LT3506/LT3506A
14
3506afc
Frequency Compensation
The LT3506 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular, the
LT3506 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. Generally a capacitor and a resistor in
series to ground determine loop gain. In addition, there
is a lower value capacitor in parallel. This capacitor is not
part of the loop compensation but is used to fi lter noise
at the switching frequency.
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 compensa-
tion 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 4 shows an equivalent circuit for the
LT3506 control loop. The error amp 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 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 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.
Soft-Start and Shutdown
The RUN/SS (Run/Soft-Start) pins are used to place the
individual switching regulators and the internal bias circuits
in shutdown mode. They also provide a soft-start function.
To shut down either regulator, pull the RUN/SS pin to ground
with an open-drain or collector. If both RUN/SS pins are
pulled to ground, the LT3506 enters its shutdown mode
with both regulators off and quiescent current reduced to
~30A. Internal 2A current sources pull up on each pin.
If either pin reaches ~0.6V, the internal bias circuits start
and the quiescent current increases to ~3.5mA.
If a capacitor is tied from the RUN/SS pin to ground, then
the internal pull-up current will generate a voltage ramp on
this pin. This voltage clamps the V
C
pin, limiting the peak
switch current and therefore input current during start-up.
A good value for the soft-start capacitor is C
OUT
/10,000,
where C
OUT
is the value of the output capacitor.
The RUN/SS pins can be left fl oating if the shutdown
feature is not used. They can also be tied together with a
single capacitor providing soft-start. The internal current
sources will charge these pins to ~2.5V.
The RUN/SS pins provide a soft-start function that limits
peak input current to the circuit during start-up. This helps
to avoid drawing more current than the input source can
Figure 4. Circuit Model for Frequency Compensation
–
+
V
FB
800mV
V
SW
V
C
LT3506
GND
3506 F04
R1
OUTPUT
ESR
C
F
C
C
R
C
1M
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
mp
2.4A/V
330umhos
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
APPLICATIONS INFORMATION
