LT1374HVIR(RevA) Просмотр технического описания (PDF) - Linear Technology
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LT1374HVIR Datasheet PDF : 28 Pages
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LT1374
APPLICATIONS INFORMATION
frequency. By contrast, the LT1374 uses a “current mode”
architecture to help alleviate phase shift created by the
inductor. The basic connections are shown in Figure 9.
Figure 10 shows a Bode plot of the phase and gain of the
power section of the LT1374, measured from the VC pin to
the output. Gain is set by the 5.3A/V transconductance of
the LT1374 power section and the effective complex
impedance from output to ground. Gain rolls off smoothly
above the 600Hz pole frequency set by the 100µF output
capacitor. Phase drop is limited to about 70°. 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 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 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°
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.
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
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
20
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
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