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参数资料
| 型号： | LX1671CLQ |
| 元件分类： | 基准电压源/电流源 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 抗辐射高效，5安培开关稳压器 |
| 文件页数： | 17/21页 |
| 文件大小： | 554K |
| 代理商： | LX1671CLQ |
Microsemi
Integrated Products, Power Management
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 17
Copyright
2000
Rev. 0.5i, 2002-07-17
W
M
.
C
LX1671
Multiple Output LoadSHARE PWM
P
RELIMINARY
I N T E G R A T E D P R O D U C T S
APPLICATION NOTE
(CONTINUED)
I
NPUT
C
APACITOR
The input capacitor and the input inductor, if used, are to filter
the pulsating current generated by the buck converter to reduce
interference to other circuits connected to the same 5V rail. In
addition, the input capacitor provides local de-coupling for the
buck converter. The capacitor should be rated to handle the RMS
current requirements. The RMS current is:
d)
d(1
I
I
L
RMS
=
Where I
L
is the inductor current and d is the duty cycle. The
maximum value occurs when d = 50% then I
RMS
=0.5I
L
. For 5V
input and output in the range of 2 to 3V, the required RMS
current is very close to 0.5I
L
.
S
OFT
-S
TART
C
APACITOR
The value of the soft-start capacitor determines how fast the
output voltage rises and how large the inductor current is required
to charge the output capacitor. The output voltage will follow the
voltage at the SS pin if the required inductor current does not
exceed the maximum allowable current for the inductor. The SS
pin voltage can be expressed as:
(
SS
1
V
V
ref
)
SS
SS
C
t/R
e
=
Where R
SS
and C
SS
are the soft-start resistor and capacitor.
The current required to charge the output capacitor during the soft
start interval is.
dt
dVss
Cout
Iout
=
Taking the derivative with respect to time results in
SS
SS
C
t/R
e
RssCss
VrefCout
Iout
=
and at t=0
RssCss
VrefCout
ax
Im
=
The required inductor current for the output capacitor to follow
the soft start voltage equals the required capacitor current plus the
load current. The soft-start capacitor should be selected to
provide the desired power on sequencing and insure that the
overall inductor current does not exceed its maximum allowable
rating.
Values of Css equal to .1uf or greater are unlikely to result in
saturation of the output inductor unless very large output capacitors
are used..
O
VER
-C
URRENT
P
ROTECTION
Current limiting occurs at current level I
CL
when the voltage
detected by the current sense comparator is greater than the current
sense comparator threshold, V
TRIP
(300mV).
TRIP
SET
SET
DS(ON)
CL
V
R
I
R
I
=
×
+
×
So,
μA
50
R
I
mV
00
3
I
R
I
V
R
DS(ON)
CL
SET
DS(ON)
CL
TRIP
SET
×
=
×
=
Example:
For 10A current limit, using FDS6670A MOSFET (10m
R
DS(ON)
):
%
1
k
02
.
10
50
10
0.0
6
10
3
0.
R
SET
=
×
×
=
.
Note: Maximum R
SET
is 6K
. Any resistor 6K
or greater will not
allow startup since I
CL
will equal zero (50uA x 6K
=300mV).
At low duty cycles where the upper gate drive is less than
350nS wide the 350nS delay for current limit enable may result in
current pulses exceeding the desired current limit set point. If the
upper MOSFET on time is less than 350nS and a short circuit
condition occurs the duty cycle will increase, since Vout will be
low. The current limit circuit will be enabled when the upper gate
drive exceeds 350nS although the actual peak current limit value
will be higher than calculated with the above equation.
Short circuit protection still exists due to the narrow pulse width
even though the magnitude of the current pulses will be higher than
the calculated value.
If OCP is not desired connect both VSX and VCX to VCC. Do
not leave them floating.
A
P
P
L
I
C
A
T
I
O
N
S
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