Datasheet

LT3681
13
3681f
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 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.
Burst Mode Operation
To enhance effi ciency at light loads, the LT3681 auto-
matically switches to Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation, the LT3681 delivers single cycle bursts of current
to the output capacitor followed by sleep periods where
the output power is delivered to the load by the output
capacitor. In addition, V
IN
and BIAS quiescent currents are
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 effi ciency.
See Figure 4.
–
+
1.265V
SW
V
C
GND
3Meg
LT3681
3681 F02
R1
OUTPUT
ESR
C
F
C
C
R
C
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
m
= 3.5mho
g
m
=
330µmho
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
Figure 2. Model for Loop Response
Figure 3. Transient Load Response of the LT3681 3.3V Application
as the Load Current is Stepped from 500mA to 1500mA.
3681 F03
I
L
1A/DIV
V
OUT
100mV/DIV
10 s/DIV
V
IN
= 12V
APPLICATIONS INFORMATION
Figure 4. Burst Mode Operation
3681 F04
I
L
0.5A/DIV
V
SW
5V/DIV
V
OUT
10mV/DIV
5 s/DIV
V
IN
= 12V; V
OUT
= 3.3V
I
LOAD
= 10mA