LT1777 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1777' PDF : 24 Pages View PDF
LT1777
APPLICATIONS INFORMATION
0MHz to 20MHz (2MHz/DIV)
1777 F04a
(a) LT1676 for Comparison
0MHz to 20MHz (2MHz/DIV)
1777 F04b
(b) LT1777 with LSENSE = 0µH
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 VIN 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 VIN of 36V, shows a substantially
nonrectangular waveform. The limited voltage slew rate is
clearly evident as transitions take a few hundred nanosec-
onds. Efficiency (POUT/PIN) 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.
0MHz to 20MHz (2MHz/DIV)
1777 F04c
(c) LT1777 with LSENSE = 2.2µH
Figure 4. Spectral Analysis of Current Waveforms in
Figures 2 and 3. (VIN = 24V, VOUT = 5V, IOUT = 400mA)
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 VSW voltage waveform with an input
12
GND
1µs/DIV
(a) VIN = 12V
1777 F05a
GND
500ns/DIV
1777 F05b
(b) VIN = 36V
Figure 5. VSW Node Voltage Behavior
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