Datasheet

20
LT1576/LT1576-5
APPLICATIONS INFORMATION
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the output. Gain is set by the 1.5A/V transconductance of
the LT1576 power section and the effective complex
impedance from output to ground. Gain rolls off smoothly
above the 160Hz pole frequency set by the 100µF output
capacitor. Phase drop is limited to about 85°. 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 1000µMho, with an output imped-
ance of 570kΩ in parallel with 2.4pF. 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 200Hz.
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 100pF, giving the
error amplifier a pole at 2.8kHz, with phase rolling off to
90° and staying there. The overall loop has a gain of 66dB
at low frequency, rolling off to unity-gain at 58kHz. Phase
shows a two-pole characteristic until the ESR of the output
capacitor brings it back above 16kHz. Phase margin is
about 77° at unity-gain.
FREQUENCY (Hz)
GAIN (µMho)
PHASE (DEG)
2000
1500
1000
500
0
–500
200
150
100
50
0
–50
10 1k 10k 1M
1576 F11
100 100k
GAIN
PHASE
R
OUT
570k
C
OUT
2.4pF
V
C
ERROR AMPLIFIER EQUIVALENT CIRCUIT
R
LOAD
= 50Ω
V
FB
1 × 10
–3
)(
–
+
1.21V
V
SW
V
C
LT1576
GND
1576 F09
R1
OUTPUT
ESR
C
F
C
C
R
C
ERROR
AMPLIFIER
FB
R2
C1
CURRENT MODE
POWER STAGE
g
m
= 1.5A/V
+
Figure 10. Response from V
C
Pin to Output
FREQUENCY (Hz)
10
GAIN (dB)
PHASE (DEG)
40
20
0
–20
–40
40
0
–40
–80
–120
100 1k
1576 F07
10k 100k
GAIN
PHASE
V
IN
= 10V
V
OUT
= 5V
I
OUT
= 500mA
Figure 12. Overall Loop Characteristics
FREQUENCY (Hz)
LOOP GAIN (dB)
LOOP PHASE (DEG)
80
60
40
20
0
–20
180
135
90
45
0
–45
10 1k 10k 1M
1576 F12
100 100k
V
IN
= 10V
V
OUT
= 5V
I
OUT
= 500mA
C
OUT
= 100µF
10V, AVX TPS
C
C
= 100pF
L = 30µH
PHASE
GAIN
Figure 9. Model for Loop Response Figure 11. Error Amplifier Gain and Phase