LT1107 Datasheet, PDF (9/12 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1107
APPLICATI S I FOR ATIO
The usual step-up configuration for the LT1107 is shown
in Figure 1. The LT1107 first pulls SW1 low causing V_IN –
V_CESAT to appear across L1. A current then builds up in L1.
At the end of the switch ON time the current in L1 is¹:
I_PEAK
=
V_{I N}
L
t_ON
L1
V_IN
R3
I_LIM
V_IN
SW1
LT1107
FB
GND
SW2
(20)
D1
V_OUT
R2
+
C1
R1
1107 F01
Figure 1. Step-Up Mode Hookup
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_OUT + V_D, the
inductor current flows through D1 into C1, increasing
V_OUT. This action is repeated as needed by the LT1107 to
keep V_FB at the internal reference voltage of 1.25V. R1 and
R2 set the output voltage according to the formula:
( ) ^VOUT
=

 ^{1 +}
R2^
R1
1.25V
(21)
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_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_{I N −}
V_SW
L
−
V_OUT
t_ON
(22)
Note 1: This simple expression neglects the effects of switch and coil
resistance. This is taken into account in the “Inductor Selection” section.
V_{IN +}
R3
C2 _100Ω
I_LIM V_IN SW1
FB
LT1107
SW2
GND
L1
D1
+
1N5818
C1
V_OUT
R2
R1
1107 F02
Figure 2. Step-Down Mode Hookup
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:
( ) ^VOUT
=

 ^{1 +}
R2^
R1
1.25V
(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 100Ω resistor pro-
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
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