LT3685 LINER [Linear Technology], LT3685 Datasheet - Page 16

• Manufacturer Part Number: LT3685
• Description: 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3690
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
addition, there may be a lower value capacitor in parallel.
This capacitor (C
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
16
C
) and a resistor (R
C
F
LT3690
V
CURRENT MODE
C
POWER STAGE
g
m
R
C
C
C
= 4.6S
3M
C
pin, as shown in Figure 1. Generally a capacitor
Figure 1. Model for Loop Response
g
m
F
= 400µS
) is not part of the loop compensation
+
–
C
) in series to ground are used. In
GND
0.8V
SW
FB
R1
R2
ELECTROLITIC
TANTALUM
POLYMER
C
PL
OR
OR
+
ESR
3690 F01
C1
OUTPUT
CERAMIC
C1
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
the output capacitor integrates this current, and that the
capacitor on the V
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
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.
100mV/DIV
2A/DIV
V
OUT
I
L
Figure 2. Transient Load Response
V
FRONT PAGE APPLICATION
IN
= 12V, I
C
pin (C
LOAD
STEPPED BETWEEN 0.6A AND 3.5A
C
20µs/DIV
) integrates the error amplifier
C
in series with C
3690 F02
C
pin. Note that
3690f
C
.