TPS7A39
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ZHCSGP0A –JULY 2017–REVISED SEPTEMBER 2017
Conditions where excessive reverse current can occur are outlined in this section, all of which can exceed the
absolute maximum rating of VOUTP > VINP + 0.3 V and VOUTN < VINN – 0.3 V:
•
•
•
If the device has a large COUTx and the input supply collapses quickly with little or no load current
The output is biased when the input supply is not established
The output is biased above the input supply
If excessive reverse current flow is expected in the application, then external protection must be used to protect
the device. 图 70 shows one approach of protecting the device.
Schottky Diode
INP
CINP
OUTP
Device
COUTP
GND
图 70. Example Circuit for Reverse Current Protection Using a Schottky Diode On Positive Rail
8.1.12 Power Dissipation (PD)
Circuit reliability demands that proper consideration is given to device power dissipation, location of the circuit on
the printed circuit board (PCB), and correct sizing of the thermal plane. The PCB area around the regulator must
be as free as possible of other heat-generating devices that cause added thermal stresses.
As a first-order approximation, power dissipation in the regulator depends on the input-to-output voltage
difference and load conditions. Use 公式 9 to approximate PD:
PD = (VINP – VOUTP) × IOUTP + (|VINN – VOUTN|) × |IOUTN
|
(9)
Careful selection of the system voltage rails minimizes power dissipation and improves system efficiency. Proper
selection allows the minimum input-to-output voltage differential to be obtained. The low dropout of the device
allows for maximum efficiency across a wide range of output voltages.
The main heat conduction path for the device is through the thermal pad on the package. As such, the thermal
pad must be soldered to a copper pad area under the device. This pad area contains an array of plated vias that
conduct heat to any inner plane areas or to a bottom-side copper plane.
The maximum power dissipation determines the maximum allowable junction temperature (TJ) for the device.
According to 公式 10, power dissipation and junction temperature are most often related by the junction-to-
ambient thermal resistance (θJA) of the combined PCB, device package, and the temperature of the ambient air
(TA).
TJ = TA + θJA × PD
(10)
Unfortunately, this thermal resistance (θJA) is highly dependent on the heat-spreading capability built into the
particular PCB design, and therefore varies according to the total copper area, copper weight, and location of the
planes. The θJA recorded in the Electrical Characteristics table is determined by the JEDEC standard, PCB, and
copper-spreading area, and is only used as a relative measure of package thermal performance. For a well-
designed thermal layout, θJA is actually the sum of the WSON package junction-to-case (bottom) thermal
resistance (θJCbot) plus the thermal resistance contribution by the PCB copper.
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