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参数资料
| 型号： | LIT1108CS8-12#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 1.5 A SWITCHING REGULATOR, 25 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | PLASTIC, SOIC-8 |
| 文件页数： | 11/12页 |
| 文件大小： | 266K |
| 代理商： | LIT1108CS8-12#PBF |
8
LT1108
U
S
A
O
PPLICATI
WU
U
I FOR ATIO
In this mode the switch is arranged in common collector or
step-down mode. The switch drop can be modeled as a
0.75V source in series with a 0.65
resistor. When the
switch closes, current in the inductor builds according to
It
V
R
e
L
Rt
L
() =
'
–(
)
–'
115
where: R' = 0.65
+ DCRL
VL = VIN – 0.75V
As an example, suppose – 5V at 100mA is to be generated
from a 4.5V to 5.5V input. Recalling Equation (14),
PL = (–5V+ 0.5V)(100mA) = 550mW.
(16)
Energy required from the inductor is
P
f
mW
kHz
J
L
OSC
==
550
19
28 9
17
.(
)
Picking an inductor value of 220
H with 0.3 DCR results
in a peak switch current of
I
VV
e
mA
PEAK
s
H
= ()
+
()
=
×
45
075
65
0 3
1
568
095
36
220
.– .
..
–
–.
0
Substituting IPEAK into Equation (04) results in
EH
A
J
L =
()(
) =
1
2
220
0 568
35 5
19
2
..
(
)
Since 35.5
J > 28.9J, the 220H inductor will work.
Finally, RLIM should be selected by looking at the Switch
Current vs RLIM curve. In this example, RLIM = 150.
STEP-UP (BOOST MODE) OPERATION
A step-up DC/DC converter delivers an output voltage
higher than the input voltage. Step-up converters are not
short-circuit protected since there is a DC path from input
to output.
The usual step-up configuration for the LT1108 is shown in
Figure 1. The LT1108 first pulls SW1 low causing VIN –
VCESAT to appear across L1. A current then builds up in L1.
At the end of the switch-ON time the current in L1 is
I
V
L
t
PEAK
IN
ON
=
*(
)
20
(18)
*Expression 20 neglects the effect of switch and coil resistance. This is taken into account in the
"Inductor Selection" section.
Immediately after switch turn-off, the SW1 voltage pin
starts to rise because current cannot instantaneously stop
flowing in L1. When the voltage reaches VOUT + VD, the
inductor current flows through D1 into C1, increasing VOUT.
This action is repeated as needed by the LT1108 to keep VFB
at the internal reference voltage of 1.245V. R1 and R2 set
the output voltage according to the formula
V
R
V
OUT =+
()
1
2
1
1 245
21
.( )
STEP-DOWN (BUCK MODE) OPERATION
A step-down DC/DC converter converts a higher voltage to
a lower voltage. The usual hookup for an LT1108 based
step-down converter is shown in Figure 2.
When the switch turns on, SW2 pulls up to VIN – VSW. This
puts a voltage across L1 equal to VIN – VSW – VOUT, causing
a current to build up in L1. At the end of the switch- ON time,
the current in L1 is equal to
Figure 1. Step-Up Mode Hookup
L1
LT1108 F01
GND
SW2
SW1
LIM
I
IN
V
D1
R3
LT1108
+
VOUT
R2
R1
C1
VIN
FB
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