LT1108 Datasheet, PDF (10/12 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1108
APPLICATI S I FOR ATIO
provided by the PNP transistor, supplies proper polarity
feedback information to the regulator.
V_IN
R3
I_LIM V_IN SW1
+
C2
FB
LT1108
SW2
GND
L1
D1 ⁺
1N5818
C1
R1
R2
– V_OUT
LT1108 • F04
Figure 4. Positive-to-Negative Converter
L1
I_LIM
V_IN
SW1
+
C2
LT1108
AO
GND
FB
SW2
– V_IN
D1
+
C1
V_OUT
R1
2N3906
R2
( ) ^VOUT ⁼
R1
R2
1.245V + 0.6V
LT1108 • F05
Figure 5. Negative-to-Positive Converter
USING THE I_LIM PIN
The LT1108 switch can be programmed to turn off at a set
switch current, a feature not found on competing devices.
This enables the input to vary over a wide range without
exceeding the maximum switch rating or saturating the
inductor. Consider the case where analysis shows the
LT1108 must operate at an 800mA peak switch current with
a 2.0V input. If V_IN rises to 4V, the peak switch current will
rise to 1.6A, exceeding the maximum switch current rating.
With the proper resistor selected (see the “Maximum
Switch Current vs R_LIM” characteristic), the switch current
will be limited to 800mA, even if the input voltage increases.
Another situation where the I_LIM feature is useful occurs
when the device goes into continuous mode operation. This
occurs in step-up mode when
V_OUT + _{VDIODE < 1 .}
(25)
V_IN − V_SW 1 − DC
When the input and output voltages satisfy this relation-
ship, inductor current does not go to zero during the switch-
OFF time. When the switch turns on again, the current ramp
starts from the non-zero current level in the inductor just
prior to switch turn-on. As shown in Figure 6, the inductor
current increases to a high level before the comparator
turns off the oscillator. This high current can cause exces-
sive output ripple and requires oversizing the output ca-
pacitor and inductor. With the I_LIM feature, however, the
switch current turns off at a programmed level as shown in
Figure 7, keeping output ripple to a minimum.
I_L
ON
SWITCH
OFF
LT1108 • F06
Figure 6. No Current Limit Causes Large Inductor
Current Build-Up
PROGRAMMED CURRENT LIMIT
I_L
ON
SWITCH
OFF
LT1108 • F07
Figure 7. Current Limit Keeps Inductor Current Under Control
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