LT3681
APPLICATIONS INFORMATION
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 LT3681 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 cur-
V
= 12V
IN
I
L
1A/DIV
V
OUT
100mV/DIV
10 s/DIV
3681 F03
Figure 3. Transient Load Response of the LT3681 3.3V Application
as the Load Current is Stepped from 500mA to 1500mA.
Burst Mode Operation
To enhance efficiency at light loads, the LT3681 auto-
matically switches to Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizingtheinputquiescentcurrent.DuringBurstMode
operation,theLT3681deliverssinglecycleburstsofcurrent
to the output capacitor followed by sleep periods where
the output power is delivered to the load by the output
rent proportional to the voltage at the V pin. Note that
C
the output capacitor integrates this current, and that the
capacitor on the V pin (C ) integrates the error ampli-
C
C
fier 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 in series with
C
C . This simple model works well as long as the value
capacitor. Inaddition, V andBIASquiescentcurrentsare
C
IN
of the inductor is not too high and the loop crossover
reduced to typically 20µA and 50µA respectively during
the sleep time. As the load current decreases towards a
no load condition, the percentage of time that the LT3681
operates in sleep mode increases and the average input
current is greatly reduced resulting in higher efficiency.
See Figure 4.
frequency is much lower than the switching frequency.
A phase lead capacitor (C ) across the feedback divider
PL
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.
LT3681
CURRENT MODE
POWER STAGE
m
SW
OUTPUT
V
I
= 12V; V
LOAD
= 3.3V
OUT
IN
ERROR
AMPLIFIER
g
= 3.5mho
= 10mA
C
R1
PL
I
L
FB
–
0.5A/DIV
g
=
m
330µmho
ESR
+
1.265V
C1
+
3Meg
C1
V
SW
5V/DIV
POLYMER
OR
TANTALUM
CERAMIC
V
GND
C
V
OUT
10mV/DIV
R
C
R2
C
F
C
C
3681 F04
5 s/DIV
3681 F02
Figure 2. Model for Loop Response
Figure 4. Burst Mode Operation
3681f
13
Linear [ Linear ]
