LT1107 Linear Technology, LT1107 Datasheet - Page 8

• Manufacturer Part Number: LT1107
• Description: Micropower DC/DC Converter Adjustable and Fixed 5V/ 12V
• Manufacturer: Linear Technology

LT1107
A
where DC = duty cycle (0.50 in step-down mode)
V
a function of V
be used for V
Once I
where t
Next, the current limit resistor R
I
The addition of this resistor keeps maximum switch cur-
rent 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 4 shows hookup for positive-to-negative conver-
sion. All of the output power must come from the inductor.
In this case,
8
PEAK
SW
PPLICATI
I
L
L
P
PEAK
L
is actually a function of switch current which is in turn
from the Maximum Switch Current vs R
PEAK
V
V
I
V
V
V
12 1 5 5
OUT
ON
SW
D
OUT
IN
IN MIN
V
600
= diode drop (0.5V for a 1N5818)
= switch ON time (7 s).
OUT
= minimum input voltage
is known, inductor value can be derived from:
2 300
= switch drop in step-down mode
= output current
LIM
= output voltage
SW
mA
IN
0 50
from the curve. For I
I
, L, time, and V
O
as a very conservative value.
PEAK
V I
mA
D OUT
V
U
SW
7
S
s
12 1 5 0 5
V
I FOR ATIO
U
OUT
64
5 0 5
OUT
H
t
LIM
ON
. To simplify, 1.5V can
W
PEAK
is selected to give
= 600mA, R
600
mA
LIM
U
curve.
(14)
12
LIM
11
13
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
switch closes, current in the inductor builds according to:
where R = 0.65 + DCR
As an example, suppose –5V at 50mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
Energy required from the inductor is:
Picking an inductor value of 100 H with 0.2 DCR results
in a peak switch current of:
Substituting I
Since 5.28 J > 3.82 J, the 100 H inductor will work.
With this relatively small input range, R
necessary and the I
the step-down case, peak switch current should be limited
to ~650mA.
Step-Up (Boost Mode) Operation
A step-up DC/DC converter delivers an output voltage
higher than the input voltage. Step-up converters are not
short-circuit protected since there is a DC path from input
to output.
P
I
E
I
f
PEAK
OSC
L
L
P
L
L
t
V
L
2
1
= V
5
325
275
100
V
R
V
63
0 65
4 5
L
IN
PEAK
kHz
mA
mW
– 0.75V
V
0 5
1
H
into Equation (04) results in:
LIM
V
e
0 75
0 325
0 2
4 4
50
pin can be tied directly to V
R t
L
V
mA
L
A
J
1
2
= 275mW
e
5 28
0 85
100
J
LIM
H
9
is not usually
s
IN
. As in
(16 )
17
18
19
15