LT1073
10
APPLICATIONS INFORMATION
Using the I
LIM
Pin
The LT1073 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
LT1073 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 (see the “Maximum Switch
Current vs R
LIM
” characteristic) selected, the switch cur-
rent will be limited to 800mA, even if the input voltage
increases. The LT1073 does this by sampling a small
fraction of the switch current and passing this current
through the external resistor. When the voltage on the I
LIM
pin drops a V
BE
below V
IN
, the oscillator terminates the
cycle. Propagation delay through this loop is about 2µs.
Another situation where the I
LIM
feature is useful is when
the device goes into continuous mode operation. This
occurs in step-up mode when
OUT
DIODE
V
– V
<
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 nonzero current level in
the inductor just prior to switch turn-on. As shown in
Figure 10. No Current Limit Causes
Large Inductor Current Build-Up
Figure 11. Current Limit Keeps Inductor Current Under Control
Figure 10, 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 11, keeping output
ripple to a minimum.
Using the Gain Block
The gain block (GB) on the LT1073 can be used as an er
-
ror amplifier, low-battery detector or linear post-regulator.
The gain block itself is a ver
y simple PNP input op amp
with an open-collector NPN output. The (
–) input of the
gain block is tied internally to the 212mV reference. The
(+) input comes out on the SET pin.
Arrangement of the gain block as a low battery detector
is straightforward. Figure 12 shows hookup. R1 and R2
need only be low enough in value so that the bias current
of the SET input does not cause large errors. 100kΩ for
R2 is adequate.
Output ripple of the LT1073, normally 150mV at 5V
OUT
,
can be reduced significantly by placing the gain block in
front of the FB input as shown in Figure 13. This effectively
reduces the comparator hysteresis by the gain of the gain
block. Output ripple can be reduced to just a few millivolts
using this technique. Ripple reduction works with step-
down or inverting modes as well.
ON
OFF
I
L
PROGRAMMED CURRENT LIMIT
1073 F11
ON
OFF
I
L
