Datasheet

8
LT1108
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
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
It
V
R
e
L
L
Rt
L
()
=
'
–()
–'
115
where: R' = 0.65 + DCR
L
V
L
= V
IN
– 0.75V
As an example, suppose –5V at 100mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
P
L
= (–5V+ 0.5V)(100mA) = 550mW. (16)
Energy required from the inductor is
P
f
mW
kHz
J
L
OSC
==
550
19
28 9 17.()µ
Picking an inductor value of 220µH with 0.3 DCR results
in a peak switch current of
I
VV
e
mA
PEAK
s
H
=
()
+
()
=
×
45 075
65 0 3
1
568
095 36
220
.–.
..
–.
0 ΩΩ
Ωµ
µ
Substituting I
PEAK
into Equation (04) results in
EHAJ
L
=
()( )
=
1
2
220 0 568 35 5 19
2
µµ.. ()
Since 35.5µJ > 28.9µJ, the 220µH inductor will work.
Finally, R
LIM
should be selected by looking at the Switch
Current vs R
LIM
curve. In this example, R
LIM
= 150.
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.
The usual step-up configuration for the LT1108 is shown in
Figure 1. The LT1108 first pulls SW1 low causing V
IN
V
CESAT
to appear across L1. A current then builds up in L1.
At the end of the switch-ON time the current in L1 is
(18)
*Expression 20 neglects the effect of switch and coil resistance. This is taken into account in the
"Inductor Selection" section.
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
OUT
+ V
D
, the
inductor current flows through D1 into C1, increasing V
OUT
.
This action is repeated as needed by the LT1108 to keep V
FB
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
IN
– V
SW
. This
puts a voltage across L1 equal to V
IN
– V
SW
– V
OUT
, causing
a current to build up in L1. At the end of the switch- ON time,
the current in L1 is equal to
Figure 1. Step-Up Mode Hookup
L1
LT1108 • F01
GND SW2
SW1
LIM
I
IN
V
D1
R3
LT1108
+
V
OUT
R2
R1
C1
V
IN
FB