LT3480
13
3480fb
APPLICATIONS INFORMATION
operation, the noise is typically very quiet to a casual ear.
If this is unacceptable, use a high performance tantalum
or electrolytic capacitor at the output.
A fi nal precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LT3480. A ceramic
input capacitor combined with trace or cable inductance
forms a high quality (under damped) tank circuit. If the
LT3480 circuit is plugged into a live supply, the input volt-
age can ring to twice its nominal value, possibly exceeding
the LT3480’s rating. This situation is easily avoided (see
the Hot Plugging Safely section).
Frequency Compensation
The LT3480 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular, the
LT3480 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 fi lter 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
application 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
conditions, including load current, input voltage and
temperature. 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 LT3480 control loop.
The error amplifi er is a transconductance amplifi er with
fi nite output impedance. The power section, consisting
of the modulator, power switch and inductor, is modeled
as a transconductance amplifi er generating an output
–
+
0.8V
SW
V
C
g
m
=
420μmho
GND
3M
LT3480
3480 F02
R1
OUTPUT
ESR
C
F
C
C
R
C
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
m
= 3.5mho
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
Figure 2. Model for Loop Response
current 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 amplifi er
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 500mA
to 1500mA and back to 500mA.
Figure 3. Transient Load Response of the LT3480 Front Page
Application as the Load Current is Stepped from 500mA to
1500mA. V
OUT
= 3.3V
3480 F03
I
L
0.5A/DIV
V
OUT
100mV/DIV
10μs/DIV
V
IN
= 12V; FRONT PAGE APPLICATION
