TPS2145, TPS2147

TPS2155, TPS2157

SLVS333 − AUGUST 2001

17

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APPLICATION INFORMATION

overcurrent (continued)

In the third condition, the load has been gradually increased beyond the recommended operating current. The

current is permitted to rise until the current-limit threshold is reached or until the thermal limit of the device is

exceeded. The TPS2145 and TPS2147 are capable of delivering current up to the current-limit threshold without

damaging the device. Once the threshold has been reached, the device switches into its constant-current mode.

OC response

The OCx open-drain output is asserted (active low) when an overcurrent condition is encountered. The output

will remain asserted until the overcurrent condition is removed. Connecting a heavy capacitive load to an

enabled device can cause momentary false overcurrent reporting from the inrush current flowing through the

device, charging the downstream capacitor. The TPS2145 and TPS2147 are designed to reduce false

overcurrent reporting. An internal overcurrent transient filter eliminates the need for external components to

remove unwanted pulses. Using low-ESR electrolytic capacitors on OUTx lowers the inrush current flow through

the device during hot-plug events by providing a low-impedance energy source, also reducing erroneous

overcurrent reporting.

power dissipation and junction temperature

The major source of power dissipation for the TPS2145 and TPS2147 comes from the internal voltage regulator

and the N-channel MOSFETs. Checking the power dissipation and junction temperature is always a good

voltage and operating temperature. As an initial estimate, use the highest operating ambient temperature of

this value, the power dissipation per switch can be calculated using:

P

Multiply this number by two to get the total power dissipation coming from the N-channel MOSFETs.

The power dissipation for the internal voltage regulator is calculated using:

P

I

–V

O(min)

I

O

The total power dissipation for the device becomes:

P

D(voltage regulator) )

2

P

D(switch)

Finally, calculate the junction temperature:

T

Where:

in this data sheet.

Compare the calculated junction temperature with the initial estimate. If they do not agree within a few degrees,

repeat the calculation, using the calculated value as the new estimate. Two or three iterations are generally

sufficient to get a reasonable answer.

(3)

(4)

(5)

(6)