Datasheet

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SLUS274A – JANUARY 1999 – REVISED APRIL 2003
8
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APPLICATION INFORMATION
Determining External Component Values (See FIgure 2)
To set R
VDD
the following must be achieved:
V
IN(min)
R
VDD
u
10 V
(
R1 ) R2
)
) 2mA
In order to estimate the minimum timing capacitor, C
T
, several things must be taken into account. For example,
given the schematic below as a possible (and at this point, a standard) application, certain external component
values must be known in order to estimate C
T(min)
.
Then use the given the values of C
OUT
, Load, R
SENSE
, VSS, and the resistors determining the voltage on the
IMAX pin, to calculate the approximate startup time of the node V
OUT
. This startup time must be faster than the
time it takes for CT to charge to 2.5 V (relative to VSS), and is the basis for estimating the minimum value of
CT. In order to determine the value of the sense resistor, R
SENSE
, assuming the user has determined the fault
current, R
SENSE
can be calculated by:
R
SENSE
+
50 mV
I
FAULT
Next, calculate the variable I
MAX
. I
MAX
is the maximum current that the device allows through the transistor, M1,
and during startup with an output capacitor the power MOSFET, M1, can be modeled as a constant current
source of value I
MAX
where:
I
MAX
+
V
IMAX
R
SENSE
where V
IMAX
= voltage on IMAX pin.
UDG–03045
R
VDD
C
OUT
Note: LOAD = I
LOAD
For Current Source Load
LOAD = R
OUT
For Resistive Load
R
PL
R
SENSE
2
8
7
6
5
PL
OUT
SENSE
VSS
IMAX
UCC3913
3
VDD
VSS
LOAD
R1
R2
M1
V
OUT
Figure 2. External Component Connections
(11)
(12)
(13)