LT1111CS85PBF LINEAR TECH, LT1111CS85PBF Datasheet - Page 12

• Manufacturer Part Number: LT1111CS85PBF
• Manufacturer: LINEAR TECH

LT1111
Using the I
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
LT1111 must operate at an 800mA peak switch current
with a 2V input. If V
will rise to 1.6A, exceeding the maximum switch current
rating. With the proper resistor selected (see the “Maxi-
mum Switch Current vs I
current will be limited to 800mA, even if the input voltage
increases.
Another situation where the I
when the device goes into continuous mode operation.
This occurs in step-up mode when:
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
however, the switch current turns off at a programmed
level as shown in Figure 10, keeping output ripple to a
minimum.
A
12
–V
PPLICATI
IN
V
OUT
+
V
IN
C2
Figure 8. Negative-to-Positive Converter
LIM
V
V
A0
DIODE
SW
GND
I
LIM
Pin
LT1111
O
IN
U
SW2
V
IN
rises to 4V, the peak switch current
1
SW1
S
FB
1
DC
LIM
I FOR ATIO
L1
U
” characteristic), the switch
LIM
R2
feature is useful occurs
ere analysis shows the
W
D1
V
OUT
+
=
( )
LT1111 • F08
R1
R2
C1
LIM
1.25V + 0.6V
U
2N3906
R1
feature,
V
OUT
( )
25
SWITCH
SWITCH
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 80 resistor R1 and out through
the I
between I
cient switch current flows to develop a V
R
turning off the switch. Delay through this circuitry is
approximately 1 s. The current trip point becomes less
accurate for switch ON times less than 3 s. Resistor
values programming switch ON time for 1 s or less will
cause spurious response in the switch circuitry although
the device will still maintain output regulation.
Figure 10. Current Limit Keeps Inductor Current Under Control
LIM
LIM
, Q3 turns on and injects current into the oscillator,
OFF
OFF
ON
ON
I
I
L
L
Figure 9. No Current Limit Causes Large Inductor
Current Build-Up
pin. The value of the external resistor connected
LIM
PROGRAMMED CURRENT LIMIT
Figure 11. LT1111 Current Limit Circuitry
V
OSCILLATOR
and V
IN
Q3
IN
sets the current limit. When suffi-
(EXTERNAL)
DRIVER
R
LIM
I
LIM
Q1
R1
80
(INTERNAL)
SW1
BE
Q2
LT1111 • F11
across R1 +
SW2
LT1111 • F10
LT1111 • F09
1111fd