Datasheet

LT3686A
22
3686afa
ApplicAtions inForMAtion
Figure 16. Model for Loop Response
Frequency Compensation
The LT3686A uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT3686A does not require the ESR of the output ca-
pacitor for stability allowing the use of ceramic capacitors
to achieve low output ripple and small circuit size. Figure
16 shows an equivalent circuit for the LT3686A control
loop. The error amp 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
node. Note that
the output capacitor integrates this current, and that the
capacitor on the V
C
node (C
C
) integrates the error ampli-
fier output current, resulting in two poles in the loop. R
C
provides a zero. With the recommended output capacitor,
the loop crossover occurs above the R
C
C
C
zero. 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. With a larger ceramic
capacitor (very low ESR), crossover may be lower and a
phase lead capacitor (CPL) across the feedback divider may
improve the phase margin and transient response. Large
electrolytic capacitors may have an ESR large enough to
create an additional zero, and the phase lead may not be
necessary. If the output capacitor is different than the rec-
ommended capacitor, stability should 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.
800mV
FB
R1 C
PL
R2
ERROR
AMPLIFIER
CURRENT MODE
POWER STAGE
GND
LT3686A
1V
1M
R
C
160k
V
C
C
C
100pF
–
+
–
+
g
m
=
200µA/V
g
m
=
2A/V
ESR
3686A F16
OUT
C1
C1
+