MT3620
Hardware User Guide
MediaTek Confidential
©
2020 MediaTek Inc.
Page 29 of 40
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One common backup power source used in many devices is a primary cell battery (e.g. a CR2032 coin
cell). In order to ensure system availability when using such a battery, designers should take care to use
a circuit that allows the 3V3_RTC to be sourced from either the battery or 3V3 system power. One
possible circuit is shown in Figure 8-11, which is explained below. The circuit guarantees that even if
the battery becomes depleted, the system will continue to start up if main system power is available.
The circuit has the added benefit of minimizing power draw on the battery when system power is
available, extending battery life.
Figure 8-11 Battery and 3V3 Power Circuit for AVDD33_RTC
Several items should be noticed here:
When the main 3V3 supply is present, D9B prevents the main supply from charging the
battery.
D9A prevents the battery from powering other parts of the system that are connected to the
main 3V3 supply when operating from the battery alone.
BAR43 Schottky diodes have a low forward voltage drop which is advantageous in this
application, since it helps maximize the useful operating life of the battery.
Lithium based primary coin cells can tolerate a small charging current, up to certain limits,
without adversely affecting their operating life. Therefore, the reverse leakage current of the
diodes should also be taken into consideration, particularly if the system will be operating in a
hot environment (>50°C), since the reverse leakage current is proportional to temperature.
R63 is included to limit the charging current to a safe level in the event D9B fails closed-
circuit.
