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LT1959 Datasheet(PDF) 19 Page - Linear Technology

Part No. LT1959
Description  4.5A, 500kHz Step-Down Switching Regulator
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT1959 Datasheet(HTML) 19 Page - Linear Technology

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19
LT1959
APPLICATIONS INFORMATION
Figure 10. Response from VC Pin to Output
FREQUENCY (Hz)
3000
2500
2000
1500
1000
500
200
150
100
50
0
–50
100
10k
100k
10M
1595 F11
1k
1M
GAIN
PHASE
ROUT
200k
COUT
12pF
VC
ERROR AMPLIFIER EQUIVALENT CIRCUIT
RLOAD = 50Ω
VFB 2 × 10
–3
)
(
Figure 11. Error Amplifier Gain and Phase
Figure 12. Overall Loop Characteristics
What About a Resistor in the Compensation Network?
It is common practice in switching regulator design to add
a “zero” to the error amplifier compensation to increase
loop phase margin. This zero is created in the external
network in the form of a resistor (RC) in series with the
compensation capacitor. Increasing the size of this resis-
tor generally creates better and better loop stability, but
there are two limitations on its value. First, the combina-
tion of output capacitor ESR and a large value for RC may
cause loop gain to stop rolling off altogether, creating a
gain margin problem. An approximate formula for RC
where gain margin falls to zero is:
R Loop
V
G
G
ESR
C
OUT
MP
MA
Gain = 1
()=()()( )( )
121
.
FREQUENCY (Hz)
40
20
0
–20
–40
40
0
–40
–80
–120
10
1k
10k
1M
1959 F10
100
100k
GAIN
PHASE
VIN = 10V
VOUT = 5V
IOUT = 2A
FREQUENCY (Hz)
80
60
40
20
0
–20
200
150
100
50
0
–50
10
1k
10k
1M
1595 F12
100
100k
GAIN
PHASE
VIN = 10V
VOUT = 5V, IOUT = 2A
COUT = 100µF, 10V, AVX TPS
CC = 1.5nF, RC = 0, L = 10µH
Error amplifier transconductance phase and gain are shown
in Figure 11. The error amplifier can be modeled as a
transconductance of 2000
µMho, with an output imped-
ance of 200k
Ω 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-
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 60
° at unity-gain.
Analog experts will note that around 4.4kHz, phase dips
very close to the zero phase margin line. This is typical of
switching regulators, especially those that operate over a
wide range of loads. This region of low phase is not a
problem as long as it does not occur near unity-gain. In
practice, the variability of output capacitor ESR tends to
dominate all other effects with respect to loop response.
Variations in ESR
will cause unity-gain to move around,
but at the same time phase moves with it so that adequate
phase margin is maintained over a very wide range of ESR
(
≥ ±3:1).


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