Datasheet

LT3680
13
Rev C
For more information www.analog.com
Frequency Compensation
The LT3680 uses current mode control to regulate the out-
put. This simplifies loop compensation. In particular, the
LT3680 does not require the ESR of the output capacitor
for stability, so you are free to use ceramic capacitors to
achieve low output ripple and small circuit size. Frequency
compensation is provided by the components tied to the
V
C
pin, as shown in Figure 2. Generally a capacitor (C
C
)
and a resistor (R
C
) in series to ground are used. In addi-
tion, there may be lower value capacitor in parallel. This
capacitor (C
F
) is not part of the loop compensation but
is used to filter noise at the switching frequency, and is
required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a
bit complicated and the best values depend on the appli
-
cation and in particular the type of output capacitor. A
practical approach is to start with one of the circuits in
this data sheet that is similar to your application and tune
the compensation network to optimize the performance.
Stability should then be checked across all operating con-
ditions, including load current, input voltage and tempera-
ture. The LT1375 data sheet contains a more thorough
discussion of loop compensation and describes how to
test the stability using a transient load. Figure 2 shows
an equivalent circuit for the LT3680 control loop. The
error amplifier is a transconductance amplifier with finite
output impedance. The power section, consisting of the
modulator, power switch and inductor, is modeled as a
transconductance amplifier generating an output cur-
rent proportional to the voltage at the V
C
pin. Note that
the output capacitor integrates this current, and that the
capacitor on the V
C
pin (C
C
) integrates the error ampli-
fier output current, resulting in two poles in the loop. In
most cases a zero is required and comes from either the
output capacitor ESR or from a resistor R
C
in series with
C
C
. This simple model works well as long as the value
of the inductor is not too high and the loop crossover
frequency is much lower than the switching frequency.
A phase lead capacitor (C
PL
) across the feedback divider
may improve the transient response. Figure 3 shows the
transient response when the load current is stepped from
1A to 3A and back to 1A.
–
+
0.8V
SW
V
C
g
m
=
500µmho
GND
3M
LT3680
3680 F02
R1
OUTPUT
ESR
C
F
C
C
R
C
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
m
= 5.3mho
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
Figure 3. Transient Load Response of the LT3680 Front Page
Application as the Load Current is Stepped from 1A to 3A.
V
OUT
= 5V
Figure 2. Model for Loop Response
3680 F03
I
L
1A/DIV
V
OUT
100mV/DIV
10µs/DIV
V
IN
= 12V
V
OUT
= 3.3V
APPLICATIONS INFORMATION
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