LT1777CS datasheet(12/24 Pages) LINER

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LT1777

APPLICATIONS INFORMATION

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 F04a

(a) LT1676 for Comparison

0MHz to 20MHz (2MHz/DIV)

1777 F04b

(b) LT1777 with LSENSE = 0µH

0MHz to 20MHz (2MHz/DIV)

1777 F04c

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

µs/DIV

1777 F05a

(a) VIN = 12V

GND

Figure 5. VSW Node Voltage Behavior

500ns/DIV

1777 F05b

(b) VIN = 36V

GND

Figure 4. Spectral Analysis of Current Waveforms in

Figures 2 and 3. (VIN = 24V, VOUT = 5V, IOUT = 400mA)

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