Application Note
BRT_AN_004
FT93x RTC External Power Switch
Circuit
Version 1.0
Issue Date: 2017-03-07
This application note describes implementing a simple circuit for the
FT930/FT931 RTC external power supply switch. The Texas Instruments
SN74AUC2G53 Single-Pole Double-Throw (SPDT) analog switch is used to
select the power supplied by the 1.5V battery or by the 1.2V internal LDO.
Use of Bridgetek Pte devices in life support and/or safety applications is entirely at the user’s risk,
and the user agrees to defend, indemnify and hold Bridgetek Pte harmless from any and all
damages, claims, suits or expense resulting from such use.
Bridgetek Pte Ltd (BRT Chip)
178 Paya Lebar Road, #07-03 Singapore 409030
Tel : +65 6547 4827 Fax : +65 6841 6071
Web Site: http://brtchip.com
Copyright © Bridgetek Pte Ltd
Application Note
BRT_AN_004 FT930 RTC External Power Switch Circuit
Version 1.0
Document Reference No.: BRT_000078
Clearance No.: BRT#068
Table of Contents
1 Introduction .............................................................. 2
2 Schematic and Circuit Description ............................. 3
2.1 Schematic............................................................................3
2.2 Circuit Description ...............................................................3
3 Power Consumption................................................... 5
4 Contact Information .................................................. 6
Appendix A – References ............................................... 7
Document References .................................................................7
Acronyms and Abbreviations.......................................................7
Appendix B – List of Tables & Figures ............................ 8
List of Tables...............................................................................8
List of Figures .............................................................................8
Appendix C – Revision History ....................................... 9
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Application Note
BRT_AN_004 FT930 RTC External Power Switch Circuit
Version 1.0
Document Reference No.: BRT_000078
Clearance No.: BRT#068
1 Introduction
The FT930/FT931 device contains an internal Real Time Clock (RTC) circuit with an external power
pin (VDDBAT, PIN 63 of FT930 / PIN 54 of FT931). It is connected to a separate 1.5V battery or
the chip’s internal +1.2V regulator output (VCC1V2).
The RTC will run continuously after it is activated. The following conditions are required for the
RTC to draw minimal power from the battery:
1. It is always powered.
2. Powered by VCC1V2 when the system is powered on.
3. Powered by the battery when the system is powered down.
4. It should not reset during a system power cycle (power on and off).
5. The switch circuit can operate at a low VCC with ultra-low power consumption.
6. The switch circuit has minimal voltage drop.
This application note describes an example external power switch circuit for the FT930/FT931 RTC.
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Application Note
BRT_AN_004 FT930 RTC External Power Switch Circuit
Version 1.0
Document Reference No.: BRT_000078
Clearance No.: BRT#068
2 Schematic and Circuit Description
2.1 Schematic
The RTC external power switch schematic is shown in Figure 2.1.
Figure 2.1 External Power Switch Circuit
2.2 Circuit Description
This circuit has two power source inputs (VCC1V2 and 1.5V Battery) and one output (VDDBAT).
VCC33 is used as a control signal via the resistor divider circuit R1 and R2.
VCC1V2 is the internal regulator output of the FT930/FT931; it connects to VCC33 internally.
U1 is a Texas Instruments SN74AUC2G53 Single-Pole Double-Throw (SPDT) analog switch. It can
operate from 0.8V to 2.7V with low power consumption. It can still operate if the battery voltage
drops to 0.9V. Table 2.1 describes its function.
CONTROL INPUTS
INH
A
ON
CHANNEL
L
L
Y1 (Battery)
L
H
Y2 (VCC1V2)
H
X
None
Table 2.1 Function Table
U1 is powered by the battery. Pin 2 (INH) connects to ground; this pin is an active-low enable pin.
Pin 5 (A) is the input select pin, connects to the R1/R2 voltage divider which divides VCC33 the
main power of the FT930/FT931 device. The voltage of pin 5 is around 1.5V after the divider,
equals the VCC of U1.
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Application Note
BRT_AN_004 FT930 RTC External Power Switch Circuit
Version 1.0
Document Reference No.: BRT_000078
Clearance No.: BRT#068
Table 2.2 shows the output voltage (VDDBAT) when the FT930/FT931 system power (VCC33)
turns on and off.
VCC33
A
VDDBAT
ON
H
VCC1V2
OFF
L
Battery
Table 2.2 VDDBAT Voltage
C1: Ceramic capacitor, capacitance is 2.2uF to 4.7uF. It maintains VDDBAT at the same level when
VCC33 switches its voltage level.
D1: Low forward voltage schottky barrier diode. ON Semiconductor RB521S30T1G. This diode
prevents VDDBAT discharge to the battery or VCC1V2 when VCC33 switches its voltage level.
R3: 560Ω resistor, it reduces U1’s current draw and prevents ringing on VDDBAT when U1 is
switching between its analog channels. It may be removed if an alternative part to U1 is used.
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Application Note
BRT_AN_004 FT930 RTC External Power Switch Circuit
Version 1.0
Document Reference No.: BRT_000078
Clearance No.: BRT#068
3 Power Consumption
Table 3.1 shows typical battery current consumption. The measurement point is the connection
between the 1.5V battery and R3.
VCC33
A
Battery Current (uA)
ON
H
<1.0
OFF
L
9
Table 3.1 Battery Current
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Application Note
BRT_AN_004 FT930 RTC External Power Switch Circuit
Version 1.0
Document Reference No.: BRT_000078
Clearance No.: BRT#068
4 Contact Information
Headquarters – Singapore
Branch Office – Taipei, Taiwan
Bridgetek Pte Ltd
178 Paya Lebar Road, #07-03
Singapore 409030
Tel: +65 6547 4827
Fax: +65 6841 6071
Bridgetek Pte Ltd, Taiwan Branch
2 Floor, No. 516, Sec. 1, Nei Hu Road, Nei Hu District
Taipei 114
Taiwan , R.O.C.
Tel: +886 (2) 8797 5691
Fax: +886 (2) 8751 9737
E-mail (Sales)
E-mail (Support)
E-mail (Sales)
E-mail (Support)
[email protected]
[email protected]
[email protected]
[email protected]
Branch Office - Glasgow, United Kingdom
Branch Office – Vietnam
Bridgetek Pte. Ltd.
Unit 1, 2 Seaward Place, Centurion Business Park
Glasgow G41 1HH
United Kingdom
Tel: +44 (0) 141 429 2777
Fax: +44 (0) 141 429 2758
Bridgetek VietNam Company Limited
Lutaco Tower Building, 5th Floor, 173A Nguyen Van
Troi,
Ward 11, Phu Nhuan District,
Ho Chi Minh City, Vietnam
Tel : 08 38453222
Fax : 08 38455222
E-mail (Sales)
E-mail (Support)
E-mail (Sales)
E-mail (Support)
[email protected]
[email protected]
[email protected]
[email protected]
Web Site
http://brtchip.com/
Distributor and Sales Representatives
Please visit the Sales Network page of the Bridgetek Web site for the contact details of our distributor(s) and
sales representative(s) in your country.
System and equipment manufacturers and designers are responsible to ensure that their systems, and any Bridgetek Pte Ltd
(BRT Chip) devices incorporated in their systems, meet all applicable safety, regulatory and system-level performance
requirements. All application-related information in this document (including application descriptions, suggested Bridgetek
devices and other materials) is provided for reference only. While Bridgetek has taken care to assure it is accurate, this
information is subject to customer confirmation, and Bridgetek disclaims all liability for system designs and for any applications
assistance provided by Bridgetek. Use of Bridgetek devices in life support and/or safety applications is entirely at the user’s
risk, and the user agrees to defend, indemnify and hold harmless Bridgetek from any and all damages, claims, suits or expense
resulting from such use. This document is subject to change without notice. No freedom to use patents or other intellectual
property rights is implied by the publication of this document. Neither the whole nor any part of the information contained in,
or the product described in this document, may be adapted or reproduced in any material or electronic form without the prior
written consent of the copyright holder. Bridgetek Pte Ltd, 178 Paya Lebar Road, #07-03, Singapore 409030. Singapore
Registered Company Number: 201542387H.
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Application Note
BRT_AN_004 FT930 RTC External Power Switch Circuit
Version 1.0
Document Reference No.: BRT_000078
Clearance No.: BRT#068
Appendix A – References
Document References
FT93x Datasheet
U1 Datasheet: SN74AUC2G53
D1 Datasheet: RB521S30T1G
Acronyms and Abbreviations
Terms
RTC
SPDT
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Description
Real Time Clock
Single-Pole Double-Throw
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Copyright © Bridgetek Pte Ltd
Application Note
BRT_AN_004 FT930 RTC External Power Switch Circuit
Version 1.0
Document Reference No.: BRT_000078
Clearance No.: BRT#068
Appendix B – List of Tables & Figures
List of Tables
Table 2.1 Function Table ...................................................................................................... 3
Table 2.2 VDDBAT Voltage ................................................................................................... 4
Table 3.1 Battery Current .................................................................................................... 5
List of Figures
Figure 2.1 External Power Switch Circuit ................................................................................ 3
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Application Note
BRT_AN_004 FT930 RTC External Power Switch Circuit
Version 1.0
Document Reference No.: BRT_000078
Clearance No.: BRT#068
Appendix C – Revision History
Document Title:
BRT_AN_004 FT930 RTC External Power Switch Circuit
Document Reference No.:
BRT_000078
Clearance No.:
BRT#068
Product Page:
http://brtchip.com/product
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Revision
1.0
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Changes
Initial Release
Date
2017-03-07
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