Datasheet

LT3690
16
3690fa
applicaTions inForMaTion
Since the LT3690 operates 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 LT3690 uses current mode control to regulate the
output. This simplifies loop compensation. In particular, the
LT3690 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.
Frequency compensation is provided by the components
tied to the V
C
pin, as shown in Figure 1. Generally a capaci-
tor (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 not part of the loop compensation
but is used to filter noise at the switching frequency, and
is required only if a phase-lead capacitor is used or if the
output capacitor has high ESR.
Loop compensation determines the stability and transient
performance. The best values for the compensation net-
work depend on the application and, in particular, the type
of output capacitor. A practical approach is to start with
one of the circuits in this data sheet that is similar to your
application 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 LT3690 control
loop. The error amplifier 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
pin. Note that
the output capacitor integrates this current, and that the
capacitor on the V
C
pin (C
C
) integrates the error amplifier
output current, resulting in two poles in the loop. In most
cases, a zero is required and comes either from 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 (CPL) across the feedback divider may improve
the transient response.
Figure 1. Model for Loop Response
Figure 2. Transient Load Response
20µs/DIV
V
OUT
100mV/DIV
I
L
2A/DIV
3690 F02
V
IN
= 12V, I
LOAD
STEPPED BETWEEN 0.6A AND 3.5A
FRONT PAGE APPLICATION
3690 F01
LT3690
GNDV
C
C
C
R
C
3M
0.8V
CURRENT MODE
POWER STAGE
g
m
= 4.6S
SW
FB
–
+
g
m
= 400µS
C
PL
R1
C1
POLYMER
OR
TANTALUM
OR
ELECTROLITIC
ESR
R2
C
F
+
C1
OUTPUT
CERAMIC