate
phase margin is maintained over a very wide range of ESR
(≥ ±3:1).
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
1374 F11
Figure 11. Error Amplifier Gain and Phase
40
GAIN
20
40
VIN = 10V
VOUT = 5V
IOUT = 2A
0
0
–40
PHASE
–20
–80
–40
10
100
1k
10k 100k
FREQUENCY (Hz)
–120
1M
1374 F10
Figure 10. Response from VC Pin to Output
22
80
200
GAIN
60
150
40
100
PHASE
20
50
0
VIN = 10V
VOUT = 5V, IOUT = 2A
COUT = 100µF, 10V, AVX TPS
CC = 1.5nF, RC = 0, L = 10µH
–20
10
100
1k
10k
FREQUENCY (Hz)
0
–50
100k 1M
1374 F12
Figure 12. Overall Loop Characteristics
1374fd
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