LT1107_15 Datasheet, PDF (10/16 Pages) Linear Technology – Micropower DC/DC Converter Adjustable and Fixed 5V, 12V

LT1107
APPLICATI S I FOR ATIO
V_OUTshould be less than 6.2V. More negative output
voltages can be accommodated as in the prior section.
In Figure 4, 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.
+V_{IN +}
C2
R3
I_LIM V_IN SW1
FB
LT1107
SW2
GND
L1
D1
+
1N5818
C1
Figure 3. Positive-to-Negative Converter
R1
R2
–V_OUT
1107 F03
L1
D1
+V_OUT
+
R3
C1
R1
+
C2
–V_IN
I_LIM
V_IN
SW1
LT1107
FB
GND
SW2
2N3906
( ) ^R2
V_OUT =
R1
R2
1.25V + 0.6V
1107 F04
Figure 4. Negative-to-Positive Converter
Using the I_LIM Pin
The LT1107 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
LT1107 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 selected (see the “Maxi-
mum 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}
(24)
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
5, 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, the
switch turns off at the programmed current as shown in
Figure 6, keeping output ripple to a minimum.
1107fa
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