Datasheet

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