LT1111
APPLICATI
V
I
L
t
=
L
R
′
S I FOR ATIO
switch closes, current in the inductor builds according to
()
–R
′
t
1 – e
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:
275mW
P
L
=
=
3.8
µ
J.
72kHz
f
OSC
(17)
(16)
I
PEAK
=
(
4.5V – 0.75V
)
1 – e
–0.85
Ω ×
7
µ
s
=
445mA .
56
µ
H
(
0.65
Ω +
0.2
Ω
)
1
56
µ
H 0.445 A
2
=
5.54
µ
J.
2
(18)
Substituting I
PEAK
into Equation (04) results in:
E
L
=
50mV/DIV
Picking an inductor value of 56µH with 0.2Ω DCR results
in a peak switch current of:
(
)(
)
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 Equiva-
lent Series Resistance (ESR) and ESL (inductance). There
are low ESR aluminum capacitors on the market specifi-
cally designed for switch mode DC/DC converters which
work much better than general-purpose units. Tantalum
50mV/DIV
50mV/DIV
U
(15)
capacitors provide still better performance at more ex-
pense. 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
W
U
UO
Figure 1. Aluminum
(19)
5µs/DIV
LT1111 • F02
Figure 2. Tantalum
5µs/DIV
LT1111 • F01
Figure 3. OS-CON
9
LINER [ LINEAR TECHNOLOGY ]
