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LT1933 Datasheet

  • LT1933

  • 600mA, 500kHz Step-Down Switching Regulator in SOT-23

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LT1933
APPLICATIO S I FOR ATIO
2.2碌F 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鈥檚 high equivalent series resistance damps the
circuit and eliminates the voltage overshoot. The extra
capacitor improves low frequency ripple filtering and can
slightly improve the efficiency of the circuit, though it is
likely to be the largest component in the circuit. An
alternative solution is shown in Figure 6c. A 1鈩?resistor is
added in series with the input to eliminate the voltage
overshoot (it also reduces the peak input current). A 0.1碌F
capacitor improves high frequency filtering. This solution
is smaller and less expensive than the electrolytic capaci-
tor. For high input voltages its impact on efficiency is
minor, reducing efficiency 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 simplifies loop compensation. In particular,
the LT1933 does not require the ESR of the output
capacitor for stability allowing the use of ceramic capaci-
tors 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 ampli-
fier with finite output impedance. The power section,
consisting of the modulator, power switch and inductor, is
modeled as a transconductance amplifier generating an
output current proportional to the voltage at the V
C
node.
Note that the output capacitor integrates this current, and
LT1933
0.7V
CURRENT MODE
POWER STAGE
SW
R1
FB
g
m
=
150碌mhos
ERROR
AMPLIFIER
500k
C
PL
g
m
1.1mho
V
C
R
C
100k
C
C
80pF
GND
OUT
SHUTDOWN
V
IN
1.245V
ESR
+
C1
R2
Figure 7. Model for Loop Response
U
that the capacitor on the V
C
node (C
C
) integrates the error
amplifier output current, resulting in two poles in the loop.
R
C
provides a zero. With the recommended output capaci-
tor, 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 thor-
ough discussion of loop compensation and describes how
to test the stability using a transient load.
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board layout. Figure 8 shows
the recommended component placement with trace,
ground plane and via locations. Note that large, switched
currents flow in the LT1933鈥檚 V
IN
and SW pins, the catch
diode (D1) and the input capacitor (C2). The loop formed
by these components should be as small as possible and
tied to system ground in only one place. These compo-
nents, along with the inductor and output capacitor,
C1
C1
C2
D1
V
OUT
SYSTEM
GROUND
VIAS TO LOCAL GROUND PLANE
OUTLINE OF LOCAL GROUND PLANE
1933 F07
1933 F08
W
U
鈥?/div>
+
U
鈥?/div>
+
Figure 8. A Good PCB Layout Ensures Proper, Low EMI Operation
1933f
13

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LT1933ES6,LT1933IS6

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