LT1936 Linear Technology, LT1936 Datasheet - Page 11

• Manufacturer Part Number: LT1936
• Description: 1.4A 500kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

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
(2a)
(2b)
(2d)
(2c)
Figure 2. Transient Load Response of the LT1936 with Different Output
Capacitors as the Load Current is Stepped from 200mA to 800mA. V
(AVX 1210ZD226MAT)
C
C
C
OUT
C
(4TPC150M)
(4TPC150M)
OUT
OUT
OUT
COMP
COMP
COMP
COMP
= 22μF ×2
= 150μF
= 150μF
V
V
V
V
= 22μF
C
C
C
C
220k
100pF
500mA/DIV
I
OUT
100mV/DIV
100mV/DIV
100mV/DIV
100mV/DIV
800mA
200mA
V
V
V
V
OUT
OUT
OUT
OUT
current proportional to the voltage at the V
the output capacitor integrates this current, and that the
capacitor on the V
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
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
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.
PL
50μs/DIV
) across the feedback divider may improve
C
pin (C
OUT
C
= 3.3V
) integrates the error amplifi er
1936 F02
www.DataSheet4U.com
C
in series with C
C
LT1936
pin. Note that
11
1936fd
C
.