External DC/DC Converter Completes Battery Charger Circuit

May 1, 2011
A high voltage controller and power manager IC combines with virtually any externally compensated DC/DC power supply IC to create a full-featured battery charger.

The LTC4000 only requires an external DC/DC converter to have a control or external-compensation pin whose voltage level varies in a positive monotonic way with its output. It can drive DC/DC converter topologies including buck, boost, buck-boost, SEPIC and flyback.

A programmable output voltage allows it to charge a variety of battery chemistries including lithium-ion/polymer/phosphate, sealed lead acid (SLA), and nickel-based. The IC also provides charge status indicators through its FLT and CHRG pins. This approach simplifies charger system applications, reduces the number of required ICs, and enables faster re-design as requirements change.

Among its other features are:

  • Programmable input and charge current with ±1% accuracy
  • Accurate programmable float voltage (±0.25% at room and ±1% over temperature)
  • Programmable C/X or timer based charge termination
  • Temperature-qualified charging using an NTC thermistor
  • Automatic recharge
  • C/10 trickle charge for deeply discharged cells
  • Bad battery detection

Fig. 1 shows a simplified typical LTC4000 configuration using an LT3845, 15V to 60V input range, synchronous DC/DC step-down controller with adjustable 100kHz to 500kHz operating frequency. The LTC4000 provides precision input and charge current regulation and operates across a 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.

The LTC4000 is housed in a low profile (0.75mm) 28-pin 4mm × 5mm QFN package and a 28-lead SSOP package. The IC is guaranteed for operation from -40°C to 125°C.


LTC4000's four different regulation loops ensure proper battery charging and system power management:

  • Input current loop uses an input current sense resistor and a resistor at IL to ensure that the IC does not exceed the programmed input current limit.
  • Charge current loop uses a current sense and a resistor at CL (24.9k) to ensure that it does not exceed the programmed battery charge current limit.
  • Battery float voltage loop uses a resistor divider from BAT to FBG via BFB (133k and 1.13M) to ensure that it does not exceed the programmed battery stack voltage.
  • Output voltage loop uses a resistor divider from CSP to FBG via OFB (1.15M and 127k) to ensure that it does not exceed programmed system output voltage. The output voltage regulation loop regulates the voltage at the CSP pin so that the output feedback voltage at the OFB pin is 1.193V.

Control of the external DC/DC converter is assumed by the loop that requires the lowest voltage on the LT3845's VC pin for its regulation. Completing the interconnection to the LT3845 is a line from its SHDN pin to the LTC4000's RST pin.

With limited input power, the LTC4000 employs an intelligent PowerPath™ topology that preferentially supplies power to the system load. 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 ranging from milliwatts to kilowatts. The LTC4000 has monitoring pins for the input current and charge current at the IIMON and IBMON pins, respectively.

Trickle charge mode drops the charge current to one- tenth of the full-charge current when charging into an over-discharged dead battery. Input current is first programmed by a sense resistor; the resistor on CL can modify that value. When trickle charging, a capacitor on the TMR pin can be used to program a bad battery 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 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 would otherwise cause unnecessary drain on the battery and could also result in unexpected DC/DC converter behavior. The ideal diode behavior is achieved by controlling an external PMOS connected to the IID pin (drain) and the CSP pin (source). The controller (A1) regulates the external PMOS by driving the gate of the PMOS device such that the voltage drop across IID and CSP is 8mV (typical). When the external PMOS ability to deliver a particular current with an 8mV drop across its source and drain is exceeded, the voltage at the gate clamps at VIGATE(ON) and the PMOS behaves like a fixed value resistor (RDS(ON)).


The LTC4000's NTC pin provides temperature qualified charging when connected to an NTC (negative temperature coefficient) 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 CHRG pins.

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. The LTC4000 features an automatic recharge cycle if the battery voltage drops below 97.6% of the programmed float voltage.

Charge termination can be configured with the TMR pin in several ways. If the TMR pin is tied to the BIAS pin, C/X termination is selected. In this case, charging is terminated when constant voltage charging reduces the charge current to the C/X level programmed at the CX pin. Connecting a capacitor to the TMR pin selects the charge timer termination and a charge termination timer is started at the beginning of constant voltage charging.

Charging terminates when the termination timer expires. When continuous charging at the float voltage is desired, tie the TMR pin to GND to disable termination.

Upon charge termination, the PMOS connected to BGATE behaves as an ideal diode from BAT to CSN. The diode function prevents charge current but provides current to the system load as needed. If the system load can be completely supplied from the input, the battery PMOS turns off. While terminated, if the input current limit is not in regulation, the output voltage regulation loop takes over to ensure that the output voltage at CSP remains in control.

To aid development and evaluation of the battery charger circuit for the LTC4000, a demo board (Fig. 2) is available for the converter.


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