Datasheet

LT1936
11
1936fd
APPLICATIONS INFORMATION
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 stability
using a transient load.
Figure 1 shows an equivalent circuit for the LT1936 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
Figure 2. Transient Load Response of the LT1936 with Different Output
Capacitors as the Load Current is Stepped from 200mA to 800mA. V
OUT
= 3.3V
COMP
C
OUT
= 22μF
(AVX 1210ZD226MAT)
(2a)
(2b)
(2c)
(2d)
V
OUT
100mV/DIV
V
OUT
100mV/DIV
V
OUT
100mV/DIV
V
OUT
100mV/DIV
I
OUT
500mA/DIV
800mA
200mA
50μs/DIV
1936 F02
V
C
COMP
C
OUT
= 22μF ×2
V
C
COMP
C
OUT
= 150μF
(4TPC150M)
V
C
COMP
220k
100pF
C
OUT
= 150μF
(4TPC150M)
V
C
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 2 compares the transient response across several
output capacitor choices and compensation schemes.
In each case the load current is stepped from 200mA to
800mA and back to 200mA.