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參數(shù)資料
| 型號(hào)： | LX1675CLQ-TR |
| 廠商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 1.5 A SWITCHING CONTROLLER, 690 kHz SWITCHING FREQ-MAX, PQCC38 |
| 封裝： | 5 X 7 MM, ROHS COMPLIANT, PLASTIC, MLPQ-38 |
| 文件頁數(shù)： | 7/19頁 |
| 文件大?。?/td> | 324K |
| 代理商： | LX1675CLQ-TR |
LX1675
PRODUCTION DATA SHEET
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 15
Copyright
2004
Rev. 1.2a, 2006-02-16
WWW
.Microse
m
i
.CO
M
Multiple Output LoadSHARE PWM
TM
APPLICATION NOTE ( C ONTINU ED)
INPUT CAPACITOR
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
L
RMS
=
Where IL is the inductor current and d is the duty cycle. The
maximum value occurs when d = 50% then IRMS =0.5IL. For 5V
input and output in the range of 2 to 3V, the required RMS
current is very close to 0.5IL.
SOFT-START CAPACITOR
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
C
t/R
e
1
V
=
ref
Where RSS and CSS 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 0.1F or greater are unlikely to result in
saturation of the output inductor unless very large output capacitors
are used.
OVER-CURRENT PROTECTION
Current limiting occurs at current level ICL when the voltage
detected by the current sense comparator is greater than the current
sense comparator threshold, VTRIP (0.0 Volts).
SET
CL
DS(ON)
TRIP
IR
-I
R
=V
So,
CL
DS(ON)
CL
DS(ON)
SET
IR
R
I50 A
××
==
Example:
For 10A current limit, using FDS6670A MOSFET (10m
Ω
RDS(ON)):
SET
6
10
0.010
R2.00K 1%
50 10
×
==
×
Note: If RSET is 0.0Ω or the CSx pin has become shorted to
ground the device will be continuously in the current limit mode. If
the CSx pin is left open then the current limit will never be enabled.
A resistor should be selected for the maximum desired current limit
and this should also provide enough current to charge up the output
filter capacitance during the soft-start time.
The current limit comparator is followed by a counter that does
not allow the hiccup mode until the current limit condition has
existed for 4 PWM cycles. If the current limit condition goes away
after a count of 2 the counter will be reset. This mode will prevent
a single cycle current or noise glitch from starting the hiccup mode
current limit.
AA
PP
LL
IICC
AA
TT
IIOO
NN
SS
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