LT1108 LINER [Linear Technology], LT1108 Datasheet - Page 9

• Manufacturer Part Number: LT1108
• Description: Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
• Manufacturer: LINER [Linear Technology]

When the switch turns off, the SW2 pin falls rapidly and
actually goes below ground. D1 turns on when SW2
reaches 0.4V below ground. D1 MUST BE A SCHOTTKY
DIODE . The voltage at SW2 must never be allowed to go
below –0.5V. A silicon diode such as the 1N4933 will allow
SW2 to go to –0.8V, causing potentially destructive power
dissipation inside the LT1108. Output voltage is deter-
mined by
R3 programs switch current limit. This is especially impor-
tant in applications where the input varies over a wide
range. Without R3, the switch stays on for a fixed time each
cycle. Under certain conditions the current in L1 can build
up to excessive levels, exceeding the switch rating and/or
saturating the inductor. The 100 resistor programs the
switch to turn off when the current reaches approximately
700mA. When using the LT1108 in step-down mode,
output voltage should be limited to 6.2V or less. Higher
output voltages can be accommodated by inserting a
1N5818 diode in series with the SW2 pin (anode connected
to SW2).
A
V
IN
PPLICATI
I
V
PEAK
OUT
+
C2
V
I
LIM
1
IN
Figure 2. Step-Down Mode Hookup
R3
100
LT1108
R
R
O
GND
V
V
2
IN
1
SW
U
L
SW2
SW1
S
1 245
FB
.
V
I FOR ATIO
OUT
U
V
D1
1N5818
t
ON
L1
W
+
C1
U
R2
R1
LT1108 • F02
V
OUT
( )
( )
22
23
HIGHER CURRENT STEP-DOWN OPERATION
Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 3. R1 serves as a
current limit sense. When the voltage drop across R1
equals 0.5V
current is limited to 2A. Inductor value can be calculated
based on formulas in the Inductor Selection Step-Down
Converter section with the following conservative expres-
sion for V
R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage. A PMOS FET can
be used in place of Q1 when V
INVERTING CONFIGURATIONS
The LT1108 can be configured as a positive-to-negative
converter (Figure 4), or a negative-to-positive converter
(Figure 5). In Figure 4, the arrangement is very similar to a
step-down, except that the high side of the feedback is
referred to ground. This level shifts the output negative. As
in the step-down mode, D1 must be a Schottky diode,
and V
voltages can be accommodated as in the prior section.
In Figure 5, the input is negative while the output is positive.
In this configuration, the magnitude of the input voltage can
be higher or lower than the output voltage. A level shift,
MAX
30V
V
IN
V
SW
OUT
+
Figure 3. Q1 Permits Higher Current Switching
The LT1108 Functions as Controller
C2
SW
V
should be less than 6.2V. More negative output
BE
:
R
1
, the switch turns off. As shown, switch
GND
V
IN
LT1108
0.15
V
R1
Q SAT
1
SW2
I
L
SW1
FB
R2
100
100
330
1 0
ZETEX ZTX749
R6
R3
.
IN
V
Q1
R5
is between 10V and 20V.
R4
D1
1N5821
V
OUT
L1
= 1.245V 1 +
LT1108
LT1108 • F03
+
C1
V
OUT
R4
R5
( )
9
24