Holtek BC45B4523, Manual, Page: 93 / 96

Rev. 1.20 93 October 28, 2020
BC45B4523
P
Reg
=I
Out,Dig
× (V
InReg
– V
OutReg
) + I
Out,Ana
× (V
InReg
– V
OutReg
)
For example, the transmitter is designed to drive a differential antenna from a 5V power supply and deliver
250mA peak output current. Regulator supplies 150mA output current by relying on 5V input.
From the “Transmitter Characteristics”, the effective resistance of the driver output is approximately at 5Ω, the
transmitter loss is
PlossTX=(0.25)2 × 5=0.312W
For the regulator, the loss is
PReg=(150mA × (5V - 3.3V))=0.255W
The temperature coefficient θ
JA
of an operating PCB is 34°C/W. (As shown in the “Absolute Maximum Rating”)
The different temperature between the silicon and ambient T
J
- T
A
is
T
J
- T
A
=(34 °C/W) × (0.314 + 0.255) W=19.3 °C
T
J
is set to 85°C. Then, the maximum T
A
is approximately 65°C.
Practically, the designed PCB may be smaller from that stated in the JEDEC51-7 standard, so the temperature
coefficient can be higher than the specification in the “Absolute Maximum Rating” (34°C/W). Designers must
provide a proper heat sink with some margins to accommodate elevated temperature coefficient.