Datasheet

LT1110
8
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Picking an inductor value of 47µH with 0.2Ω DCR results
in a peak switch current of
I
V
emA
PEAK
s
H
=−








=
−•
45
10
1 862 08
10 10
47
.
.
.()
.
W
Wm
m
Substituting I
PEAK
into Equation 05 results in
EHAJ
L
=
()( )
=
1
2
47 0 862 17 5 09
2
µµ...()
Since 17.5µJ > 13.7µJ, the 47µH inductor will work. This
trial-and-error approach can be used to select the opti-
mum inductor. Keep in mind the switch current maximum
rating of 1.5A. If the calculated peak current exceeds this,
an external power transistor can be used.
A resistor can be added in series with the I
LIM
pin to invoke
switch current limit. The resistor should be picked such
that the calculated I
PEAK
at minimum V
IN
is equal to the
Maximum Switch Current (from Typical Performance
Characteristic curves). Then, as V
IN
increases, switch
current is held constant, resulting in increasing efficiency.
Inductor Selection — Step-Down Converter
The step-down case (Figure 5) differs from the step-up in
that the inductor current flows through the load during
both the charge and discharge periods of the inductor.
Current through the switch should be limited to ~800mA
in this mode. Higher current can be obtained by using an
external switch (see Figure 6). The I
LIM
pin is the key to
successful operation over varying inputs.
After establishing output voltage, output current and input
voltage range, peak switch current can be calculated by the
formula
I
I
DC
VV
VV V
PEAK
OUT OUT D
IN SW D
=
+
+






2
10
–
()
where DC = duty cycle (0.69)
V
SW
= switch drop in step-down mode
V
D
= diode drop (0.5V for a 1N5818)
I
OUT
= output current
V
OUT
= output voltage
V
IN
= minimum input voltage
V
SW
is actually a function of switch current which is in turn
a function of V
IN
, L, time and V
OUT
. To simplify, 1.5V can
be used for V
SW
as a very conservative value.
Once I
PEAK
is known, inductor value can be derived from
L
VVV
I
t
IN MIN SW OUT
PEAK
ON
=•
––
()11
where t
ON
= switch ON time (10µs).
Next, the current limit resistor R
LIM
is selected to give
I
PEAK
from the R
LIM
Step-Down Mode curve. The addition
of this resistor keeps maximum switch current constant as
the input voltage is increased.
As an example, suppose 5V at 250mA is to be generated
from a 9V to 18V input. Recalling Equation (10),
I
mA
mA
PEAK
=
()
+
+






=
2 250
069
505
91505
498 12
.
.
–. .
.( )
Next, inductor value is calculated using Equation (11)
L
mA
sH=•=
9155
498
10 50 13
–.–
.()µµ
Use the next lowest standard value (47µH).
Then pick R
LIM
from the curve. For I
PEAK
= 500mA,
R
LIM
= 82Ω.
Inductor Selection — Positive-to-Negative Converter
Figure 7 shows hookup for positive-to-negative conver-
sion. All of the output power must come from the inductor.
In this case,
PV VI
L OUT D OUT
=+
()()
|| . ()14
In this mode the switch is arranged in common collector
or step-down mode. The switch drop can be modeled as
a 0.75V source in series with a 0.65Ω resistor. When the