Datasheet

LT3685
13
3685fb
forms a high quality (under damped) tank circuit. If the
LT3685 circuit is plugged into a live supply, the input volt-
age can ring to twice its nominal value, possibly exceeding
the LT3685’s rating. This situation is easily avoided (see
the Hot Plugging Safely section).
Frequency Compensation
The LT3685 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular, the
LT3685 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 2. Generally a capacitor (C
C
)
and a resistor (R
C
) in series to ground are used. In addi-
tion, there may be lower value capacitor in parallel. This
capacitor (C
F
) is not part of the loop compensation but
is used to fi lter 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. Designing the compensation network is a
bit complicated and the best values depend on the ap-
plication 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 2
shows an equivalent circuit for the LT3685 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 cur-
rent 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 ampli-
fi 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
) across the feedback divider
may improve the transient response. Figure 3 shows the
transient response when the load current is stepped from
500mA to 1500mA and back to 500mA.
–
+
0.8V
SW
V
C
g
m
=
420μmho
GND
3M
LT3685
3685 F02
R1
OUTPUT
ESR
C
F
C
C
R
C
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
m
= 3.5mho
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
Figure 2. Model for Loop Response
APPLICATIONS INFORMATION
Figure 3. Transient Load Response of the LT3685 Front Page
Application as the Load Current is Stepped from 500mA to
1500mA. V
OUT
= 3.3V
3685 F03
I
L
0.5A/DIV
V
OUT
100mV/DIV
10μs/DIV
V
IN
= 12V; FRONT PAGE APPLICATION