LT1108CS8 Linear Technology, LT1108CS8 Datasheet - Page 8

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

Specifications of LT1108CS8

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adjustable
• 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

LT1108
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 100mA 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 220 H with 0.3 DCR results
in a peak switch current of
Substituting I
Since 35.5 J > 28.9 J, the 220 H inductor will work.
Finally, R
Current vs R
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.
8
A
I
PEAK
PPLICATI
P
I
E
f
L
L
P
OSC
L
=
L
t
V
(
L
568
2
1
LIM
– 5V + 0.5V
0
= V
4 5
.
550
V
.
65
220
R
19
LIM
mA
should be selected by looking at the Switch
PEAK
L
'
IN
V
kHz
mW
– .
– 0.75V
curve. In this example, R
O
1
H
0 75
into Equation (04) results in
–
0 3
U
e
.
0 568
)(
S
– '
V
28 9
.
R t
L
100mA
L
.
I FOR ATIO
U
1
A
J
–
2
e
)
– .
= 550mW.
0 95
35 5
220
W
.
LIM
J
H
36
= 150 .
s
U
(16)
(18)
( )
( )
( )
15
17
19
The usual step-up configuration for the LT1108 is shown in
Figure 1. The LT1108 first pulls SW1 low causing V
V
At the end of the switch-ON time the current in L1 is
Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches V
inductor current flows through D1 into C1, increasing V
This action is repeated as needed by the LT1108 to keep V
at the internal reference voltage of 1.245V. R1 and R2 set
the output voltage according to the formula
STEP-DOWN (BUCK MODE) OPERATION
A step-down DC/DC converter converts a higher voltage to
a lower voltage. The usual hookup for an LT1108 based
step-down converter is shown in Figure 2.
When the switch turns on, SW2 pulls up to V
puts a voltage across L1 equal to V
a current to build up in L1. At the end of the switch- ON time,
the current in L1 is equal to
*Expression 20 neglects the effect of switch and coil resistance. This is taken into account in the
"Inductor Selection" section.
V
CESAT
IN
I
V
PEAK
OUT
to appear across L1. A current then builds up in L1.
V
GND
I
L
1
LIM
IN
R3
Figure 1. Step-Up Mode Hookup
LT1108
t
R
R
ON
2
1
V
SW2
IN
*
SW1
FB
1 245
.
L1
V
IN
D1
– V
SW
R2
R1
– V
IN
OUT
OUT
– V
+
+ V
, causing
LT1108 • F01
SW
C1
D
. This
V
, the
OUT
IN
( )
OUT
( )
20
21
FB
–
.