LT1110 Linear Technology, LT1110 Datasheet - Page 9

• Manufacturer Part Number: LT1110
• Description: Micropower DC-DC Converter Adjustable and Fixed 5V/ 12V
• Manufacturer: Linear Technology

switch closes, current in the inductor builds according to
where R' = 0.65 + DCR
As an example, suppose –5V at 75mA 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 10.8 J > 5.9 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 ~800mA.
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
PPLICATI
I
I
P
PEAK
E
f
L
P
OSC
L
L
L
V
L
2
1
|
= V
0 65
4 5
413
.
.
56
5
70
V
R
IN
V
PEAK
V
L
'
kHz
mW
– .
– 0.75V
|
H
O
0 75
1
0 2
into Equation (04) results in
0 5
LIM
–
.
U
0 621
.
V
e
.
S
V
pin can be tied directly to V
5 9
– '
R t
.
L
1
L
A
I FOR ATIO
75
–
U
J
e
.
2
mA
– .
0 85
56
10 8
H
.
413
10
W
s
J
LIM
mW
.
is not usually
621
.
mA
U
IN
.
. As in
( )
( )
( )
( )
( )
18
15
16
17
19
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 LT1110
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 240m . 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 70m .
Figure 3 shows the circuit with a 16V OS-CON unit. ESR is
now only 20m .
Figure 1. Aluminum
Figure 2. Tantalum
Figure 3. OS-CON
5 s/DIV
5 s/DIV
5 s/DIV
LT1110 • TA20
LT1110 • TA21
LT1110 • TA19
OUT
LT1110
jumps by
9