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Bidirectional DC/DC Regulator and Supercapacitor Charger

Linear Technology » LTC3110, LT8608

Victor Khasiev, Linear Technology

Design Note 558


The LTC®3110 bidirectional buck-boost DC/DC regulator charges and balances a supercapacitor when a bus voltage is present (for example 3.3 V) and discharges the supercapacitor into the load when the bus fails. The LTC3110 maintains the bus’s nominal level (3.3 V in the example in Figure 1) even if the supercapacitor voltage is above or below the nominal bus voltage. Supporting the load this way allows data backup and retention during a power interruption – important in a wide variety of industrial and automotive applications.

Bidirectional DC/DC Regulator and Supercapacitor Charger
Figure 1. LTC3110 Application, VSYS = 3.3 V, VCAP = 5.0 V.

Low Profile Data Backup Power Supply

Industrial and automotive applications are distinguished by their lack of stable, high quality voltage sources. In automotive environments, a 12 V nominal voltage rail can change from 4 V to above 40 V during cold starts and load dumps. The 24 V equipment bus used in factories and plants suffers voltage spikes and brownouts just from cycling electric motors and solenoids.

Industrial applications, however, share something in common with consumer electronics: the trend toward increased portability and mobility. Space is increasingly limited for electronics inside modern industrial equipment. The volume, in particular the thickness, of a data storage unit is at a premium in industrial assemblies and modules. With that in mind, the solution herein aims to minimize the profiles of the selected components. Table 1 shows two passive component group options, arranged by thickness: 1 mm and 2.5 mm. The required semiconductor parts are not listed because the controllers, MOSFETs and resistors do not exceed 1 mm thickness.

Table 1. Converter Components for 1 mm and 2.5 mm Thickness Solutions
Height L1 L2 CS1 CIN1
Vendor Wurth Elektronik Wurth Elektronik CAP-XX TDK
1 mm 74437321022 74437321056 2 × HS103F 2 × C3216JB1H475K085AB
2.5 mm 74437334022 74437334056 HS206F C3225X7R2A225K230AB

Figure 2 shows a low profile energy storage solution for data backup or wireless data transmission in situations where the input voltage changes in an extremely wide range from 4 V to 40 V, with possible input voltage interruption.

Bidirectional DC/DC Regulator and Supercapacitor Charger
Figure 2. LTC3110, High Voltage Application, VIN = 4 V to 40 V, VSYS = 3.3 V, VCAP = 5.0 V.

This solution uses two converters to produce a complete industrial/automotive application. The converter, based on the LTC3110 supercapacitor charger (SCCH), charges the storage capacitor when the input voltage is present and provides a regulated system output during power interruption. Under normal operating conditions, when a 4 V to 40 V input supply is present, sensitive electronic loads and the LTC3110 are supported by a step-down converter (SDC) based on the LT8608.

The LT8608 is a low quiescent current, high efficiency, highly integrated synchronous buck converter featuring a wide input voltage range extending from 3 V to 42 V. Its 42 V upper limit minimizes the volume required for system protection circuitry, practically eliminating the need for traditional transient voltage suppressor diodes (TVS). The SDC maintains the system voltage within 3.3 V bus rail limits.

If the input voltage is disconnected, the SCCH takes over, powering the regulated 3.3 V rail. The LT8608 supplies 3.5 V to the system bus under normal circumstances; the LTC3110 supplies 3.2 V when it registers a power failure and supports the load. These supply voltages are well within the recommended limits for 3.3 V logic power supplies. The /CHRG flag can be used to alert digital systems to disconnect non-essential loads and begin data backup or wireless transmission.

Figure 3 shows the beginning of the switchover process; with the input voltage disconnected, the SDC turns off while the SCCH is turning on.

Bidirectional DC/DC Regulator and Supercapacitor Charger
Figure 3. Switchover [VSYS Load Voltage, VIN Input Voltage, /CHRG Flag Status].


LTC3110 is a highly integrated, high performance supercapacitor charger and balancer, which can be implemented in low profile solutions for data retention and backup in automotive and industrial applications.

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