Datasheet

LT1073
9
APPLICATIO S I FOR ATIO
WUUU
V
R
R
mV
OUT
=+






()
1
2
1
212
Step-Down (Buck Mode) Operation
A step-down DC/DC converter converts a higher voltage to
a lower voltage. It is short-circuit protected because the
switch is in series with the output. Step-down converters
are characterized by low output voltage ripple but high
input current ripple. The usual hookup for an LT1073-
based step-down converter is shown in Figure 7.
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
i
VV V
L
t
PEAK
IN SW OUT
ON
=
––
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 LT1073. Output voltage is deter-
mined by
V
R
R
mV
OUT
=+






()
1
2
1
212
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 400mA. When using the LT1073 in step-
down mode, output voltage should be limited to 6.2V or
less.
Inverting Configurations
The LT1073 can be configured as a positive-to-negative
converter (Figure 8), or a negative-to-positive converter
(Figure 9). In Figure 8, 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.
Figure 7. Step-Down Mode Hookup
V
IN
+
R3
220Ω
L1
D1
1N5818
V
OUT
C1
+
C2
R2
R1
1073 FO7
LT1073
I
LIM
V
IN
SW1
SW2
FB
GND
Figure 8. Positive-to-Negative Converter
Figure 9. Negative-to-Positive Converter
+V
IN
+
R3
L1
D1
1N5818
–V
OUT
C1
+
C2
R1
R2
1073 FO8
LT1073
I
LIM
V
IN
SW1
SW2
FB
GND
–V
IN
+
L1
D1
+V
OUT
V
OUT
= ( )212mV + 0.6V
R1
C1
+
C2
R2
1073 F09
LT1073
I
LIM
V
IN
SW1
FBAO
SW2GND
2N3906
R1
R2