TPS40050

TPS40051

TPS40053

SLUS540F − DECEMBER 2002 − REVISED JUNE 2004

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9

Figure 2

fSW − Switching Frequency − kHz

SWITCHING FREQUENCY

vs

TIMING RESISTANCE

0

100

0

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400

600

800

1000

200

300

400

500

600

100

0

200

300

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500

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200

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600

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1000

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FEED-FORWARD IMPEDANCE

vs

SWITCHING FREQUENCY

Figure 3

fSW − Switching Frequency − kHz

VIN = 25 V

VIN = 15 V

VIN = 9 V

UVLO OPERATION

The TPS4005x uses variable (user programmable) UVLO protection. The UVLO circuit holds the soft-start low

until the input voltage has exceeded the user programmable undervoltage threshold.

The TPS4005x uses the feed-forward pin, KFF, as a user programmable low-line UVLO detection. This variable

low-line UVLO threshold compares the PWM ramp duration to the oscillator clock period. An undervoltage

condition exists if the TPS4005x receives a clock pulse before the ramp has reached 90% of its full amplitude.

The ramp duration is a function of the ramp slope, which is directly related to the current into the KFF pin. The

KFF current is a function of the input voltage and the resistance from KFF to the input voltage. The KFF resistor

can be referenced to the oscillator frequency as descibed in equation (3):

R

58.14

R

W

where:.

The variable UVLO function uses a three−bit full adder to prevent spurious shut-downs or turn-ons due to spikes

or fast line transients. When the adder reaches a total of seven counts in which the ramp duration is shorter

than the clock cycle a powergood signal is asserted and a soft-start initiated, and the upper and lower

MOSFETS are turned off.

(3)