A new life for linear chargers

Steve Knoth, Linear Technology


One can create an ultra-low consumption current, high voltage linear battery charger

Switchmode battery chargers are popular choices in applications due to their topology flexibility, multi-chemistry charging, high charging efficiencies which minimize heat to enable fast charge times and wide operating voltage ranges. Nevertheless, some downsides of switching chargers include relatively high cost, more complicated inductor-based designs, potential noise generation and larger footprint solutions.

Traditional linear topology battery chargers are often valued for their compact footprints, simplicity and lower cost. Drawbacks of traditional linear chargers have included limited input and battery voltage ranges, higher relative current consumption, excessive power dissipation, limited charge termination algorithms and lower relative efficiency (efficiency ~ [VOUT/VIN] * 100%).

Modern lead acid (LA), wireless power, energy harvesting, solar charging, remote sensor and embedded automotive applications have been traditionally powered by switchmode chargers for the positive reasons stated above; however an opportunity exists for an ultra-low consumption current, high voltage linear battery charger that negates its usually associated drawbacks.

Cutting-edge applications demand more effective linear chargers

Some leading edge application spaces, where innovative, high voltage and ultra-low quiescent current linear chargers can be beneficial include the following:

• Sealed lead acid (SLA) applications with low charge current. Many remote sensor & control applications benefit from the wide temperature range of an SLA battery.

These remote applications are usually very low power and don’t need to charge quickly, therefore low charge currents can be used – they just need to keep the battery topped off.

• For wireless power, charging is done at very low power levels, typically less than 100mW.

• In energy harvesting applications with any micro-powered source, low quiescent current is essential to avoid competing with the downstream load current demands.

• Solar charging has voltages, both from the panel and the battery, that vary widely. For low power applications a linear charger works well.

• Remote sensors for monitoring or control, typically found in low power industrial applications, have batteries used primarily for backup. As a result, charge time is rarely important and input / battery voltages vary widely depending on the specific application. A low IQ linear charger would charger fits well here.

• Embedded automotive applications have input voltages >30V, with some even higher. For example, consider GPS location systems used as anti-theft deterrents; a linear charger with the typical 12V to 2-in-series Li-Ion (7.4V typical) with added protection to much higher voltages would be valuable for these applications.

A novel linear charger solution

It is clear that a linear topology IC charging solution that solves the applications and associated issues already discussed needs to possess many of the following attributes:

• Low quiescent current - more energy is transferred from weak / intermittent input sources to the battery, reducing power dissipation. Further, low battery IQ also extends the lifetime of the battery when charging has terminated and when an input is not present.

• Wide input voltage range to accommodate a variety of power sources

• Wide battery charge voltage range to address multiple battery stacks

• Ability to charge multiple battery chemistries (Lithium, lead acid, Nickel)

• Simple and autonomous operation with  onboard charge termination (no µC needed)

• Input voltage regulation for solar input sources

• Small and low profile solution footprints

• Advanced packaging for improved thermal performance and space efficiency

For example, Linear’s recent LTC4079 linear battery charger has most of these attributes. The LTC4079 is a 60V, constant-current/constant-voltage 250mA multi-chemistry battery charger with low quiescent current (only 4µA while charging); its linear topology offers a simple inductor-less design and accepts a wide 2.7V to 60V input voltage range. A resistor-programmable 1.2V to 60V battery charging voltage range with tight ±0.5% voltage accuracy and onboard adjustable charge termination makes the LTC4079 suitable for many battery chemistries, including Li-Ion/polymer, Li-Iron Phosphate (LiFePO4), Nickel and lead acid.

The charge current is adjustable from 10mA to 250mA via an external resistor and unlike competing chargers the device maintains high accuracy at low charge currents. While 3-stage lead acid charging is possible using a few external components, the relatively low charge current of the LTC4079 makes it more suitable for float charging lead acid batteries. Similarly, no fast charge Nickel termination algorithms are implemented, so Nickel batteries should only be trickled charged with the LTC4079. Applications include embedded automotive and industrial systems, backup battery charging, energy harvesting applications and thin film battery-based products.

The LTC4079’s input voltage regulation function can regulate the IN pin to a constant voltage or to a constant differential voltage above the battery. These features can be used to prevent the input voltage of a current limited power source, such as a weak battery or solar panel from collapsing below the undervoltage lockout (UVLO) voltage. The charge current is reduced as the input voltage falls to the programmed threshold. This regulation mechanism allows the charge current to be selected based on the battery requirement, while letting the LTC4079 take care of situations when the input source cannot provide the full, programmed charge current (see Figure 1).

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Figure 1. LTC4079 Typical Application Circuit for Charging a 7.4V Li-Ion Battery

The LTC4079’s thermal regulation feature ensures maximum charge current up to the specified limit without the risk of overheating. Charging can be terminated via C/10 or an onboard adjustable timer. Other features include NTC thermistor temperature-qualified charging, bad battery detection, and automatic recharge with sampled feedback in standby for negligible battery drain and an open-drain CHRG pin status output. Once the battery is charged, the battery voltage is sampled via a feedback network every 3 seconds to minimize battery drain, thus prolonging battery run time.

Figure 2 shows the LTC4079’s typical full Li-Ion charge cycle, with C/10 charge termination. The LTC4079 is housed in a low profile (0.75mm) 10-pin 3mm x 3mm DFN package with backside metal pad for excellent thermal performance. The device is guaranteed for operation from –40°C to 125°C. Its key features are:

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Figure 2. LTC4079 Li-Ion Charge Cycle

•          Wide Input Voltage Range: 2.7V to 60V

•          Adjustable Battery Voltage: 1.2V to 60V

•          Adjustable Charge Current: 10mA to 250mA

•          Low Quiescent Current While Charging: IIN = 4µA

•          Ultralow Battery Drain When Shutdown or Charged: IBAT < 0.01µA

•          Auto Recharge

•          Input Voltage Regulation for High Impedance Sources

•          Thermal Regulation Maximizes Output Current without Overheating

•          Constant Voltage Feedback with ±0.5% Accuracy

•          NTC Thermistor Input for Temperature Qualified Charging

•          Adjustable Safety Timer

•          Charging Status Indication

•          Thermally Enhanced 10-Lead (3mm × 3mm)  DFN Package

Solar Charging - Input Voltage Regulation & Differential Voltage Regulation

Solar panel-based charging applications are increasing for many varieties of battery chemistries, but Li-Ion/polymer/phosphate and lead acid are the most popular. The LTC4079’s voltage regulation can handle these cases without an issue. The IC can regulate a constant voltage on the IN pin when charging from a current-limited power source such as a weak battery or a solar panel. This feature can be used to prevent the input voltage from collapsing below the UVLO, or to maintain the input source voltage at peak power. The charge current is reduced as the input voltage falls to the threshold set by an external resistor divider from the input power source to the EN pin and GND, as shown in Figure 3.

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Figure 3. Setting LTC4079’s Input Voltage Regulation  

The input voltage regulation threshold, VIN(REG) is calculated as follows:

This regulation mechanism allows the charge current to be selected based on the battery’s requirement and the maximum power available from the charging source. The LTC4079 automatically reduces the charge current when the input source cannot provide the programmed charge current.

The LTC4079’s Differential Voltage Regulation (VIN-VBAT provides an additional method to keep the input voltage from collapsing when the input power comes from a weak power source. If the input voltage falls close to the battery voltage, the differential voltage regulation loop in LTC4079 keeps the input voltage above the battery voltage by 160mV (typical value) by reducing the charge current as the input voltage to battery differential voltage falls. In both of the above regulation conditions, the input source must provide at least the quiescent current of the device to prevent UVLO. The charge timer is paused whenever the charge current is reduced due to input voltage regulation or differential voltage regulation conditions.

Nickel Charging

The LTC4079’s design can also handle Nickel batteries. For nickel-chemistry batteries (e.g. Nickel Cadmium [NiCd] and Nickel metal hydride [NiMH]), the possibility of overcharging must be considered. A typical method is to trickle charge with low currents for a long period of time. Since NiCd and NiMH batteries can absorb a C/300 charge rate indefinitely, shorter duration charging is possible using a timed charge algorithm. It is advisable to charge the battery to no more than 125% of its capacity. For example, a 1000mAh NiMH battery can be charged at a 100mA charge current setting for 12-14 hours. The constant voltage regulation safely tapers the charge current to near zero once the battery reaches its full capacity.

Linear Technology