LT1111 LINER [Linear Technology], LT1111 Datasheet - Page 12

• Manufacturer Part Number: LT1111
• Description: Micropower DC/DC Converter Adjustable and Fixed 5V, 12V
• Manufacturer: LINER [Linear Technology]

LT11 11
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.
12
A
–V
PPLICATI
IN
V
+
OUT
V
IN
C2
Figure 8. Negative-to-Positive Converter
LIM
V
A0
V
DIODE
GND
SW
I
LIM
Pin
LT1111
O
IN
U
SW2
V
IN
rises to 4V, the peak switch current
SW1
S
1
FB
1
LIM
I FOR ATIO
L1
DC
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
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
through the I
connected between I
When sufficient switch current flows to develop a V
across R1 + R
oscillator, turning off the switch. Delay through this cir-
cuitry is approximately 1 s. The current trip point be-
comes 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.
SWITCH
Figure 10. Current Limit Keeps Inductor Current Under Control
OFF
OFF
ON
ON
I
I
Figure 9. No Current Limit Causes Large Inductor
Current Build-Up
L
L
PROGRAMMED CURRENT LIMIT
Figure 11. LT1111 Current Limit Circuitry
V
OSCILLATOR
LIM
IN
LIM
Q3
, Q3 turns on and injects current into the
pin. The value of the external resistor
LIM
(EXTERNAL)
DRIVER
R
and V
LIM
IN
I
LIM
Q1
sets the current limit.
R1
80
(INTERNAL)
resistor R1 and out
SW1
Q2
LT1111 • F11
SW2
LT1111 • F10
LT1111 • F09
BE