Datasheet

LT1110
11
Converter” section with the following conservative ex-
pression for V
SW
:
VVV V
SW R SAT
=+ ≈
1
09 24.. ()
R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage.
Inverting Configurations
The LT1110 can be configured as a positive-to-negative
converter (Figure 7), or a negative-to-positive converter
(Figure 8). In Figure 7, 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
OUT
should be less than 6.2V. More negative out-
put voltages can be accommodated as in the prior section.
LT1110 • TA03
–V
OUT
+
C2
+
C1
D1
1N5818
+V
IN
R1
R2
L1
GND
SW2
SW1
LIM
I
IN
V
R3
FB
LT1110
Figure 7. Positive-to-Negative Converter
In Figure 8, 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, provided by the PNP transistor, supplies proper
polarity feedback information to the regulator.
dissipation inside the LT1110. Output voltage is deter-
mined by
V
R
R
mV
OUT
=+






()
1
2
1
220 23.()
R3 programs switch current limit. This is especially im-
portant 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 220Ω resistor pro-
grams the switch to turn off when the current reaches
approximately 800mA. When using the LT1110 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).
Higher Current Step-Down Operation
Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 6. R1 serves as a
current limit sense. When the voltage drop across R1
equals a V
BE
, the switch turns off. For temperature com-
pensation a Schottky diode can be inserted in series with
the I
LIM
pin. This also lowers the maximum drop across R1
to V
BE
– V
D
, increasing efficiency. As shown, switch
current is limited to 2A. Inductor value can be calculated
based on formulas in the “Inductor Selection Step-Down
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Figure 6. Q1 Permits Higher-Current Switching.
LT1110 Functions as Controller.
LT1110 • TA16
D1
1N5821
+
+
V
OUT
V
IN
25V
MAX
L1
R1
0.3Ω
R2
220
Q1
MJE210 OR
ZETEX ZTX789A
R3
330
R4
R5
C1
V
OUT
= 220mV ( 1 + )

R4
R5
LT1110
GND
SW2
SW1
V
IN
I
L
FB
C2
Figure 8. Negative-to-Positive Converter
L1
LT1110 • TA04
GND SW2
FB
SW1
LIM
I
IN
V
D1
AO
+V
OUT
R2
V
OUT
= 220mV + 0.6V
R1
R2
( )
R1
2N3906
–V
IN
+
C1
LT1110
+
C2