LT1108
PPLICATI
provided by the PNP transistor, supplies proper polarity
feedback information to the regulator.
O U
W
U
A
S I FOR ATIO
Another situation where the ILIM feature is useful occurs
whenthedevicegoesintocontinuousmodeoperation.This
occurs in step-up mode when
V
IN
R3
V
+
V
1
OUT
DIODE
<
.
(25)
I
V
IN
SW1
FB
LIM
V − V
1− DC
IN
SW
+
C2
LT1108
GND
When the input and output voltages satisfy this relation-
ship,inductorcurrentdoesnotgotozeroduringtheswitch-
OFFtime. Whentheswitchturnsonagain, thecurrentramp
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 ILIM feature, however, the
switch current turns off at a programmed level as shown in
Figure 7, keeping output ripple to a minimum.
L1
SW2
R1
R2
+
D1
1N5818
C1
–V
OUT
LT1108 • F04
Figure 4. Positive-to-Negative Converter
D1
L1
V
OUT
+
R1
C1
I
V
2N3906
LIM
IN
SW1
+
LT1108
C2
AO
GND
FB
SW2
I
L
R2
R1
R2
V
=
1.245V + 0.6V
OUT
(
)
LT1108 • F05
–V
IN
ON
SWITCH
Figure 5. Negative-to-Positive Converter
OFF
LT1108 • F06
USING THE ILIM PIN
Figure 6. No Current Limit Causes Large Inductor
Current Build-Up
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
LT1108mustoperateatan800mApeakswitchcurrentwith
a 2.0V input. If VIN rises to 4V, the peak switch current will
riseto1.6A, exceedingthemaximumswitchcurrentrating.
With the proper resistor selected (see the “Maximum
SwitchCurrent vs RLIM” characteristic), the switch current
willbelimitedto800mA,eveniftheinputvoltageincreases.
PROGRAMMED CURRENT LIMIT
I
L
ON
SWITCH
OFF
LT1108 • F07
Figure 7. Current Limit Keeps Inductor Current Under Control
10
Linear [ Linear ]
