Datasheet

LT1073
10
In Figure 9, the input is negative while the output is
positive. In this configuration, the magnitude of the input
voltage can be higher or lower than the output voltage. A
level shift, provided by the PNP transistor, supplies proper
polarity feedback information to the regulator.
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
APPLICATIO S I FOR ATIO
WUUU
Figure 10. No Current Limit Causes
Large Inductor Current Build-Up
Figure 11. Current Limit Keeps Inductor Current Under Control
VV
VV DC
OUT DIODE
IN SW
+
<
––
1
1
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, 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 pro-
grammed 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 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 (–) 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.
1073 F10
ON
OFF
SWITCH
I
L
PROGRAMMED CURRENT LIMIT
1073 F11
ON
OFF
SWITCH
I
L