LT3506/LT3506A
U U
W U
APPLICATIO S I FOR ATIO
Frequency Compensation
LT3506
CURRENT MODE
POWER STAGE
V
SW
The LT3506 uses current mode control to regulate the
output.Thissimplifiesloopcompensation.Inparticular,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.
OUTPUT
ERROR
g
mp
2.4A/V
C
R1
AMPLIFIER
PL
FB
–
330umhos
ESR
+
V
FB
800mV
1MΩ
C1
+
GND
V
C
C1
Frequency compensation is provided by the components
tied to the V pin. Generally a capacitor and a resistor in
POLYMER
OR
TANTALUM
CERAMIC
C
R2
R
C
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 filter noise
at the switching frequency.
C
F
C
C
3506 F04
Loop compensation determines the stability and transient
performance.Designingthecompensationnetworkisabit
complicatedandthebestvaluesdependontheapplication
and in particular the type of output capacitor. A practical
approachistostartwithoneofthecircuitsinthisdatasheet
that is similar to your application and tune the compensa-
tionnetworktooptimizetheperformance. Stabilityshould
thenbecheckedacrossalloperatingconditions, including
load current, input voltage and temperature. The LT1375
data sheet contains a more thorough discussion of loop
compensationanddescribeshowtotestthestabilityusing
atransientload.Figure4showsanequivalentcircuitforthe
LT3506 control loop. The error amp is a transconductance
amplifierwithfiniteoutputimpedance. Thepowersection,
consisting of the modulator, power switch and inductor,
is modeled as a transconductance amplifier generating an
Figure 4. Circuit Model for Frequency Compensation
Soft-Start and Shutdown
The RUN/SS (Run/Soft-Start) pins are used to place the
individualswitchingregulatorsandtheinternalbiascircuits
inshutdownmode.Theyalsoprovideasoft-startfunction.
Toshutdowneitherregulator,pulltheRUN/SSpintoground
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
~30μA. Internal 2μA 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
theinternalpull-upcurrentwillgenerateavoltagerampon
this pin. This voltage clamps the V pin, limiting the peak
C
output current proportional to the voltage at the V pin.
C
switchcurrentandthereforeinputcurrentduringstart-up.
Note that the output capacitor integrates this current, and
A good value for the soft-start capacitor is C /10,000,
OUT
that the capacitor on the V pin (C ) integrates the error
C
C
where C
is the value of the output capacitor.
OUT
amplifier output current, resulting in two poles in the loop.
The RUN/SS pins can be left floating 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.
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
The RUN/SS pins provide a soft-start function that limits
peakinputcurrenttothecircuitduringstart-up. Thishelps
to avoid drawing more current than the input source can
capacitor (C ) across the feedback divider may improve
PL
the transient response.
3506afb
ꢀꢃ
Linear [ Linear ]
