LT1111_03 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
( ) ^IL
t
=
V_L

_1–
e
–R′t 
L
R′ _

(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:
specifically designed for switch mode DC/DC converters
which work much better than general-purpose units.
Tantalum capacitors provide still better performance at
more expense. 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.
4.5V – 0.75V ^{ –0.85Ω × 7µs }
(( ^{)) IPEAK =}
0.65Ω + 0.2Ω
_{1– e}

56µH



=
445mA.
(18)
Substituting I_PEAK into Equation (4) results in:
( ) ( ) ^EL
=
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
equivalent series resistance (ESR) and ESL (inductance).
There are low ESR aluminum capacitors on the market
5µs/DIV
Figure 1. Aluminum
LT1111 • F01
5µs/DIV
Figure 2. Tantalum
LT1111 • F02
5µs/DIV
Figure 3. OS-CON
LT1111 • F01
1111fd
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