MC45 Hardware Interface Description
P R E L I M I N A R Y
MC45_HD_01_V00.02a Page 26 of 90 12.08.2002
3.2.2 Minimizing power losses
When designing the power supply for your application please pay specific attention to power
losses. Ensure that the input voltage V
BATT+
never drops below 3.3 V on the MC45 board, not
even in a transmit burst where current consumption can rise to peaks of 2A. It should be
noted that MC45 switches off when exceeding these limits. Any voltage drops that may occur
in a transmit burst should not exceed 400mV. For further details see Chapter 6.4.
The best approach to reducing voltage drops is to use a board-to-board connection as
recommended, and a low impedance power source. The resistance of the power supply lines
on the host board and of a battery pack should also be considered.
Note: If the application design requires an adapter cable between both board-to-board
connectors, use a flex cable as short as possible in order to minimize power
losses.
Example: If the length of the flex cable reaches the maximum length of 200mm, this
connection may cause, for example, a resistance of 50m! in the BATT+ line and
50m! in the GND line. As a result, a 2A transmit burst would add up to a total
voltage drop of 200mV. Plus, if a battery pack is involved, further losses may
occur due to the resistance across the battery lines and the internal resistance of
the battery.
Figure 2: Power supply limits during transmit burst
Transmit
burst 2A
Transmit
burst 2A
min. 3.3V
max. 400mV
BATT+
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