LT1111 LINER [Linear Technology], LT1111 Datasheet - Page 8

• Manufacturer Part Number: LT1111
• Description: Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
• Manufacturer: LINER [Linear Technology]

LT11 11
Picking an inductor value of 47 H with 0.2 DCR results
in a peak switch current of:
Substituting I
Since 9.1 J > 6.7 J, the 47 H 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,
consider using the LT1110. The 70% duty cycle of the
LT1110 allows more energy per cycle to be stored in the
inductor, resulting in more output power.
A resistor can be added in series with the I
switch current limit. The resistor should be picked so the
calculated I
Switch Current (from Typical Performance Characteristic
curves). Then, as V
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 ~650mA
in this mode. Higher current can be obtained by using an
external switch (see Figure 6). The I
successful operation over varying inputs.
After establishing output voltage, output current and input
voltage range, peak switch current can be calculated by the
formula:
where DC = duty cycle (0.50)
8
A
I
PEAK
PPLICATI
E
I
PEAK
L
V
V
SW
D
2
1
= diode drop (0.5V for a 1N5818)
1 0
4 5
PEAK
.
= switch drop in step-down mode
.
47
2
I
DC
PEAK
V
OUT
H
at minimum V
O
1
into Equation 04 results in:
–
U
0 623
IN
V
e
.
S
IN
– .
increases, switch current is held
V
1 0
OUT
–
47
I FOR ATIO
A
U
V
SW
2
H
IN
7
V
is equal to the Maximum
s
9 1
D
.
V
W
LIM
D
J
623
pin is the key to
LIM
mA
pin to invoke
.
U
( )
( )
( )
09
08
10
V
a function of V
be used for V
Once I
where t
Next, the current limit resistor R
I
of this resistor keeps maximum switch current constant as
the input voltage is increased.
As an example, suppose 5V at 300mA is to be generated
from a 12V to 24V input. Recalling Equation (10),
Next, inductor value is calculated using Equation (11):
Use the next lowest standard value (56 H).
Then pick R
= 56 .
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,
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 0.65 resistor. When the
PEAK
I
SW
PEAK
P
L
L
L
is actually a function of switch current which is in turn
from the R
=
PEAK
I
V
V
12 1 5 5
OUT
V
ON
(
OUT
IN
IN MIN
600
V
2 300
= minimum input voltage
= switch-on time (7 s).
– . –
is known, inductor value can be derived from:
OUT
= output current
LIM
= output voltage
0 50
SW
mA
IN
.
from the curve. For I
, L, time, and V
LIM
+ V
mA
I
as a very conservative value.
PEAK
V
Step-Down Mode curve. The addition
D
SW
7
)(
12 1 5 0 5
I
s
OUT
V
– .
5 0 5
64
)
OUT
OUT
H
.
.
LIM
. To simplify, 1.5V can
t
ON
.
PEAK
is selected to give
= 600mA, R
600
mA
(14)
( )
( )
( )
LIM
12
13
11