LT1110 Linear Technology, LT1110 Datasheet - Page 12

• Manufacturer Part Number: LT1110
• Description: Micropower DC-DC Converter Adjustable and Fixed 5V/ 12V
• Manufacturer: Linear Technology

Using the I
The LT1110 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
LT1110 must operate at an 800mA peak switch current
with a 2.0V input. If V
to 1.6A, exceeding the maximum switch current rating.
With the proper resistor selected (see the “Maximum
Switch Current vs R
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.
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
switch current flows to develop a V
turns on and injects current into the oscillator, turning off
the switch. Delay through this circuitry is approximately
800ns. The current trip point becomes less accurate for
12
LT1110
A
PPLICATI
V
LIM
OUT
V
I N
pin. The value of the external resistor connected
LIM
LIM
V
V
DIODE
and V
SW
Pin
O
IN
LIM
U
set the current limit. When sufficient
IN
S
1
” characteristic), the switch current
rises to 4V, peak current will rise
1
I FOR ATIO
DC
U
LIM
.
feature is useful occurs
BE
across R1 + R
W
LIM
U
feature,
LIM
( )
, Q3
25
switch ON times less than 3 s. Resistor values program-
ming switch ON time for 800ns or less will cause spurious
response in the switch circuitry although the device will
still maintain output regulation.
Using the Gain Block
The gain block (GB) on the LT1110 can be used as an error
amplifier, low battery detector or linear post regulator. The
gain block itself is a very simple PNP input op amp with an
open collector NPN output. The negative input of the gain
block is tied internally to the 220mV reference. The posi-
tive input comes out on the SET pin.
SWITCH
SWITCH
Figure 10. Current Limit Keeps Inductor Current Under Control
OFF
OFF
ON
ON
Figure 9. No Current Limit Causes Large Inductor
Current Build-Up
I
I
L
L
Figure 11. LT1110 Current Limit Circuitry
PROGRAMMED CURRENT LIMIT
V
OSCILLATOR
IN
Q3
(EXTERNAL)
DRIVER
R
LIM
I
LIM
Q1
R1
80
(INTERNAL)
SW1
LT1110 • TA17
Q2
SW2
LT1110 • TA05
LT1110 • TA06