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Time

Output1

Output2

PVDD1

PVDD2

Multiphase Operation

Bypass and FIltering

Over-Temperature Protection and Junction Temperature Rise

TPS54283,, TPS54286

SLUS749C – JULY 2007 – REVISED OCTOBER 2007

Figure 34. Waveforms Resulting from Cascading PVDD1 from Output 2

In this configuration, the following conditions must be maintained:

1. Output 2 must be of a voltage high enough to maintain regulation of Output 1 under all load conditions.

2. The sum of the current drawn by Output 2 load plus the current into PVDD1 must be less than the overload

protection current level of Output 2.

3. The method of output sequencing must be such that the voltage on Output 2 is sufficient to support Output 1

before Output 1 is enabled. This requrement may be accomplished by:

a delay of the enable function

selecting sequential sequencing of Output 1 starting after Output 2 is in regulation

The

TPS5428x

not

designed

operate

two-phase

single-output

voltage

converter.

See

http://www.power.ti.com for appropriate device selection.

As with any integrated circuit, supply bypassing is important for jitter-free operation. To improve the noise

immunity of the converter, ceramic bypass capacitors must be placed as close to the package as possible.

1. PVDD1 to GND: Use a 10-

µF ceramic capacitor

2. PVDD2 to GND: Use a 10-

µF ceramic capacitor

3. BP to GND: Use a 4.7-

µF to 10-µF ceramic capacitor

The over-temperature thermal protection limits the maximum power to be dissipated at a given operating ambient

temperature. In other words, at a given device power dissipation, the maximum ambient operating temperature is

limited by the maximum allowable junction operating temperature. The device junction temperature is a function

of power dissipation, and the thermal impedance from the junction to the ambient. If the internal die temperature

should reach the thermal shutdown level, the TPS5428x shuts off both PWMs and remains in this state until the

die temperature drops below the hysteresis value, at which time the device restarts.

The first step to determine the device junction temperature is to calculate the power dissipation. The power

dissipation is dominated by the two switching MOSFETs and the BP internal regulator. The power dissipated by

each MOSFET is composed of conduction losses and output (switching) losses incurred while driving the

external rectifier diode. To find the conduction loss, first find the RMS current through the upper switch MOSFET.

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SLMA002 Datasheet(PDF) 28 Page - Texas Instruments

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