LT1375/LT1376
U
W U U
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).
Erroramplifiertransconductancephaseandgainareshown
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-
selvesdonotcontributeexcessphaseshifttotheloop,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
3000
2500
2000
1500
1000
500
200
150
100
50
LT1375
LT1376
CURRENT MODE
POWER STAGE
PHASE
GAIN
V
SW
FB
OUTPUT
g
= 2A/V
ERROR
m
AMPLIFIER
R1
R2
–
V
C
ESR
C1
+
C
R
OUT
12pF
OUT
200k
2.42V
V
2 •10–3
(
)
FB
+
V
C
GND
ERROR AMPLIFIER EQUIVALENT CIRCUIT
= 50Ω
0
R
LOAD
1k
R
C
C
–50
F
100
10k
100k
1M
10M
C
C
FREQUENCY (Hz)
1375/76 F14
1375/76 F12
Figure 12. Model for Loop Response
Figure 14. Error Amplifier Gain and Phase
40
20
0
40
80
60
200
150
100
50
V
V
I
= 10V
IN
GAIN
= 5V
OUT
OUT
= 500mA
0
GAIN
40
–40
–80
–120
20
PHASE
PHASE
V
V
C
C
= 10V
OUT
OUT
–20
–40
IN
0
0
= 5V, I
= 500mA
= 100µF, 10V, AVX TPS
OUT
= 3.3nF, R = 0, L = 10µH
C
C
–20
–50
10
100
1k
10k
100k
1M
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
FREQUENCY (Hz)
1375/76 F13
1375/76 F15
Figure 15. Overall Loop Characteristics
Figure 13. Response from V Pin to Output
C
13756fd
20
Linear [ Linear ]
