Datasheet

10
LT1777
APPLICATIONS INFORMATION
WUU
U
Deciding upon a value for the sense inductor involves
evaluating the trade-off between overall efficiency (P
OUT
/
P
IN
) and switch current slew rate. Larger sense inductors
yield lower current slew rates which offer reduced high
frequency RFI emissions, but at the expense of poorer
efficiency.
The question is “What is the allowed range of values for a
sense inductor in a given application?” There is really no
minimum limit to the sense inductor, i.e., its value is
allowed to be zero. (In other words, the physical sense
inductor ceases to exist and is replaced by a short circuit.)
This will yield the highest efficiency possible in a given
situation. Although an explicit current slew rate no longer
exists, the naturally less aggressive nature of the LT1777
will often yield quieter supply operation than other stan-
dard switching regulators.
As far as the maximum allowable value for the sense
inductor, this is dictated by the current ramp rate in the
main inductor during the conventional part of the switch-
ing cycle. It is generally overconservative to limit the
switch current slew rate to that exhibited by the main
inductor. This would potentially yield a triangular current
waveform. Efficiency would be greatly reduced at little
further gain in noise performance. Stated mathematically,
maximum slew rate in the main inductor occurs at maxi-
mum input voltage as:
dI
dt
Max V V
L
IN OUT
MAIN
=
–
The sense inductor experiences 2V
BE
of applied voltage.
This is perhaps 1.0V at a maximum hot condition. If we use
an additional factor of two to be conservative, this yields
a maximum sense inductor value as follows:
05
05
.
–
,
.
–
V
L
Max V V
L
or
Max L L
V
Max V V
SENSE
IN OUT
MAIN
SENSE MAIN
IN OUT
=
=






As an example, a maximum input voltage of 36V, an output
voltage of 5V and a main inductor value of 220µH yields a
maximum suggested sense inductor value of 3.5µH.
Circuit behavior versus sense inductor value is shown in
the oscilloscope photos in Figure 2. The circuit and oper-
ating conditions are similar to the Typical Application on
the first page of this data sheet with the exception that the
sense inductor is allowed to assume the series of values:
0µH, 0.47µH, 1µH and 2.2µH. Figure 2a shows a close-up
of the leading edge (turn-on) of the current waveform.
Values of 0µH and 0.47µH are found to yield a dI/dt of
about 2.2A/µs, while 1µH yields 1.4A/µs and 2.2µH yields
0.6A/µs. Figure 2b shows the trailing edge (turn-off) of the
Figure 2. V
SW
Node Current Behavior vs L
SENSE
Value.
L
SENSE
= 0µH, 0.47µH, 1.0µH and 2.2µH
100mA/DIV
200ns/DIV
1777 F02a
(a) Leading Edge
100mA/DIV
200ns/DIV
1777 F02b
(b) Trailing Edge