LT1110_15 Datasheet, PDF (12/16 Pages) Linear Technology – Micropower DC-DC Converter Adjustable and Fixed 5V, 12V

LT1110
APPLICATI S I FOR ATIO
Using the I_LIM Pin
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_IN rises to 4V, peak current will rise
to 1.6A, exceeding the maximum switch current rating.
With the proper resistor selected (see the “Maximum
Switch Current vs R_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
V_OUT + _{VDIODE < 1 .}
(25)
V_IN − V_SW 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.
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 set 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 circuitry is approximately
800ns. The current trip point becomes less accurate for
switch ON times less than 3µs. 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.
I_L
ON
SWITCH
OFF
LT1110 • TA05
Figure 9. No Current Limit Causes Large Inductor
Current Build-Up
I_L ^{PROGRAMMED CURRENT LIMIT}
ON
SWITCH
OFF
LT1110 • TA06
Figure 10. Current Limit Keeps Inductor Current Under Control
R_LIM
(EXTERNAL)
V_IN
Q3
DRIVER
OSCILLATOR
I_LIM
R1
80Ω
(INTERNAL)
SW1
Q1
Q2
SW2
LT1110 • TA17
Figure 11. LT1110 Current Limit Circuitry
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.
12