lx1675 Microsemi Corporation, lx1675 Datasheet - Page 15

• Manufacturer Part Number: lx1675
• Description: Multiple Output Loadshare?? Pwm Production Data Sheet
• Manufacturer: Microsemi Corporation

Copyright © 2004
Rev. 1.2a, 2006-02-16
Where R
The current required to charge the output capacitor during the soft
Taking the derivative with respect to time results in
and at t = 0
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:
maximum value occurs when d = 50% then I
input and output in the range of 2 to 3V, the required RMS
current is very close to 0.5I
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:
start interval is.
The required inductor current for the output capacitor to follow
the soft start voltage equals the required capacitor current plus the
load current.
provide the desired power on sequencing and insure that the
overall inductor current does not exceed its maximum allowable
rating.
I
S
NPUT
OFT
The input capacitor and the input inductor, if used, are to filter
Where I
The value of the soft-start capacitor determines how fast the
-S
C
TART
TM
SS
APACITOR
and C
L
is the inductor current and d is the duty cycle. The
C
APACITOR
The soft-start capacitor should be selected to
SS
V
Iout
are the soft-start resistor and capacitor.
SS
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
I
=
Im
=
RMS
Iout =
V
VrefCout
ax
L
RssCss
ref
=
.
=
I
Cout
L
(
1
VrefCout
RssCss
−
d(1
e
dVss
e
−
−
dt
−
t/R
t/R
d)
SS
A P P L I C A T I O N N O T E
SS
C
C
SS
SS
RMS
)
=0.5I
Integrated Products Division
L
Microsemi
. For 5V
®
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.
saturation of the output inductor unless very large output capacitors
are used.
O
detected by the current sense comparator is greater than the current
sense comparator threshold, V
R
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.
DS(ON)
VER
Note: If R
The current limit comparator is followed by a counter that does
Multiple Output LoadSHARE™ PWM
Values of C
Current limiting occurs at current level I
So,
Example:
For 10A current limit, using FDS6670A MOSFET (10mΩ
( C O N T I N U E D )
-C
):
URRENT
SET
P
SS
R
is 0.0Ω or the CSx pin has become shorted to
R
RODUCTION
I
P
SET
equal to 0.1µF or greater are unlikely to result in
SET
SET
ROTECTION
=
•R
=
I
10 0.010
SET
CL
50 10
×
×
I
×
-I
R
SET
TRIP
CL
DS(ON)
−
D
6
•R
ATA
(0.0 Volts).
=
DS(ON)
=
2.00KΩ 1%
I
S
CL
HEET
50 µA
×
=V
R
TRIP
DS(ON)
CL
when the voltage
LX1675
Page 15