UCC2913

UCC3913

SLUS274A – JANUARY 1999 – REVISED APRIL 2003

8

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APPLICATION INFORMATION

Determining External Component Values (See FIgure 2)

To set RVDD the following must be achieved:

V

IN(min)

R

VDD

u

10 V

(R1

) R2)

) 2mA

In order to estimate the minimum timing capacitor, CT, 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 CT(min).

Then use the given the values of COUT, Load, RSENSE, VSS, and the resistors determining the voltage on the

IMAX pin, to calculate the approximate startup time of the node VOUT. 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, RSENSE, assuming the user has determined the fault

current, RSENSE can be calculated by:

R

SENSE +

50 mV

I

FAULT

Next, calculate the variable IMAX. IMAX 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 IMAX where:

I

MAX +

V

IMAX

R

SENSE

where VIMAX = voltage on IMAX pin.

UDG–03045

RVDD

COUT

Note: LOAD = ILOAD For Current Source Load

LOAD = ROUT For Resistive Load

RPL

RSENSE

2

8

7

6

5

PL

OUT

SENSE

VSS

IMAX

UCC3913

3

VDD

VSS

LOAD

R1

R2

M1

VOUT

Figure 2. External Component Connections

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