MB3771
13. Back-up Power Supply System (VCC = 5 V)
• Use CMOS Logic and connect VDD of CMOS logic with VCCO.
• The back-up battery works after CS goes high as V2 < V1.
• During tPO, memory access is prohibited.
• CS‘s threshold voltage V1 is determined by the following equation:
V1 = VF + (R1 + R2 + R3) × VSB/R3
When V1 is 4.45 V or less, connect 7 pin with VCC.
When V1 is 4.45 V or more, 7 pin can be used to open.
• The voltage to change V2 is provided as the following equation:
V2 = VF + (R1 + R2 + R3) × VSC/ (R2 + R3)
However, please set V2 to 3.5 V or more.
VCC
V1
V2
t
CS
TPO
t
VCCO
t
VCC
D1
R4 >1 kΩ
V F 0.6 V
R 1: 100 kΩ
R 2: 6.2 kΩ
R 5: 100 kΩ
MB3771
R 6: 100 kΩ
1
2
3
8
7
6
VCCO
CT
4
5
CS
R3: 56 kΩ
*: Diode has been added to prevent Comp.C from malfunctionig when VCC voltage is low.
Set V1 and V2 with care given to VF temperature characteristics (typically negative temperature
characteristics).
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