LT3695 Series
17
3695fa
Audible Noise
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can sometimes cause
problems when used with the LT3695 regulators due to
their piezoelectric nature. When in Burst Mode operation,
the LT3695 regulators’ switching frequency depends on the
load current, and at very light loads the LT3695 regulators
can excite the ceramic capacitor at audio frequencies, gen-
erating audible noise. Since the LT3695 regulators operate
at a lower current limit during Burst Mode operation, the
noise is typically very quiet. If this is unacceptable, use
a high performance tantalum or electrolytic capacitor at
the output.
Frequency Compensation
The LT3695 regulators use current mode control to
regulate the output. This simplifi es loop compensation.
In particular, the LT3695 regulators do 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, as shown in Figure 1.
Generally a capacitor (C
C
) and a resistor (R
C
) in series to
ground are used. In addition, there may be a lower value
capacitor in parallel. This capacitor (C
F
) is used to fi lter
noise at the switching frequency, and is required only if a
phase-lead capacitor (C
PL
, LT3695 only) is used or if the
output capacitor has high ESR.
APPLICATIONS INFORMATION
Loop compensation determines the stability and transient
performance. Optimizing the design of the compensation
network depends on the application and type of output
capacitor. A practical approach is to start with one of the
circuits in this data sheet that is similar to your applica-
tion and tune the compensation 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 1
shows an equivalent circuit for the LT3695 regulators
control loop. The error amplifi er 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 R
C
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
, LT3695 only) across the feedback divider
may improve the transient response. Figure 2 shows the
transient response when the load current is stepped from
300mA to 650mA and back to 300mA.
0.8V
LT3695
C
F
R
C
V
C
3M
3695 F01
CURRENT MODE
POWER STAGE
g
m
= 1.25S
C
C
C
PL
C1
R2
R1
SW
FB
GND
OUTPUT
CERAMIC
POLYMER
OR
TANTALUM
OR
ELECTROLITIC
ESR
+
C1
–
+
g
m
= 430μS
Figure 1. Model for Loop Response. Note That R1 and R2 Are
Integrated in the LT3695-3.3 and LT3695-5
Figure 2. Transient Load Response of the LT3695
Regulators. A 3.3V
OUT
Typical Application with V
IN
= 12V
as the Load Current Is Stepped from 300mA to 650mA
V
OUT
100mV/DIV
I
LOAD
0.5A/DIV
20μs/DIV
3695 F02
