LT1108CS8-5 Linear Technology, LT1108CS8-5 Datasheet - Page 7

• Manufacturer Part Number: LT1108CS8-5
• Description: IC DC/DC CONV FIXED OUT 5V 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost), Invertingr

Specifications of LT1108CS8-5

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 400mA
• Frequency - Switching: 19kHz
• Voltage - Input: 2 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Power - Output: 500mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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 18.3 J > 16.6 J, the 100 H inductor will work. This
trial-and-error approach can be used to select the optimum
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
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.
Step-Down Converter
The step-down case (Figure 2) 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 3). The I
operation over varying inputs.
After establishing output voltage, output current and input
voltage range, peak switch current can be calculated by the
formula:
A
I
PEAK
PPLICATI
E
I
f
PEAK
P
OSC
L
L
2
1
1 0
315
PEAK
2
.
100
2
19
V
PEAK
I
DC
OUT
kHz
mW
at minimum V
O
H
1
into Equation 04 results in
–
U
IN
V
6 605
e
16 6
S
IN
.
– .
increases, switch current is held
V
1 0
.
OUT
–
100
I FOR ATIO
LIM
U
V
A
J
SW
IN
2
pin is the key to successful
H
36
V
is equal to the Maximum
D
s
18 3
V
W
.
D
605
J
LIM
mA
pin to invoke
U
( )
( )
( )
( )
07
08
09
10
where DC = duty cycle (0.60)
V
a function of V
be used for V
Once I
where t
Next, the current limit resistor R
from the R
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 (330 H).
Then pick R
R
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,
I
SW
LIM
PEAK
V
V
I
V
V
P
L
L
OUT
SW
D
OUT
IN
L
is actually a function of switch current which is in turn
= 220 .
=
= diode drop (0.5V for a 1N5818)
PEAK
= minimum input voltage
12 1 5 5
ON
= switch drop in step-down mode
= output current
V
(
= output voltage
IN MIN
500
V
2 300
LIM
= switch-ON time (36 s).
– . –
OUT
is known, inductor value can be derived from
LIM
SW
0 60
mA
Step-Down Mode curve. The addition of this
IN
.
from the curve. For I
, L, time, and V
+ V
as a very conservative value.
mA
I
PEAK
V
D
SW
36
)(
I
12 1 5 0 5
OUT
s
V
– .
5 0 5
)
OUT
396
LIM
OUT
.
is selected to give I
H
. To simplify, 1.5V can
t
PEAK
ON
.
= 500mA,
500
LT1108
mA
PEAK
(14)
( )
( )
7
( )
12
13
11