LT1933
15
1933fe
APPLICATIONS INFORMATION
Figure 6 shows the waveforms that result when an LT1933
circuit is connected to a 24V supply through six feet of
24-gauge twisted pair. The fi rst plot is the response with
a 2.2µF ceramic capacitor at the input. The input voltage
rings as high as 35V and the input current peaks at 20A.
One method of damping the tank circuit is to add another
capacitor with a series resistor to the circuit. In Figure 6b
an aluminum electrolytic capacitor has been added. This
capacitor’s high equivalent series resistance damps the
circuit and eliminates the voltage overshoot. The extra
capacitor improves low frequency ripple fi ltering and can
slightly improve the effi ciency of the circuit, though it is
likely to be the largest component in the circuit. An alterna-
tive solution is shown in Figure 6c. A 1 resistor is added
in series with the input to eliminate the voltage overshoot
(it also reduces the peak input current). A 0.1µF capacitor
improves high frequency fi ltering. This solution is smaller
and less expensive than the electrolytic capacitor. For high
input voltages its impact on effi ciency is minor, reducing
effi ciency less than one half percent for a 5V output at full
load operating from 24V.
Frequency Compensation
The LT1933 uses current mode control to regulate the
output. This simplifi es loop compensation. In particular,
the LT1933 does not require the ESR of the output capaci-
tor for stability allowing the use of ceramic capacitors to
achieve low output ripple and small circuit size.
Figure 7 shows an equivalent circuit for the LT1933 control
loop. The error amp 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
node. Note that
the output capacitor integrates this current, and that the
capacitor on the V
C
node (C
C
) integrates the error ampli-
fi er output current, resulting in two poles in the loop. R
C
provides a zero. With the recommended output capacitor,
the loop crossover occurs above the R
C
C
C
zero. 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. With a larger ceramic
capacitor (very low ESR), crossover may be lower and a
phase lead capacitor (C
PL
) across the feedback divider may
improve the phase margin and transient response. Large
electrolytic capacitors may have an ESR large enough to
create an additional zero, and the phase lead may not be
necessary.
If the output capacitor is different than the recommended
capacitor, stability should be checked across all operating
conditions, including load current, input voltage and tem-
perature. The LT1375 data sheet contains a more thorough
discussion of loop compensation and describes how to
test the stability using a transient load.
Figure 7. Model for Loop Response
–
+
–
+
1.245V
SW
V
C
LT1933
GND
1933 F07
R1
OUT
ESR
ERROR
AMPLIFIER
CURRENT MODE
POWER STAGE
FB
R2
500k
R
C
100k
C
C
80pF
C1
C1
g
m
=
150µmhos
g
m
+
C
PL
0.7V
1.1mho
