Datasheet

LT3686
22
3686fc
ApplicAtions inForMAtion
Figure 16. Model for Loop Response
Frequency Compensation
The LT3686 uses current mode control to regulate the
output. This simplifies loop compensation. In particular,
the LT3686 does not require the ESR of the output capaci-
tor 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 LT3686 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 current
proportional to the voltage at the V
C
node. Note that the
output capacitor integrates this current, and that the capaci-
tor on the V
C
node (C
C
) integrates the error amplifier 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 recommended
capacitor, stability should be checked across all operating
conditions, including load current, input voltage and tem-
perature. The LT1375 data sheet contains a more thorough
discussion of loop compensation and describes how to
test the stability using a transient load.
800mV
SW
FB
R1 C
PL
R2
ERROR
AMPLIFIER
CURRENT MODE
POWER STAGE
GND
LT3686
1V
1M
R
C
160k
V
C
C
C
100pF
–
+
–
+
g
m
=
200µA/V
g
m
=
2A/V
ESR
3686 F16
OUT
C1
C1
+