Bidirectional DC/DC Regulator and Supercapacitor Charger

Design Note DN558: Introduction

The LTC3110 bidirectional buck-boost DC/DC regulator charges and balances a supercapacitor when a bus voltage is present (for example 3.3V) and discharges the supercapacitor into the load when the bus fails. The LTC3110 maintains the bus’s nominal level (3.3V 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.

Figure 1. LTC3110 Application, VSYS = 3.3V, VCAP = 5.0V

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 12V nominal voltage rail can change from 4V to above 40V during cold starts and load dumps. The 24V 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: 1mm and 2.5mm. The required semiconductor parts are not listed because the controllers, MOSFETs and resistors do not exceed 1mm thickness.

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 4V to 40V, with possible input voltage interruption.

Figure 2. LTC3110, High Voltage Application, VIN = 4V to 40V, VSYS = 3.3V, VCAP = 5.0V Figure 3. Switchover [VSYS Load Voltage, VIN Input Voltage, CHRG Flag Status] 1ms/

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 4V to 40V 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 3V to 42V. Its 42V 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 3.3V bus rail limits.

If the input voltage is disconnected, the SCCH takes over, powering the regulated 3.3V rail. The LT8608 supplies 3.5V to the system bus under normal circumstances; the LTC3110 supplies 3.2V when it registers a power failure and supports the load. These supply voltages are well within the recommended limits for 3.3V 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.

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.

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

Victor Khasiev

Victor Khasiev

Victor Khasievは、アナログ・デバイセズのシニア・アプリケーション・エンジニアです。パワー・エレクトロニクスの分野を担当しており、AC/DC変換とDC/DC変換の両方に関する豊富な経験を持ちます。また、車載用途や産業用途をターゲットとするアナログ・デバイセズのIC製品の使い方に関して、複数の記事を執筆しています。それらの記事では、昇圧、降圧、SEPIC、反転、負電圧、フライバック、フォワードに対応するコンバータや、双方向バックアップ電源などを取り上げています。効果的な力率改善の手法と高度なゲート・ドライバに関して2件の特許を保有しています。日々の業務では、製品に関する質問への回答を通じた顧客のサポート、電源回路の設計/検証、プリント回路基板のレイアウト、システムの最終テストの実行とトラブルシューティングなどに取り組んでいます。