TPS54383, TPS54386
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SLUS774B–AUGUST 2007–REVISED OCTOBER 2007
PVDD2
Output2
PVDD1
Output1
T - Time
Figure 35. 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. a delay of the enable function
b. selecting sequential sequencing of Output 1 starting after Output 2 is in regulation
Multiphase Operation
The TPS5438x is not designed to operate as a two-channel multiphase converter. See http://www.power.ti.com
for appropriate device selection.
Bypass and FIltering
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
Over-Temperature Protection and Junction Temperature Rise
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 TPS5438x 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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