11
LT1777
APPLICATIONS INFORMATION
WUU
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current waveform. The four sense inductor values of 0µH,
0.47µH, 1µH and 2.2µH yield dI/dt rates of roughly
4.5A/µs, 2.2A/µs, 1.4A/µs and 0.6A/µs, respectively.
These photos show that there is a minimum effective value
for sense inductance, which is 0.47µH for a typical part at
room temperature as shown. This value inductor has a
small effect on the trailing edge rate, but essentially no
effect on the rising edge. Minimum effective sense induc-
tance value means that inductors much smaller than this
value will have substantially the same performance as zero
inductance, such that these inductors serve no useful
purpose.
In summary,
1. The LT1777 uses an external sense inductor to set a
theoretical limit for current ramp rate according to the
formula:
Max dI dt
V
L
BE
SENSE
/ =
2
2. Allowable range for the sense inductor runs from a
minimum of 0 to a maximum of:
Max L L
V
Max V V
SENSE MAIN
IN OUT
=
05.
–
3. The minimum effective inductor size is typically 0.47µH.
Harmonic Behavior
The LT1676 is a high efficiency “cousin” to the LT1777. An
additional set of oscilloscope photographs in Figure 3
show the leading edge and trailing edge of the current
waveform when this part is substituted for the LT1777.
(No sense inductor is used with the LT1676.) The leading
and trailing edges of the LT1676 current waveform are
much faster than that of the LT1777, even when the
LT1777 uses a sense inductor of 0µH. The 10% to 90%
rise time/fall time is on the order of 10ns to 20ns, too fast
to measure accurately at the horizontal sweep rate of
200ns/DIV.
While this time-based analysis demonstrates that the
current waveform of the LT1777 is quieter than standard
high efficiency buck converters, some users may prefer to
see a direct comparison on a frequency domain basis.
Figures 4a, 4b, and 4c show a spectral analysis of the
current waveforms. The horizontal axis is 2MHz/DIV (0MHz
to 20MHz), and the vertical axis is 10dB/DIV. All photos
were taken with V
IN
= 24V and V
OUT
= 5V at 400mA. Figure
4a is of the LT1676 and is for comparison purposes.
Figures 4b and 4c are of the LT1777 with a sense inductor
of 0µH and 2.2µH, respectively. A decrease in high fre-
quency energy is seen when going from the LT1676 to the
LT1777 with no sense inductor, and a further improve-
ment with a 2.2µH sense inductor. For example, at 10MHz,
the LT1777 shows an improvement of about –10dB with
0µH and perhaps –25dB with 2.2µH.
100mA/DIV
200ns/DIV
1777 F03a
(a) Leading Edge
Figure 3. LT1676 Current Behavior for Comparison Purposes Only
100mA/DIV
200ns/DIV
1777 F03b
(b) Trailing Edge
