LT1111 Datasheet, PDF (9/16 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1111
APPLICATI S I FOR ATIO
switch closes, current in the inductor builds according to
( ) ^I_L
t
=
V_L
R′

^{1 –}
e
–R′t
L


(15)
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
(16)
Energy required from the inductor is:
P_{L =} 275mW _{= 3.8µJ.}
(17)
f_OSC 72kHz
Picking an inductor value of 56µH with 0.2Ω DCR results
in a peak switch current of:
( ) ^{4.5V – 0.75V}  ^{–0.85Ω × 7µs} 
( ) ^{IPEAK = 0.65Ω + 0.2Ω} ^{1 – e 56µH}  ^{= 445mA.}
(18)
Substituting I_PEAK into Equation (04) results in:
( ) ( ) ^E_L
=
1
2
56µH
0.445A ² = 5.54µJ.
(19)
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
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
Figure 1. Aluminum
LT1111 • F01
5µs/DIV
Figure 2. Tantalum
LT1111 • F02
5µs/DIV
Figure 3. OS-CON
LT1111 • F01
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