Design Article
Charging high capacity batteries from 5V sources
David Simmons, Linear Technology Corp.
11/6/2012 10:30 AM EST
Seamless handling of multiple input connectors
The LTC4155 optionally accepts input from two power sources, solving the challenge of intelligently routing power from two different physical connectors to the product. When both input sources are connected simultaneously, the decision of which source to use is based on a user-programmable priority. As long as each input voltage is within the valid operating range, either one may be selected without concern for which voltage is higher than the other. This allows, for instance, a 4.5V/2A AC adapter to be favored over a 5V/500mA USB port. If the USB connection is removed and a 5V/3A AC adapter is connected to the same port, the input source priority can be modified over I2C to switch to the new higher power source.
The LTC4155 supports independent I2C programmable input current limits for each of its two power inputs. When the higher priority input source is disconnected, charging can continue uninterrupted, with automatic reduction to the new lower maximum input current limit. No immediate attention is required from the system microcontroller.
Depending on the external components selected for the input multiplexer, overvoltage and reverse voltage protection up to ±77V can be easily implemented if required for the application. Additionally, the LTC4155 can produce a USB On-The-Go 5V current-limited supply to the USB connector using no additional external components.
Programmability and telemetry for advanced charging algorithms
The LTC4155 provides continuous I2C status reporting, allowing system software to have a complete view of the state of input power sources, fault conditions, battery charge cycle state, battery temperature, and several other parameters.
Key charge parameters can be changed under I2C control to implement customized charge algorithms. Unlike microcontroller-based or other programmable charge algorithms, all possible LTC4155 settings available under software I2C control are intrinsically safe for the battery. Float voltage can never be programmed above 4.2V or below 4.05V. Similarly, battery charge current is programmable to one of 15 possible settings, but software may never increase the limit above the level set by the designer—via a programming resistor chosen to match the battery capacity and maximum charge rate.
Continuous battery temperature data is available to system software to dynamically adapt system or charger behavior to manage extreme operational corners. For instance, float voltage and/or charge current may be reduced under I2C control to increase the battery safety margin at high ambient temperatures. Similarly, charge current or total system load current can be reduced in response to high temperature to reduce additional heating within the product enclosure.
Like all other aspects of battery charger programmability, the LTC4155 implements an intrinsically safe charging solution without (or despite) any software intervention. Battery charging is always paused when the cell temperature falls below 0°C or rises above 40°C. Additionally, a fault interrupt may be optionally generated whenever cell temperature rises above 60°C. Figure 4 shows the transfer function of the LTC4155 battery temperature data converter, with the autonomous charger cut-out temperature thresholds highlighted.
The LTC4155 optionally accepts input from two power sources, solving the challenge of intelligently routing power from two different physical connectors to the product. When both input sources are connected simultaneously, the decision of which source to use is based on a user-programmable priority. As long as each input voltage is within the valid operating range, either one may be selected without concern for which voltage is higher than the other. This allows, for instance, a 4.5V/2A AC adapter to be favored over a 5V/500mA USB port. If the USB connection is removed and a 5V/3A AC adapter is connected to the same port, the input source priority can be modified over I2C to switch to the new higher power source.
The LTC4155 supports independent I2C programmable input current limits for each of its two power inputs. When the higher priority input source is disconnected, charging can continue uninterrupted, with automatic reduction to the new lower maximum input current limit. No immediate attention is required from the system microcontroller.
Depending on the external components selected for the input multiplexer, overvoltage and reverse voltage protection up to ±77V can be easily implemented if required for the application. Additionally, the LTC4155 can produce a USB On-The-Go 5V current-limited supply to the USB connector using no additional external components.
Programmability and telemetry for advanced charging algorithms
The LTC4155 provides continuous I2C status reporting, allowing system software to have a complete view of the state of input power sources, fault conditions, battery charge cycle state, battery temperature, and several other parameters.
Key charge parameters can be changed under I2C control to implement customized charge algorithms. Unlike microcontroller-based or other programmable charge algorithms, all possible LTC4155 settings available under software I2C control are intrinsically safe for the battery. Float voltage can never be programmed above 4.2V or below 4.05V. Similarly, battery charge current is programmable to one of 15 possible settings, but software may never increase the limit above the level set by the designer—via a programming resistor chosen to match the battery capacity and maximum charge rate.
Continuous battery temperature data is available to system software to dynamically adapt system or charger behavior to manage extreme operational corners. For instance, float voltage and/or charge current may be reduced under I2C control to increase the battery safety margin at high ambient temperatures. Similarly, charge current or total system load current can be reduced in response to high temperature to reduce additional heating within the product enclosure.
Like all other aspects of battery charger programmability, the LTC4155 implements an intrinsically safe charging solution without (or despite) any software intervention. Battery charging is always paused when the cell temperature falls below 0°C or rises above 40°C. Additionally, a fault interrupt may be optionally generated whenever cell temperature rises above 60°C. Figure 4 shows the transfer function of the LTC4155 battery temperature data converter, with the autonomous charger cut-out temperature thresholds highlighted.
Figure 4: Transfer function of the LTC4155 battery temperature data converter, with the autonomous charger cut-out temperature thresholds highlighted.
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