Datasheet

12
LT1777
APPLICATIONS INFORMATION
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voltage of 12V, and then 36V. Once again the circuit is the
Typical Application shown on the first page of this data
sheet, with an output load of 400mA.
Figure 5a, with V
IN
of 12V, shows a relatively rectangular
voltage waveform. The limited voltage slew rate still allows
for nearly vertical switching edges, so little power is
wasted. A positive-going step before the leading edge and
a negative-going step after the trailing edge can be seen.
These are evidence of the internal current limiting circuitry
at work.
Figure 5b, with V
IN
of 36V, shows a substantially
nonrectangular waveform. The limited voltage slew rate is
clearly evident as transitions take a few hundred nanosec-
onds. Efficiency (P
OUT
/P
IN
) is reduced as a result of the
slower transitions. For comparison purposes, the oscillo-
scope photo in Figure 6 shows the performance of the high
efficiency LT1676. Voltage transitions are well under
100ns and the waveform appears quite rectangular.
10dB/DIV
0MHz to 20MHz (2MHz/DIV)
1777 F04a
(a) LT1676 for Comparison
10dB/DIV
0MHz to 20MHz (2MHz/DIV)
1777 F04b
(b) LT1777 with L
SENSE
= 0µH
10dB/DIV
0MHz to 20MHz (2MHz/DIV)
1777 F04c
(c) LT1777 with L
SENSE
= 2.2µH
Voltage Waveform Behavior
Unlike current behavior, voltage slew rate of the LT1777 is
not adjustable by the user. No component selection or
other action is required. Nevertheless, it is instructive to
examine typical behavior. The oscilloscope photos in
Figure 5 show the V
SW
voltage waveform with an input
2V/DIV
1µs/DIV
1777 F05a
(a) V
IN
= 12V
GND
Figure 5. V
SW
Node Voltage Behavior
10V/DIV
500ns/DIV
1777 F05b
(b) V
IN
= 36V
GND
Figure 4. Spectral Analysis of Current Waveforms in
Figures 2 and 3. (V
IN
= 24V, V
OUT
= 5V, I
OUT
= 400mA)