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參數(shù)資料
| 型號： | LIT1108CS8-5#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1.5 A SWITCHING REGULATOR, 25 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封裝： | PLASTIC, SOIC-8 |
| 文件頁數(shù)： | 2/12頁 |
| 文件大?。?/td> | 266K |
| 代理商： | LIT1108CS8-5#PBF |
10
LT1108
U
S
A
O
PPLICATI
WU
U
I FOR ATIO
Another situation where the ILIM feature is useful occurs
when the device goes into continuous mode operation. This
occurs in step-up mode when
V
VV
DC
OUT
DIODE
IN
SW
+
<
1
25
.(
)
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 6, the inductor
current increases to a high level before the comparator
turns off the oscillator. This high current can cause exces-
sive output ripple and requires oversizing the output ca-
pacitor and inductor. With the ILIM feature, however, the
switch current turns off at a programmed level as shown in
Figure 7, keeping output ripple to a minimum.
provided by the PNP transistor, supplies proper polarity
feedback information to the regulator.
Figure 4. Positive-to-Negative Converter
Figure 5. Negative-to-Positive Converter
LT1108 F04
+
C2
+
C1
D1
1N5818
VIN
R1
R2
L1
GND
SW2
SW1
R3
FB
LT1108
ILIM
VIN
–VOUT
L1
LT1108 F05
GND
SW2
FB
SW1
D1
AO
R2
VOUT =
1.245V + 0.6V
R1
R2
R1
2N3906
+
C1
LT1108
+
C2
ILIM
VIN
()
VOUT
–VIN
USING THE ILIM PIN
The LT1108 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
LT1108 must operate at an 800mA peak switch current with
a 2.0V input. If VIN 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 “Maximum
Switch Current vs RLIM” characteristic), the switch current
will be limited to 800mA, even if the input voltage increases.
LT1108 F06
OFF
ON
SWITCH
IL
LT1108 F07
ON
OFF
SWITCH
PROGRAMMED CURRENT LIMIT
IL
Figure 7. Current Limit Keeps Inductor Current Under Control
Figure 6. No Current Limit Causes Large Inductor
Current Build-Up
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