LT1578 Linear Technology, LT1578 Datasheet - Page 20

• Manufacturer Part Number: LT1578
• Description: 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT1578/LT1578-2.5
APPLICATIONS
the output. Gain is set by the 1.5A/V transconductance of
the LT1578 power section and the effective complex
impedance from output to ground. Gain rolls off smoothly
above the 160Hz pole frequency set by the 100 F output
capacitor. Phase drop is limited to about 85 . Phase
recovers and gain levels off at the zero frequency ( 16kHz)
set by capacitor ESR (0.1 ).
Error amplifier transconductance phase and gain are shown
in Figure 11. The error amplifier can be modeled as a
transconductance of 1000 Mho, with an output imped-
ance of 570k
applications, the compensation network from the V
ground has a much lower impedance than the output
impedance of the amplifier at frequencies above 200Hz.
20
LT1578
GND
CURRENT MODE
POWER STAGE
C
Figure 10. Response from V
g
–20
–40
F
m
40
20
0
= 1.5A/V
Figure 9. Model for Loop Response
10
V
R
C
C
C
C
in parallel with 2.4pF. In all practical
U
100
PHASE
AMPLIFIER
FREQUENCY (Hz)
ERROR
–
+
INFORMATION
U
GAIN
1k
1.21V
V
SW
FB
V
V
I
OUT
C
IN
OUT
10k
W
Pin to Output
= 10V
= 500mA
= 5V
1578 F07
100k
R1
R2
40
0
–40
–80
–120
+
U
ESR
C1
C
pin to
OUTPUT
1578 F09
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. The
phase plot shows a two-pole characteristic until the ESR
of the output capacitor brings it back to single pole above
16kHz. Phase margin is about 77 at unity-gain.
–500
2000
1500
1000
–20
Figure 11. Error Amplifier Gain and Phase
500
80
60
40
20
0
Figure 12. Overall Loop Characteristics
0
10
10
V
V
I
C
10V, AVX TPS
C
L = 30 H
OUT
V
R
IN
OUT
OUT
C
ERROR AMPLIFIER EQUIVALENT CIRCUIT
FB 1 10
LOAD
= 100pF
= 10V
= 500mA
= 5V
= 100 F
100
100
= 50
–3
FREQUENCY (Hz)
FREQUENCY (Hz)
PHASE
GAIN
1k
1k
R
570k
OUT
10k
10k
GAIN
100k
100k
C
2.4pF
PHASE
OUT
V
1578 F12
C
1578 F11
1M
1M
180
135
90
45
0
–45
200
150
100
50
0
–50