LT11
11
APPLICATI
S I FOR ATIO
U
(15)
(16)
switch closes, current in the inductor builds according to
I
L
()
V
L
1 – e
t
=
R
′
–R
′
t
L
where R′ = 0.65Ω + DCR
L
V
L
= V
IN
– 0.75V
As an example, suppose –5V at 50mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
P
L
=
(
-5V+0.5V
)(
50mA
)
= 275mW
Energy required from the inductor is:
P
L
275mW
=
=
3.8
µ
J.
f
OSC
72kHz
Picking an inductor value of 56µH with 0.2Ω DCR results
in a peak switch current of:
(
4.5V – 0.75V
)
1 – e
–0.85
Ωµ×
7
µ
s
=
445mA .
56 H
I
PEAK
=
(
0.65
Ω +
0.2
Ω
)
(18)
Substituting I
PEAK
into Equation (4) results in:
E
L
=
1
56
µ
H 0.445A
2
50mV/DIV
(
)(
)
2
=
5.54
µ
J.
Since 5.54µJ > 3.82µJ, the 56µH inductor will work.
With this relatively small input range, R
LIM
is not usually
necessary and the I
LIM
pin can be tied directly to V
IN
. As in
the step-down case, peak switch current should be limited
to ~650mA.
Capacitor Selection
Selecting the right output capacitor is almost as important
as selecting the right inductor. A poor choice for a filter
capacitor can result in poor efficiency and/or high output
ripple. Ordinary aluminum electrolytics, while inexpensive
and readily available, may have unacceptably poor
equivalent series resistance (ESR) and ESL (inductance).
There are low ESR aluminum capacitors on the market
50mV/DIV
50mV/DIV
W
U
UO
(17)
specifically designed for switch mode DC/DC converters
which work much better than general-purpose units.
Tantalum capacitors provide still better performance at
more expense. We recommend OS-CON capacitors from
Sanyo Corporation (San Diego, CA). These units are
physically quite small and have extremely low ESR. To
illustrate, Figures 1, 2, and 3 show the output voltage of an
LT1111 based converter with three 100µF capacitors. The
peak switch current is 500mA in all cases. Figure 1 shows
a Sprague 501D, 25V aluminum capacitor. V
OUT
jumps by
over 120mV when the switch turns off, followed by a drop
in voltage as the inductor dumps into the capacitor. This
works out to be an ESR of over 0.24Ω. Figure 2 shows the
same circuit, but with a Sprague 150D, 20V tantalum
capacitor replacing the aluminum unit. Output jump is
now about 35mV, corresponding to an ESR of 0.07Ω.
Figure 3 shows the circuit with a 16V OS-CON unit. ESR
is now only 0.02Ω.
5µs/DIV
LT1111 • F01
Figure 1. Aluminum
(19)
5µs/DIV
LT1111 • F02
Figure 2. Tantalum
5µs/DIV
LT1111 • F01
Figure 3. OS-CON
1111fd
9
LINER [ LINEAR TECHNOLOGY ]
