Backup Power Supply Solutions using an OR Circuit
Torex Semiconductor Europe Ltd
Power Management IC, DC/DC Converters, Battery Chargers, and Voltage Regulators
Battery backup circuits are circuits that immediately shift the load to the battery when there’s no main power supply. However, if there is a main supply, the load shifts to the power supply as the backup battery is isolated or disconnected from the system.
A simple backup circuit can be implemented using a standard primary cell, a Supercap (Electric Double Layer Capacitor, EDLC for short) or a Lithium-Titanium-Oxide (LTO) battery.? The latter two energy storage devices can be quickly re-charged using an inexpensive LDO voltage regulator rather than a more complex and expensive dedicated battery charger IC.
Depending on the VBAT level and the required system voltage, there are different solutions to consider when creating a backup circuit as illustrated in the article below.
Backup Power Supply with Primary Coin Battery
Key System Requirements:
Below shows the typical power architecture to meet the key system requirements if using a standard primary cell, highlighting how our products can be used in a simple backup circuit:
Load Switch with Ideal Diode Function
(1) The XC8110/XC8111 are load switch IC with an ideal diode function that also feature chip enable (CE), over current limit, inrush current limit and thermal shutdown. The circuit above automatically switches between the main input rail and the battery/EDLC output using the XC8110/XC8111 ideal diode in an OR circuit configuration. The XC8110/XC8111 is perfect for OR circuits because of its low VF and 0 μA VIN Iq when reverse biased. The XC8110 supports an output current of 500 mA whereas the XC8111 can support up to 1 A. Both versions have a quiescent current of only 3.6 μA and a stand-by current of 0.65 μA. The series is IEC62368-1 certified and is available in WLP-4-02 (0.82 x 0.82 x 0.5 mm), USP-6B06 or SOT-25 packages.
This solution for the OR circuit is the best solution for applying the backup battery voltage directly to the MCU and compared to using a Schottky Barrier Diode, the VF/leakage current loss is significantly reduced.
Step-Up DC/DC
(2) The XC9148 is a PWM/PFM synchronous boost converter optimised to provide high efficiencies under all load conditions. This DC/DC integrates a 0.17 Ω Nch driver transistor and a 0.2 Ω synchronous Pch switching transistor and is available with either a 1.2 MHz or 3.0 MHz switching frequency. Output currents of 750 mA minimum can be realised with only two low-cost ceramic capacitors and an inductor placed externally. The internal circuit can start operation from input voltages as low as 0.9 V and once started will continue to operate down to 0.65 V
The XC9148C type is ideal for use in an OR circuit as it compares the input voltage BAT and output voltage VOUT to optimally control the orientation of the parasitic diode of the Pch synchronous switching FET even during standby mode, so that the input side and output side do not connect through the parasitic diode of the Pch synchronous switching FET. Even if an external voltage higher than the BAT voltage is applied to the output side, by controlling this parasitic diode, the input side and output side do not connect, and an output OR connection is possible.
If space on the PCB is limited, then the XCL105C boost Micro DC/DC with integrated inductor can be used instead of the XC9148C. Available in a small DFN3030-10B (3.0 x 3.0 x 1.7 mm) package, the XCL105C provides the same high performance to that of the XC9148C. This solution for the OR circuit is the best solution for applying a step-up voltage, such as 3.3V, to the MCU.
N-Ch MOSFET for Shutdown during Shipment
(3) The XP222N N-ch MOSFET used together with a resistor is a simple and effective method to extend battery life during shipment as it cuts off the supply from the backup battery. Turning on the MOSFET after final testing is complete turns off the CE of the XC8110/XC8111 which significantly reduces the discharge from the battery during shipment or storage.
LDO for Main Power Side
(4) The XC6227 is suitable for the upper side LDO if used because it includes reverse current prevention, so an external blocking diode is not required. The XC6227 series is a high speed, low noise 6.0 V 700 mA LDO voltage regulator with VOUT selectable within a range of 0.8 V ~ 5.0V and PSRR is 65dB @ 1kHz.
Alternatively, a general step-down DC/DC or LDO can be used for an OR connection, but if selected a Schottky Barrier Diode should also be used for reverse current prevention.
Backup Power Supply with LTO Battery or Supercapacitor (EDLC)
Key System Requirements:
Below is a typical power architecture that meets the key system requirements and which shows how our products can be used in a seamless and efficient circuit:
Load Switch with Ideal Diode Function
(1) The XC8110/XC8111 is covered in detail above but just to reiterate, this solution for the OR circuit is the best solution for applying the backup battery voltage directly to the MCU and compared to using a Schottky Barrier Diode, the VF/leakage current loss is significantly reduced.
Step-Up DC/DC
(2) The XC9148 is also covered in detail above. This solution for the OR circuit is the best solution for applying a step-up voltage, such as 3.3V, to the MCU. XC9148C (or XCL105C with integrated inductor) are the recommended versions for use in an OR circuit.
It should be noted that with both the XC9148C & XCL105C, when the output voltage is higher than the input voltage from the battery/EDLC, there is almost no consumption current (typ. 0.12 μA) from the battery/EDLC, because its operation current is taken from the output side.
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In this case, the IC only stops boosting because the output voltage is higher than the set value in the operating state, so as soon as the main power voltage is lost, the IC automatically starts boosting to maintain the output voltage.
LDO, Buck DC/DC for Main Power Side
(4) The XC6227 is also covered above and is recommended because it includes reverse current prevention, so an external blocking diode is not required.
LDO Voltage Regulator for LTO Battery Charging
(5) The XC6240/XC6242 series are low power LDO voltage regulators that typically consume only 0.8 μA when operating and they sink only 0.24 μA in standby (battery sink current).? The XC6240/XC6242 have a fixed VOUT of 2.63 V ± 1.5 % which is an ideal voltage for charging LTO batteries.
The operating temperature range for the XC6240 is -40 °C ~ 85 °C, whereas the XC6242 is -40 °C ~ 105 °C.
For charging a supercap, a LDO with an externally adjustable current limit and reverse current protection is the best solution and for this the XC6230 series is an ideal choice with the XC6230B version (no CL discharge) being the recommended type. The XC6230B operating voltage range is 1.7 V ~ 6.0 V, output voltage is set externally within a range of 1.2 V ~ 5.0V and load currents of up to 2 A can be supported. Current limit is also externally set within a range of 0.3 A ~ 2.5A.???
LTO Battery Voltage Monitor
(6) The XC6140C/XC6142C series are ultra-low power CMOS output voltage detectors that typically consume 104 nA when operating. The detect voltage threshold (VDF) is user selectable between 1.6 V ~ 2.2 V which is suitable for LTO type batteries. Detect Release Voltage (VDR) is fixed at 2.475 V and this is the optimum value for this type of cell.?
Further Reading
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