UCC2919

UCC3919

SLUS374C − JULY 1999 − REVISED NOVEMBER 2005

9

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APPLICATION INFORMATION

overload threshold

There is a third threshold which, if exceeded, will declare a fault and shutdown the external MOSFET

(again, this is with respect to CSP). This feature protects the circuit in the event that the external MOSFET is

in cases where the UCC3919 is already powered up (on the backplane) and capacitors are added across the

output bus. In this case, the load current could rise too quickly for the linear amplifier to reduce the voltage on

same period as defined above before retrying. The overload current is:

I

OVERLOAD +

V

CSP *

V

IMAX )

0.2

R

SENSE

+ I

MAX )

0.2

R

SENSE

power limiting

A power limiting feature is included which allows the power dissipated in the external MOSFET to be held

relatively constant during a short, for different values of input voltage. This is accomplished by connecting a

resistor from the output (source of the external MOSFET) to PL. When the output voltage drops due to a short

or overload, an internal bias current is generated which is equal to:

I

PL ^

V

IN *

V

OUT *

V

PL

R

PL

simplified schematic of the circuit internal to the UCC3919 is shown in Figure 2.) The result is that the on time

of the MOSFET during current limit is reduced as the input voltage is increased. This reduces the effective duty

cycle, holding the average power dissipated constant.

RPL

CT

VDD

UCC3919

POWER LIMIT

1X

1X

TO

GATE

TO

LOAD

VDD

SD

PL

FLT

IPL

UDG−98124

Figure 2. Power Limiting Circuit

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