LT1777 Linear Technology, LT1777 Datasheet - Page 12

• Manufacturer Part Number: LT1777
• Description: Low Noise Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
LT1777
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
12
Figure 4. Spectral Analysis of Current Waveforms in
Figures 2 and 3. (V
(c) LT1777 with L
(b) LT1777 with L
(a) LT1676 for Comparison
U
0MHz to 20MHz (2MHz/DIV)
0MHz to 20MHz (2MHz/DIV)
0MHz to 20MHz (2MHz/DIV)
SW
IN
= 24V, V
voltage waveform with an input
INFORMATION
U
SENSE
OUT
SENSE
= 5V, I
= 2.2 H
W
= 0 H
OUT
1777 F04a
1777 F04b
1777 F04c
= 400mA)
U
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
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
nonrectangular waveform. The limited voltage slew rate is
clearly evident as transitions take a few hundred nanosec-
onds. Efficiency (P
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.
GND
GND
Figure 5. V
IN
of 12V, shows a relatively rectangular
OUT
IN
SW
(a) V
/P
(b) V
of 36V, shows a substantially
Node Voltage Behavior
IN
500ns/DIV
1 s/DIV
) is reduced as a result of the
IN
IN
= 12V
= 36V
1777 F05a
1777 F05b