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User's Manual Part 1

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MC75 Hardware Interface Description
Strictly confidential / Draft
s
MC75_V00.190a Page 22 of 91 15.02.2005
3.1 Power Supply
MC75 needs to be connected to a power supply at the B2B connector (5 pins each BATT+
and GND).
The power supply of MC75 has to be a single voltage source at BATT+. It must be able to
provide the peak current during the uplink transmission.
All the key functions for supplying power to the device are handled by the power
management section of the analog controller. This IC provides the following features:
• Stabilizes the supply voltages for the GSM baseband using low drop linear voltage
regulators.
• Switches the module's power voltages for the power up and down procedures.
• Delivers, across the VEXT pin, a regulated voltage for an external application. This
voltage is not available in Power-down mode.
• SIM switch to provide SIM power supply.
3.1.1 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 3.2 V on the MC75 board, not
even in a transmit burst where current consumption can rise to typical peaks of 2A. It should
be noted that MC75 switches off when exceeding these limits. Any voltage drops that may
occur in a transmit burst should not exceed 400mV.
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 100mm, this
connection may cause, for example, a resistance of 30m in the BATT+ line and
30m in the GND line. As a result, a 2A transmit burst would add up to a total
voltage drop of 120mV. 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 including its protection circuit.
Figure 3: Power supply limits during transmit burst
Transmit
burst 2A
Transmit
burst 2A
Ripple
Drop
min. 3.2V
BATT+