Datasheet

LT1110
9
switch closes, current in the inductor builds according to
I
V
R
e
L
L
Rt
L
+
()
=






'
–()
–'
115
where R' = 0.65Ω + DCR
L
V
L
= V
IN
– 0.75V
As an example, suppose –5V at 75mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
PVVmAmW
L
=− +
()()
=||..()5 0 5 75 413 16
Energy required from the inductor is
P
f
mW
kHz
J
L
OSC
==
413
70
59 17.. ()µ
Picking an inductor value of 56µH with 0.2Ω DCR results
in a peak switch current of
I
VV
emA
PEAK
s
H
=
()
+
()






=
•
45 075
065 02
1 621 18
085 10
56
.–.
..
–.()
–.
ΩΩ
Ωµ
µ
Substituting I
PEAK
into Equation (04) results in
EHAJ
L
=
()( )
=
1
2
56 0 621 10 8 19
2
µµ...()
Since 10.8µJ > 5.9µJ, 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 ~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
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 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 240mΩ. 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 70mΩ.
Figure 3 shows the circuit with a 16V OS-CON unit. ESR is
now only 20mΩ.
5 s/DIV
50mV/DIV
LT1110 • TA19
µ
Figure 1. Aluminum
5 s/DIV
50mV/DIV
LT1110 • TA20
µ
Figure 2. Tantalum
5 s/DIV
50mV/DIV
LT1110 • TA21
µ
Figure 3. OS-CON
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO