LT1110CS8-5 Linear Technology, LT1110CS8-5 Datasheet - Page 8

• Manufacturer Part Number: LT1110CS8-5
• Description: IC DC/DC CONV FIXED OUT 5V 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost)r

Specifications of LT1110CS8-5

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 400mA
• Frequency - Switching: 70kHz
• Voltage - Input: 1 ~ 30 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Power - Output: 500mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Picking an inductor value of 47 H with 0.2 DCR results
in a peak switch current of
Substituting I
Since 17.5 J > 13.7 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,
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 such
that the calculated I
Maximum Switch Current (from Typical Performance
Characteristic curves). Then, as V
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
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 (0.69)
8
LT1110
A
I
PEAK
PPLICATI
E
I
PEAK
L
V
V
I
OUT
SW
D
2
1
= diode drop (0.5V for a 1N5818)
1 0
4 5
.
= switch drop in step-down mode
= output current
2
47
.
I
DC
PEAK
W
V
OUT
H
O
1
into Equation 05 results in
U
PEAK
0 862
V
e
.
S
IN
V
1 0
.
–
at minimum V
OUT
47
I FOR ATIO
A
W
U
V
m
SW
2
10
H
m
V
17 5
s
D
V
.
IN
W
LIM
D
862
increases, switch
J
IN
pin is the key to
.
LIM
is equal to the
mA
pin to invoke
.
U
( )
( )
( )
08
09
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 250mA is to be generated
from a 9V to 18V input. Recalling Equation (10),
Next, inductor value is calculated using Equation (11)
Use the next lowest standard value (47 H).
Then pick R
R
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
SW
LIM
L
I
L
P
PEAK
is actually a function of switch current which is in turn
L
= 82 .
from the R
PEAK
V
V
9 1 5 5
V
ON
OUT
IN
IN MIN
498
– . –
|
= minimum input voltage
= switch ON time (10 s).
V
is known, inductor value can be derived from
2 250
OUT
= output voltage
mA
LIM
SW
IN
0 69
–
LIM
.
, L, time and V
|
I
as a very conservative value.
PEAK
from the curve. For I
V
mA
Step-Down Mode curve. The addition
SW
V
10
D
–
9 1 5 0 5
s
I
V
OUT
– .
5 0 5
OUT
50
OUT
.
.
H
LIM
. To simplify, 1.5V can
t
.
ON
.
is selected to give
PEAK
498
mA
= 500mA,
. ( )
( )
( )
( )
12
13
14
11