LT1107 Linear Technology, LT1107 Datasheet - Page 9

• Manufacturer Part Number: LT1107
• Description: Micropower DC/DC Converter Adjustable and Fixed 5V/ 12V
• Manufacturer: Linear Technology

A
The usual step-up configuration for the LT1107 is shown
in Figure 1. The LT1107 first pulls SW1 low causing V
V
At the end of the switch ON time the current in L1 is
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
inductor current flows through D1 into C1, increasing
V
keep V
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 LT1107 based
step-down converter is shown in Figure 2.
When the switch turns on, SW2 pulls up to V
puts a voltage across L1 equal to V
causing a current to build up in L1. At the end of the switch
ON time, the current in L1 is equal to:
Note 1: This simple expression neglects the effects of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
CESAT
OUT
PPLICATI
V
I
I
PEAK
PEAK
OUT
. This action is repeated as needed by the LT1107 to
V
FB
IN
to appear across L1. A current then builds up in L1.
at the internal reference voltage of 1.25V. R1 and
V
V
1
GND
I
L
LIM
IN
IN
R3
LT1107
Figure 1. Step-Up Mode Hookup
R
R
t
ON
O
2
1
V
SW
SW2
V
L
U
IN
SW1
1 25
FB
S
V
L1
V
OUT
I FOR ATIO
U
t
ON
D1
W
IN
R2
R1
– V
OUT
IN
+
SW
– V
1107 F01
+ V
C1
U
SW
V
– V
OUT
D
. This
, the
1
OUT
IN
:
21
22
20
–
,
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 LT1107. Output voltage is deter-
mined by:
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 100
grams the switch to turn off when the current reaches
approximately 700mA. When using the LT1107 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).
Inverting Configurations
The LT1107 can be configured as a positive-to-negative
converter (Figure 3), or a negative-to-positive converter
(Figure 4). In Figure 3, 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
IN
V
OUT
+
C2
I
LIM
1
R3
100
Figure 2. Step-Down Mode Hookup
LT1107
GND
R
R
V
IN
2
1
SW1
SW2
1 25
FB
V
D1
1N5818
L1
+
C1
resistor pro-
LT1107
R1
R2
1107 F02
V
9
23
OUT