Datasheet

10
LT1108
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
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
V
VV DC
OUT DIODE
IN SW
+
−
<
−
1
1
25.()
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.
provided by the PNP transistor, supplies proper polarity
feedback information to the regulator.
Figure 4. Positive-to-Negative Converter
Figure 5. Negative-to-Positive Converter
LT1108 • F04
+
C2
+
C1
D1
1N5818
V
IN
R1
R2
L1
GND
SW2
SW1
R3
FB
LT1108
I
LIM
V
IN
–V
OUT
L1
LT1108 • F05
GND SW2
FB
SW1
D1
AO
R2
V
OUT
= 1.245V + 0.6V
R1
R2
R1
2N3906
+
C1
LT1108
+
C2
I
LIM
V
IN
()
V
OUT
–V
IN
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.
LT1108 • F06
OFF
ON
SWITCH
I
L
LT1108 • F07
ON
OFF
SWITCH
PROGRAMMED CURRENT LIMIT
I
L
Figure 7. Current Limit Keeps Inductor Current Under Control
Figure 6. No Current Limit Causes Large Inductor
Current Build-Up