lt3686a Linear Technology Corporation, lt3686a Datasheet - Page 22

• Manufacturer Part Number: lt3686a
• Description: 37v/1.2a Step-down Regulator In 3mm
• Manufacturer: Linear Technology Corporation

LT3686A
APPLICATIONS INFORMATION
Frequency Compensation
The LT3686A uses current mode control to regulate the
output. This simplifi es 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 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 cur-
rent proportional to the voltage at the V
the output capacitor integrates this current, and that the
capacitor on the V
fi er output current, resulting in two poles in the loop. R
provides a zero. With the recommended output capacitor,
22
C
node (C
LT3686A
GND
C
) integrates the error ampli-
R
160k
C
C
100pF
C
V
C
1V
C
1M
node. Note that
AMPLIFIER
Figure 16. Model for Loop Response
ERROR
–
+
g
2A/V
m
g
200μA/V
CURRENT MODE
=
POWER STAGE
m
=
–
+
C
800mV
the loop crossover occurs above the R
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.
FB
R1
R2
C
PL
+
ESR
C1
3686A F16
OUT
C1
C
C
C
zero. This simple
3686af