lx1672 Microsemi Corporation, lx1672 Datasheet - Page 16

• Manufacturer Part Number: lx1672
• Description: Multiple Output Loadshare Pwm
• Manufacturer: Microsemi Corporation

Copyright © 2000
Rev. 1.0, 2005-08-10
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.
S
I
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
Iout =
Iout
V
Im
are the soft-start resistor and capacitor.
SS
ax
I
=
=
RMS
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
Cout
=
V
VrefCout
RssCss
L
VrefCout
ref
=
.
RssCss
I
dVss
L
(
1
dt
−
d(1
e
e
−
−
−
t/R
t/R
d)
SS
SS
A P P L I C A T I O N N O T E
C
C
SS
SS
RMS
)
=0.5I
Integrated Products Division
L
. For 5V
®
Microsemi
Values of Css equal to .1µF or greater are unlikely to result in
saturation of the output inductor unless very large output capacitors
are used..
Note: Maximum R
O
detected by the current sense comparator is greater than the current
sense comparator threshold, V
R
allow startup since I
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 V
enabled when the upper gate drive exceeds 350nS although the
actual peak current limit value will be higher than calculated with
the above equation.
even though the magnitude of the current pulses will be higher than
the calculated value.
leave them floating.
If OCP is not desired connect both VS X and VC X to VCC. Do not
DS(ON)
VER
At higher PWM frequencies or low duty cycles, where the upper
Short circuit protection still exists due to the narrow pulse width
Current limiting occurs at current level I
So,
Example:
For 10A current limit, using FDS6670A MOSFET (10mΩ
Multiple Output LoadSHARE™ PWM
( C O N T I N U E D )
-C
R
):
URRENT
SET
=
P
V
TRIP
RODUCTION
P
I
OUT
R
CL
SET
ROTECTION
SET
CL
−
×
will be low. The current limit circuit will be
is 6KΩ. Any resistor 6KΩ or greater will not
I
R
will equal zero (50µA x 6KΩ = 300mV).
I
=
CL
SET
DS(ON)
×
0.
3
R
50
−
DS(ON)
+
TRIP
10
×
I
D
SET
10
×
(300mV).
ATA
0.0
−
×
=
6
R
3
10
00
SET
S
=
mV
HEET
=
4
K
V
−
50
CL
TRIP
Ω
I
µA
CL
when the voltage
×
R
DS(ON)
LX1672
Page 16