Author:
Reported by Cliff Keys, Editorial Director, Editor-in-Chief, Power Systems Design
Date
04/17/2011
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I had the recent pleasure to talk with Tony Armstrong, Linear's Director of Product Marketing for Power Products, about the company's latest high voltage controller and power manager, the LTC4000, that converts virtually any externally compensated DC/DC power supply into a full-featured battery charger. The LTC4000 is capable of driving typical DC/DC converter topologies, including buck, boost, buck-boost, SEPIC and flyback. The device offers precision input and charge current regulation and operates across a wide 3V to 60V input and output voltage range, compatible with a variety of different input voltage sources, battery stacks and chemistries. Typical applications include high power battery charger systems, high performance portable instruments, battery back-up systems, industrial battery-equipped devices and notebook/subnotebook computers.
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The LTC4000 features an intelligent PowerPath™ topology that preferentially provides power to the system load when input power is limited. The LTC4000 controls external PFETs to provide low loss reverse current protection, efficient charging and discharging of the battery and instant-on operation to ensure that system power is available at plug-in even with a dead or deeply discharged battery. External sense resistors and precision sensing enable accurate currents at high efficiency, allowing the LTC4000 to work with converters that span the power range from milliwatts to kilowatts. Detailed overview The LTC4000 is designed to simplify the conversion of any externally compensated DC/DC converter into a high performance battery charger with PowerPath control. It only requires the DC/DC converter to have a control or external-compensation pin (usually named VC or ITH) whose voltage level varies in a positive monotonic way with its output. The output variable can be either output voltage or output current. The LTC4000 includes four different regulation loops: input current, charge current, battery float voltage and output voltage (A4-A7). Whichever loop requires the lowest voltage on the ITH pin for its regulation controls the external DC/DC converter. The input current regulation loop ensures that the programmed input current limit (using a resistor at IL) is not exceeded at steady state. The charge current regulation loop ensures that the programmed battery charge current limit (using a resistor at CL) is not exceeded. The float voltage regulation loop ensures that the programmed battery stack voltage (using a resistor divider from BAT to FBG via BFB) is not exceeded. The output voltage regulation loop ensures that the programmed system output voltage (using a resistor divider from CSP to FBG via OFB) is not exceeded. The LTC4000 also provides monitoring pins for the input current and charge current at the IIMON and IBMON pins respectively. The LTC4000 features an ideal diode controller at the input from the IID pin to the CSP pin and a PowerPath controller at the output from the BAT pin to the CSN pin. The output PowerPath controller behaves as an ideal diode controller when not charging. When charging, the output PowerPath controller has two modes of operation. If VOFB is greater than VOUT(INST_ON), BGATE is driven low. When VOFB is less than VOUT(INST_ON), a linear regulator implements the instant-on feature. This feature provides regulation of the BGATE pin so that a valid voltage level is immediately available at the output when the LTC4000 is charging an over-discharged, dead or short faulted battery. The state of the ENC pin determines whether charging is enabled. When ENC is grounded, charging is disabled and the battery float voltage loop is disabled. Charging is enabled when the ENC pin is left floating or pulled high (?1.5V). The LTC4000 offers several user configurable battery charge termination schemes. The TMR pin can be configured for either C/X termination, charge timer termination or no termination. After a particular charge cycle terminates, the LTC4000 features an automatic recharge cycle if the battery voltage drops below 97.6% of the programmed float voltage. Trickle charge mode drops the charge current to one tenth of the normal charge current (programmed using a resistor from the CL pin to GND) when charging into an over discharged or dead battery. When trickle charging, a capacitor on the TMR pin can be used to program a time out period. When this bad battery timer expires and the battery voltage fails to charge above the low battery threshold (VLOBAT), the LTC4000 will terminate charging and indicate a bad battery condition through the status pins (FLT and CHRG). The LTC4000 also includes an NTC pin, which provides temperature qualified charging when connected to an NTC thermistor thermally coupled to the battery pack. To enable this feature, connect the thermistor between the NTC and the GND pins, and a corresponding resistor from the BIAS pin to the NTC pin. The LTC4000 also provides a charging status indicator through the FLT and the CHRG pins. Aside from biasing the thermistor-resistor network, the BIAS pin can also be used for a convenient pull up voltage. This pin is the output of a low dropout voltage regulator that is capable of providing up to 0.5mA of current. The regulated voltage on the BIAS pin is available as soon as the IN pin is within its operating range (?3V). Input Ideal Diode The input ideal diode feature provides low loss conduction and reverse blocking from the IID pin to the CSP pin. This reverse blocking prevents reverse current from the output (CSP pin) to the input (IID pin) which causes unnecessary drain on the battery and in some cases may result in unexpected DC/DC converter behavior. The LTC4000's full-featured controller charges a variety of battery chemistries including lithium-ion/polymer/phosphate, sealed lead acid (SLA), and nickel-based. The device also provides charge status indicators through its FLT and CHRG pins. Other features of the battery charger include: ±0.2% programmable float voltage, selectable timer or C/X current termination, temperature-qualified charging using an NTC thermistor, automatic recharge, C/10 trickle charge for deeply discharged cells and bad battery detection.
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Summary of Features: LTC4000
