LT1111 LINER [Linear Technology], LT1111 Datasheet - Page 9

• Manufacturer Part Number: LT1111
• Description: Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
• Manufacturer: LINER [Linear Technology]

switch closes, current in the inductor builds according to
where R = 0.65 + DCR
As an example, suppose –5V at 50mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
Energy required from the inductor is:
Picking an inductor value of 56 H with 0.2 DCR results
in a peak switch current of:
Substituting I
Since 5.54 J > 3.82 J, the 56 H inductor will work.
With this relatively small input range, R
necessary and the I
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
A
I
PEAK
PPLICATI
P
I
E
f
L
L
P
OSC
L
=
L
V
t
(
L
2
0 65
1
4 5
= V
-5V +0.5V
.
.
275
V
R
56
V
72
IN
L
PEAK
– .
kHz
mW
– 0.75V
0 75
H
1
0 2
O
.
–
into Equation (04) results in:
LIM
V
U
0 445
e
)(
.
–
S
pin can be tied directly to V
3 8
50mA
R t
1
L
.
–
L
I FOR ATIO
A
e
U
J
– .
.
0 85
2
)
56
= 275mW
5 54
H
.
7
W
s
J
LIM
.
445
is not usually
mA
U
.
IN
. As in
(16)
( )
( )
( )
( )
15
17
19
18
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 100 F capacitors. The peak
switch current is 500mA in all cases. Figure 1 shows a
Sprague 501D, 25V aluminum capacitor. V
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 0.24 . 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 0.07 .
Figure 3 shows the circuit with a 16V OS-CON unit. ESR
is now only 0.02 .
Figure 1. Aluminum
Figure 2. Tantalum
Figure 3. OS-CON
5 s/DIV
5 s/DIV
5 s/DIV
LT1111 • F01
LT1111 • F02
LT1111 • F01
OUT
jumps by
LT1111
9