Neoway WM620 - Vrtc; Figure 3-5 RTC Reference Design

WM620 Hardware User Guide
Copyright © Neoway Technology Co., Ltd
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Reference components:
 Q1 can be IRML6401 or Rds(on) p-MOSFET which has higher withstand voltage and drain current.
 Q2: a common NPN transistor, e.g. MMBT3904; or a digital NPN transistor, e.g. DTC123. If digital
transistor is used, delete R1 and R2.
 C4: 100 μF tantalum capacitor rated at 6.3V; or 470 μF aluminum capacitor.
Protection
It is strongly recommended to add a TVS diode (V
RWM
=5 V) to the VBAT power supply. For some stable
power supplies, zener diodes can decrease the power supply overshoot. SMAJ5.0A/C, SMBJ5.0A/C,
MMSZ5231B1T1G from ONSEMI, and PZ3D4V2 from Prisemi are options.
Trace
The primary loop lines for VBAT on PCB must support the safe transmission of 2 A current and ensure
no obvious loop voltage decrease. Therefore, the trace width is required greater than 1.5 mm and the
ground should be as complete as possible.
3.1.2 VRTC
VRTC is the power supply pin of RTC inside the module to ensure that the clock works properly even
though the module encounters exceptional power failure. It can be connected to external battery or
supercapacitor. When VBAT works properly, VRTC outputs 2 mA current to charge the backup battery or
the capacitor. When VBAT is disconnected, the battery or capacitor will supply power for RTC in short
time. If using a capacitor rather than a backup battery, connect the capacitor to the VRTC and then ground
the capacitor. The capacitor must be charged before it supplies power for RTC in an exceptional situation.
Figure 3-5 shows the reference design of the VRTC power supply.
Figure 3-5 RTC reference design
The capacity of the supercapacitor is dependent on the duration of exceptional power failures. Table 3-2
shows the capacity matching the power failure duration.
Backup
Battery
WM620
RTC
Circuit
2K
Capacitor
VRTC
R1
C1