LT1374C Linear Technology, LT1374C Datasheet - Page 20

• Manufacturer Part Number: LT1374C
• Description: 4.5A/ 500kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT1374
APPLICATIONS
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 V
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 100 F output
capacitor. Phase drop is limited to about 70 . Phase
recovers and gain levels off at the zero frequency ( 16kHz)
set by capacitor ESR (0.1 ).
Error amplifier transconductance phase and gain are shown
in Figure 11. The error amplifier can be modeled as a
transconductance of 2000 Mho, with an output imped-
ance of 200k
20
LT1374
GND
CURRENT MODE
Figure 10. Response from V
POWER STAGE
C
g
F
–20
–40
m
40
20
0
= 5.3A/V
Figure 9. Model for Loop Response
10
V
R
C
C
C
C
in parallel with 12pF. In all practical
100
U
PHASE
GAIN
AMPLIFIER
FREQUENCY (Hz)
ERROR
–
+
INFORMATION
1k
U
2.42V
V
SW
10k
FB
C
W
V
V
I
Pin to Output
100k
OUT
IN
OUT
= 10V
= 2A
= 5V
1374 F10
1M
R1
R2
40
0
–40
–80
–120
+
U
ESR
C1
C
OUTPUT
pin to
1374 F09
applications, the compensation network from V
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.
Figure 11. Error Amplifier Gain and Phase
3000
2500
2000
1500
1000
500
–20
Figure 12. Overall Loop Characteristics
80
60
40
20
0
100
10
V
R
V
V
C
C
ERROR AMPLIFIER EQUIVALENT CIRCUIT
FB 2 10
LOAD
IN
OUT
OUT
C
= 1.5nF, R
= 10V
= 5V, I
= 100 F, 10V, AVX TPS
1k
100
= 50
–3
OUT
FREQUENCY (Hz)
C
FREQUENCY (Hz)
PHASE
GAIN
= 0, L = 10 H
10k
= 2A
1k
GAIN
R
200k
OUT
100k
10k
100k
1M
C
12pF
PHASE
OUT
V
C
1374 F11
1374 F12
10M
1M
200
150
100
50
0
–50
200
150
100
50
0
–50
C
pin to