LT11
11
APPLICATI
S I FOR ATIO
L1
D1
+
C1
I
LIM
V
IN
SW1
+
C2
A0
GND
–V
IN
LT1111
FB
SW2
R2
LT1111 • F08
V
OUT
= R1 1.25V + 0.6V
R2
( )
Figure 8. Negative-to-Positive Converter
Using the I
LIM
Pin
The LT1111 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 wh ere analysis shows the
LT1111 must operate at an 800mA peak switch current
with a 2V 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 “Maxi-
mum Switch Current vs I
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:
I
L
V
OUT
+
V
DIODE
V
IN
−
V
SW
<
1
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
9, the inductor current increases to a high level before the
comparator turns off the oscillator. This high current can
cause excessive output ripple and requires oversizing the
output capacitor and inductor. With the I
LIM
feature,
however, the switch current turns off at a programmed
level as shown in Figure 10, keeping output ripple to a
minimum.
12
U
V
OUT
R1
2N3906
I
L
SWITCH
ON
OFF
LT1111 • F09
W
U
UO
Figure 9. No Current Limit Causes Large Inductor
Current Build-Up
PROGRAMMED CURRENT LIMIT
SWITCH
ON
OFF
LT1111 • F10
Figure 10. Current Limit Keeps Inductor Current Under Control
(25)
Figure 11 details current limit circuitry. Sense transistor
Q1, whose base and emitter are paralleled with power
switch Q2, is ratioed such that approximately 0.5% of
Q2’s collector current flows in Q1’s collector. This current
is passed through internal 80Ω resistor R1 and out
through the I
LIM
pin. The value of the external resistor
connected between I
LIM
and V
IN
sets the current limit.
When sufficient switch current flows to develop a V
BE
across R1 + R
LIM
, Q3 turns on and injects current into the
oscillator, turning off the switch. Delay through this cir-
cuitry is approximately 1µs. The current trip point be-
comes less accurate for switch ON times less than 3µs.
Resistor values programming switch ON time for 1µs or
less will cause spurious response in the switch circuitry
although the device will still maintain output regulation.
R
LIM
(EXTERNAL)
V
IN
Q3
DRIVER
OSCILLATOR
Q1
I
LIM
R1
80Ω
(INTERNAL)
SW1
Q2
SW2
LT1111 • F11
Figure 11. LT1111 Current Limit Circuitry
LINER [ LINEAR TECHNOLOGY ]
