Datasheet
SLUS274A – JANUARY 1999 – REVISED APRIL 2003
7
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APPLICATION INFORMATION
During a fault, C
T
charges at a rate determined by the internal charging current and the external timing capacitor.
Once C
T
charges to 2.5 V, the fault comparator switches and sets the fault latch. Setting of the fault latch causes
both the output to switch off and the charging switch to open. C
T
must now discharge with the 1-µA current
source, I2, until 0.5 V is reached. Once the voltage at CT reaches 0.5 V, the fault latch resets, which re-enables
the output and allows the fault circuitry to regain control of the charging switch. If a fault is still present, the fault
comparator closes the charging switch causing the cycle to begin. Under a constant fault, the duty cycle is given
by:
Duty Cycle +
1 mA
I
PL
) 36 mA
Average power dissipation in the pass element is given by:
P
FET(avg)
+ V
FET
IMAX
ǒ
1 mA
I
PL
) 36 mA
Ǔ
Where VFET >> 5 V I
PL
can be approximated as :
I
PL
^
V
FET
R
PL
and where I
PL
>> 36 µA, the duty cycle can be approximated as :
Duty Cycle +
1 mA R
PL
V
FET
Therefore, the maximum average power dissipation in the MOSFET can be approximated by:
P
FET(avg)
+ V
FET
IMAX
ǒ
1 mA R
PL
V
FET
Ǔ
+ IMAX 1 mA R
PL
Notice that in the approximation, V
FET
cancels. therefore, average power dissipation is limited in the N-channel
MOSFET pass element.
Overload Comparator
The linear amplifier in the UCCx913 ensures that the output N-channel MOSFET does not pass more than I
MAX
(which is V
IMAX
/R
S
). In the event the output current exceeds the programmed IMAX by 0.2 V/R
S
(
which can only
occur if the output MOSFET is not responding to a command from the device) the CT pin begins charging with
I3, 1 mA, and continue to charge to approximately 8 V. This allows a constant fault to show up on the SD/FLT
pin, and also since the voltage on CT charges past 2.5 V only in an overload fault mode, it can be used for
detection of output FET failure or to build in redundancy in the system.
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