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參數(shù)資料
| 型號(hào)： | LIT1108CS8-12#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(yè)數(shù)： | 12/12頁(yè) |
| 文件大小： | 266K |
| 代理商： | LIT1108CS8-12#PBF |
9
LT1108
U
S
A
O
PPLICATI
WU
U
I FOR ATIO
HIGHER CURRENT STEP-DOWN OPERATION
Output current can be increased by using a discrete PNP
pass transistor as shown in Figure 3. R1 serves as a
current limit sense. When the voltage drop across R1
equals 0.5VBE, the switch turns off. As shown, switch
current is limited to 2A. Inductor value can be calculated
based on formulas in the Inductor Selection Step-Down
Converter section with the following conservative expres-
sion for VSW:
VV
V
SW
R
Q SAT
=+
≈
11
10
24
.(
)
R2 provides a current path to turn off Q1. R3 provides base
drive to Q1. R4 and R5 set output voltage. A PMOS FET can
be used in place of Q1 when VIN is between 10V and 20V.
Figure 2. Step-Down Mode Hookup
LT1108 F02
GND
SW2
SW1
ILIM
R3
100
FB
VOUT
+
C2
+
C1
D1
1N5818
VIN
R2
R1
L1
LT1108
VIN
LT1108 F03
D1
1N5821
+
VOUT
VIN
30V
MAX
L1
R1
0.15
R2
100
Q1
ZETEX ZTX749
R3
330
R4
R5
C1
LT1108
GND
SW2
SW1
VIN
IL
FB
C2
R6
100
VOUT = 1.245V 1 +
R4
R5
()
INVERTING CONFIGURATIONS
The LT1108 can be configured as a positive-to-negative
converter (Figure 4), or a negative-to-positive converter
(Figure 5). In Figure 4, the arrangement is very similar to a
step-down, except that the high side of the feedback is
referred to ground. This level shifts the output negative. As
in the step-down mode, D1 must be a Schottky diode,
and
VOUTshould be less than 6.2V. More negative output
voltages can be accommodated as in the prior section.
In Figure 5, 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,
I
V
VV
L
t
PEAK
IN
SW
OUT
ON
=
()
22
When the switch turns off, the SW2 pin falls rapidly and
actually goes below ground. D1 turns on when SW2
reaches 0.4V below ground.
D1 MUST BE A SCHOTTKY
DIODE. The voltage at SW2 must never be allowed to go
below –0.5V. A silicon diode such as the 1N4933 will allow
SW2 to go to –0.8V, causing potentially destructive power
dissipation inside the LT1108. Output voltage is deter-
mined by
V
R
V
OUT =+
()
1
2
1
1 245
23
.(
)
R3 programs switch current limit. This is especially impor-
tant in applications where the input varies over a wide
range. Without R3, the switch stays on for a fixed time each
cycle. Under certain conditions the current in L1 can build
up to excessive levels, exceeding the switch rating and/or
saturating the inductor. The 100
resistor programs the
switch to turn off when the current reaches approximately
700mA. When using the LT1108 in step-down mode,
output voltage should be limited to 6.2V or less. Higher
output voltages can be accommodated by inserting a
1N5818 diode in series with the SW2 pin (anode connected
to SW2).
Figure 3. Q1 Permits Higher Current Switching
The LT1108 Functions as Controller
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參數(shù)描述 |
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| LIT-2013 | 制造商:JST Manufacturing 功能描述: |
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| LITERATURE ED 5 | 制造商:SOLA/HEVI-DUTY 功能描述:_ |
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