Datasheet

LT1913
14
1913f
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 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 com-
pensation 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 stabil-
ity using a transient load. Figure 2 shows an equivalent
circuit for the LT1913 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 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 amplifi 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 cur-
rent is stepped from 1A to 3A and back to 1A.
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age that is higher than the input voltage. In most cases
a 0.47µF capacitor will work well. Figure 2 shows three
ways to arrange the boost circuit. The BOOST pin must be
–
+
0.8V
SW
V
C
g
m
=
525µmho
GND
3M
LT1913
1913 F02
R1
OUTPUT
ESR
C
F
C
C
R
C
ERROR
AMPLIFIER
FB
R2
C1
C1
CURRENT MODE
POWER STAGE
g
m
= 5.3mho
+
POLYMER
OR
TANTALUM
CERAMIC
C
PL
Figure 3. Transient Load Response of the LT1913 Front Page
Application as the Load Current is Stepped from 1A to 3A.
V
OUT
= 5V
Figure 2. Model for Loop Response
1913 F03
I
L
1A/DIV
V
OUT
100mV/DIV
10µs/DIV
APPLICATIONS INFORMATION