參數(shù)資料
型號: LT1107IS8
英文描述: DC to DC Converter
中文描述: DC至DC轉換器
文件頁數(shù): 10/16頁
文件大小: 206K
代理商: LT1107IS8
10
LT1107
1107fa
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.
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
V
V
V
DC
OUT
DIODE
IN
SW
+
<
1
1
24
(
)
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.
D1
L1
I
LIM
V
IN
FB
SW2
GND
C1
LT1107
1107 F04
+V
OUT
–V
IN
R2
SW1
C2
2N3906
R1
V
OUT
= R1
( )
R3
+
+
D1
1N5818
L1
R3
I
LIM
V
IN
FB
SW2
GND
C1
LT1107
1107 F03
+V
IN
–V
OUT
R1
R2
SW1
C2
+
+
Figure 3. Positive-to-Negative Converter
U
S
A
O
PPLICATI
U
U
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