LT1107 Linear Technology, LT1107 Datasheet - Page 10

• Manufacturer Part Number: LT1107
• Description: Micropower DC/DC Converter Adjustable and Fixed 5V/ 12V
• Manufacturer: Linear Technology

LT1107
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.
A
Using the I
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
will rise to 1.6A, exceeding the maximum switch current
rating. With the proper resistor selected (see the “Maxi-
mum Switch Current vs R
10
–V
V
+V
PPLICATI
OUT
IN
IN
+
+
should be less than 6.2V. More negative output
C2
C2
Figure 3. Positive-to-Negative Converter
Figure 4. Negative-to-Positive Converter
LIM
I
LIM
I
GND
LIM
Pin
R3
O
R3
LT1107
LT1107
GND
V
IN
IN
U
SW2
rises to 4V, the peak switch current
V
S
IN
SW1
SW2
SW1
FB
FB
I FOR ATIO
LIM
L1
U
” characteristic), the switch
D1
1N5818
R2
L1
D1
W
V
OUT
+
+
=
C1
R1
R2
C1
1.25V + 0.6V
U
2N3906
R2
R1
R1
+V
–V
1107 F04
1107 F03
OUT
OUT
SWITCH
current 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
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
switch turns off at the programmed current as shown in
Figure 6, keeping output ripple to a minimum.
SWITCH
Figure 6. Current Limit Keeps Inductor Current Under Control
V
OUT
OFF
OFF
ON
V
ON
Figure 5. No Current Limit Causes Large Inductor
Current Build-Up
I
IN
L
I
L
PROGRAMMED CURRENT LIMIT
V
V
DIODE
SW
1
1
DC
LIM
feature is useful occurs
LIM
feature, the
1107 F06
1107 F05
24