LT1073_15 Datasheet, PDF (9/16 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1073
APPLICATIO S I FOR ATIO
( ) ^VOUT
=

¹⁺
R2^
R1
212mV
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
V_IN
R3
220Ω
+
C2
I_LIM V_IN SW1
FB
LT1073
SW2
GND
D1
1N5818
L1
+
C1
V_OUT
R2
R1
1073 FO7
Figure 7. Step-Down Mode Hookup
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_PEAK
=
V_IN
–
V_SW
L
–
V_OUT
t_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
( ) ^VOUT
=

¹⁺
R2^
R1
212mV
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.
+V_{IN +}
C2 R3
I_LIM V_IN SW1
FB
LT1073
L1
SW2
GND
R1
D1
1N5818
+
C1 R2
–V_OUT
1073 FO8
Figure 8. Positive-to-Negative Converter
+
C2
L1
I_LIM
V_IN
SW1
LT1073
AO
FB
GND SW2
D1
+
C1
+V_OUT
R1
2N3906
R2
–V_IN
( ) V_OUT =
R1
R2
212mV + 0.6V
1073 F09
Figure 9. Negative-to-Positive Converter
9