Battery Charger's Unique Input Regulation Loop Simplifies Solar Panel Maximum Power Point Tracking

Advances in battery technology and improvements in device performance have made it possible to build complex electronics that can operate for long periods of time between charges. Even so, for some devices, recharging the battery by plugging into an electrical outlet on the grid is sometimes impossible. Roadside emergency phones, navigation buoys, and remote weather monitoring stations are just a few of the applications that don’t have access to the grid and therefore must harvest energy from the surrounding environment.

Solar panels have great potential as energy harvesting power sources, they only need batteries to store the harvested energy and continue to provide power when the light is dim. Solar panels are relatively expensive, so extracting the maximum power from the panels is critical to minimizing the size of the panels. The trickier question is how to balance the size of the solar panel with the power required. The characteristics of solar panels require that the panel output power, which varies with load, be carefully managed to effectively optimize the panel output power under a variety of different lighting conditions.

Solar panels have a specific operating point that produces the most power for a given amount of illumination (see Figure 1). The practice of maintaining operation at this peak power point as lighting conditions change is called maximum peak power point tracking (MPPT). To perform the MPPT function, complex algorithms are often required, such as periodically changing the load on the panel while directly measuring the panel's output voltage and output current, calculating the panel's output power, and then forcing operation at an operating point that provides peak output power as lighting and/or temperature conditions change. Such algorithms typically require complex circuitry and microprocessor control methods.

Figure 1: Current vs. voltage and power vs. voltage for a solar panel at different lighting levels.
The panel's output voltage remains relatively constant at the maximum power point (V _{MP ) regardless of lighting level.}

However, there is an interesting relationship between the output voltage of a solar panel and the power produced by that panel. At the maximum power point, the output voltage of a solar panel remains relatively constant regardless of the amount of illumination. Therefore, forcing the output voltage to remain at this peak power voltage (V _MP ) while the panel is operating will allow the panel to produce peak output power. Therefore, by taking advantage of this V _MP characteristic, rather than using complex maximum peak power tracking circuits and algorithms, the battery charger can maintain peak power delivery.

The LT3652 is a complete monolithic step-down charger for multiple battery chemistries that operates from an input voltage up to 32V (40V absolute maximum) and charges battery packs with float voltages up to 14.4V. The LT3652 contains
an innovative input regulation circuit that uses a simple and automatic method to control the charger's input supply voltage, which is useful when using a power source with poor stability (such as a solar panel). The LT3652HV is a high voltage version of the charger that can charge battery packs with float voltages up to 18V.

Input Regulation Loop Keeps Solar Panel Operating at Peak Power Point
The LT3652’s input regulation loop linearly reduces the output battery charge current if the input supply voltage drops toward the programmed value. This closed-loop regulation circuit follows the charge current, and therefore the input supply load, so that the input supply voltage is maintained at or above the programmed value. When powered by a solar panel, the LT3652 can be operated in maximum peak power tracking mode by simply setting the minimum input voltage value equal to the panel’s peak supply voltage, V _MP . The desired peak power voltage is set by a resistor divider.

If the power required by the LT3652 for charging exceeds the power available from the solar panel, the LT3652's input regulation loop reduces the charge current accordingly. This may occur because the desired battery charge current increases, or because the solar panel illumination decreases. In either case, the regulation loop maintains the solar panel input voltage equal to the set V _MP , as set by the resistor divider on the VIN_REG pin.

This input regulation loop is a simple and elegant way to force a particular solar panel to operate at its peak power point. This input voltage regulation loop can also be used to optimize operation when using other power sources with poor stability, such as where the input supply may collapse under overcurrent conditions.

The integrated and full-featured battery charger
LT3652 operates at a fixed 1MHz switching frequency with a constant current/constant voltage (CC/CV) charging characteristic. The device can be programmed with an external resistor to provide up to 2A of charging current with a charging current accuracy of ±5%. The IC is particularly suitable for the voltage range associated with popular and inexpensive "12V system" solar panels, which have a typical open circuit voltage of approximately 25V.

The charger uses a 3.3V float voltage feedback reference, so a resistor divider can be used to set the desired battery float voltage in the range of 3.3V to 14.4V (up to 18V when using the LT3652HV). The float voltage feedback accuracy of the LT3652 is ±0.5%. The wide output voltage range of the LT3652 is suitable for many battery chemistries and configurations, including up to 3 series Li-Ion/Polymer batteries, up to 4 series LiFePO4 (lithium iron phosphate) batteries, and up to 6 series sealed lead acid (SLA) batteries. A high voltage version of the charger, the LT3652HV, is also available. The LT3652HV operates with an input voltage of up to 34V and can charge to an 18V float voltage, suitable for 4-cell Li-Ion/Polymer or 5-cell LiFePO4 battery packs.

The LT3652 contains a programmable safety timer to terminate charging after the desired time has been reached. The timer can be started by simply connecting a capacitor to the TIMER pin. By shorting the TIMER pin to ground, the LT3652 can be configured to terminate charging when the charge current drops below 10% (C/10) of the programmed maximum value, with a C/10 detection accuracy of ±2.5%. Termination using a safety timer allows Top-Off charging when the current is below C/10. Once charging is terminated, the LT3652 enters a low current (85µA) standby mode. If the battery voltage drops below 2.5% of the programmed float voltage, the automatic recharge function initiates a new charge cycle. The LT3652 is available in flat, 12-lead 3mm x 3mm DFN and MSOP packages.

The LT3652 has a precise threshold shutdown pin that allows simple implementation of undervoltage lockout using a resistor divider. When in low current shutdown mode
, the LT3652 draws only 15µA from the input supply. The IC also supports temperature-qualified charging by monitoring the battery temperature using a thermistor connected to the device's NTC pin. The device has two binary-coded open-collector status pins that show the operating status of the LT3652 battery charger, /CHRG and /FAULT. These status pins can drive LEDs to signal the charger status visually, or can be used as logic-level signals for control systems.

Simple Solar-Powered Battery Charger
Figure 2 shows a 2A two-cell LiFePO4 battery charger with Power Path management _. This circuit supplies power to the system load from the battery when the solar panel is not fully illuminated and directly from the solar panel when the solar panel can provide the required power to the system load. The input voltage regulation loop is set for a solar panel with a 17V peak power input. The charger uses C/10 termination, so the charging circuit is disabled when the required battery charge current drops below 200mA. The LT3652 charger also uses two LEDs for status and fault signals. These binary coded pins signal battery charging, standby or shutdown mode as well as battery temperature fault and bad battery fault.

Figure 2: A 2A solar panel power manager with 17V peak power tracking and for two
LiFePO4 _batteries

The input voltage regulation point is set with a resistor divider between the solar panel output and the VIN_REG pin. The maximum output charging current is reduced when the solar panel output drops sharply toward 17V, which corresponds to 2.7V at the VIN_REG pin. This servo loop acts in this way to dynamically reduce the power demand of the charger system to the maximum power that the solar panel can provide, thereby maintaining the power utilization of the solar panel close to 100%, as shown in Figure 3.

Figure 3: The 17V input voltage regulation threshold tracks the solar panel peak power to over 98%

Want higher efficiency? Replace the blocking diode with a blocking FET
Using the LT3652 with battery voltages above 4.2V requires a blocking diode. The voltage drop across this diode creates a power loss term that reduces charging efficiency. This power loss term can be greatly reduced by replacing the blocking diode with a P-channel FET, as shown in Figure 4.

Figure 4: A 2A three-cell LiFePO ₄ charger uses P-channel FETs for input isolation to improve high-current charging efficiency.

Figure 4 shows a 3-cell LiFePO4 2A charger with a 10.8V float voltage. This charger has a 14.5V input voltage regulation threshold and is enabled by the SHDN pin when V _IN ≥ 13V. The termination of the charge cycle is controlled by a 3-hour timer period. When used, the blocking diode, which is usually connected in series with the input supply for reverse voltage protection, is replaced by a FET. In addition, a 10V Zener diode is used to implement a clamp to prevent exceeding the V _GS maximum of the FET. If the specified V _IN range does not exceed the V _GS maximum of the input FET, then this clamp is not required.

During high current charging in a normal charge cycle (I _CHG > C/10), the /CHRG status pin is held low. In the charger shown in Figure 4, this /CHRG signal is used to pull the gate of the isolation FET low, thereby achieving a low impedance power path without the isolation diode voltage drop, which improves conversion efficiency. Figure 5 shows that adding this isolation FET improves efficiency by 4% compared to operating with a Schottky isolation diode.

Figure 5: Efficiency comparison of Schottky isolation diode and isolation FET when charging a 10.8V three-cell LiFePO4 battery _with a 15V input

If the timer is used to achieve termination, once a charge current of < C/10 is reached, the body diode of the FET provides a conduction path and the /CHRG pin becomes high impedance. If necessary, the Schottky isolation diode can be left in parallel with the isolation FET to improve conversion efficiency during the Top-Off portion of the timer controlled charge cycle. Using FETKEY as an isolation component also improves Top-Off efficiency.

Afraid of the Dark? Use an Ideal Diode for Low-Light Applications
When the LT3652 is actively charging, the IC presents an internal load to the switching loop to ensure closed-loop operation under all conditions. This internal load is provided by allowing the BAT pin to sink 2mA whenever a charge cycle is active. In a solar panel powered battery charger, low-light conditions can cause the input solar panel voltage to drop sharply below the input regulation threshold, causing the output charge current to drop to zero. If the charger remains enabled during this condition (i.e., the panel voltage remains above the UVLO threshold), the internal battery load results in a net current drain from the battery. This is obviously undesirable, but fortunately, this condition can be eliminated by using a unidirectional pass component to prevent current from flowing back from the battery.

Linear Technology has created a high efficiency pass component IC, the LTC4411 ideal diode, which has an effective forward voltage drop of nearly zero. Since the forward voltage drop of the device is very low when conducting, the impact on the overall efficiency of the charger and the final float voltage is negligible.

Figure 6 shows an LT3652 solar-powered battery charger that uses an LTC4411 ideal diode IC for low-light reverse protection. In low-light conditions, if the panel voltage drops sharply below the input regulation threshold, the LT3652 reduces the battery charge current to zero. As long as the input voltage remains above the UVLO threshold, the charger remains enabled but remains in a zero charge current state. The LT3652 attempts to sink 2mA into the BAT pin, but the LTC4411 prevents reverse conduction from the battery.

Figure 6: Solar-powered 2A Li-Ion battery charger with ideal diode output pass element;
the LTC4411 ideal diode IC prevents reverse conduction in low-light conditions

Need a boost? No problem. A two-stage buck-boost battery charger
The LT3652 can be used in both boost and boost/buck charger applications using a front-end boost DC/DC converter. The front-end converter generates a local high voltage supply for the LT3652 to use as an input supply. The LT3652’s input regulation loop works perfectly when two converters are used.

Figure 7 shows a 1.5A single-cell Li-Ion battery charger powered by a low voltage solar panel and with a 4.2V float voltage. The charger is designed to operate with a solar panel with a peak power voltage of 3.8V.

Figure 7: A low voltage solar panel powers a 1.5A single-cell Li-Ion buck/boost charger.
The LT3479 boosts the 3.8V output of the solar panel to operate the LT3652 charger.
The LT3652’s closed-loop operating system includes a boost converter to regulate the LT3479’s input to the solar panel’s 3.8V V _MP .

An LT3479 step-up switching converter operating at 1MHz is used in the front end to generate an 8V supply that powers the LT3652. The charger operates with input voltages as low as the input regulation threshold of 3.8V and up to the LT3479 maximum input voltage of 24V. When the input voltage approaches (or exceeds) 8V, the LT3479 step-up converter is no longer regulated and ends up operating at 0% duty cycle, effectively shorting the input supply through the Schottky pass diode to the LT3652. Because the input regulation loop monitors the LT3479 input, the LT3652 reduces the charge current when the input voltage drops sharply toward the input regulation threshold, thereby reducing the current demand of the LT3479 step-up converter. The input voltage varies with this regulation point while the step-up converter and LT3652 charger together extract peak power from the solar panel.

Need more charge current? Use more LT3652 chargers
Multiple LT3652 chargers can be used in parallel to create a charger with charge current capability exceeding that of a single LT3652. In the application shown in Figure 8, a network of three 2A LT3652 chargers are connected in parallel to create a 6A, 12.3V float voltage, 3-cell Li-Ion charger with C/10 termination. This charger is compatible with solar power sources and has a 20V input regulation threshold. This charger also uses an input blocking FET to improve charging efficiency.

Figure 8: 6A three-cell Li-Ion charger using three LT3652 charger ICs

The three LT3652 charger ICs share a common floating voltage feedback network and a common input regulation network. It is recommended to use a feedback network with a 250kΩ equivalent resistance to compensate for the input bias current into the LT3652 V _FB pin. Since the three LT3652s in this charger share the same feedback network, the input bias current is also shared through the network, so the network equivalent resistance is reduced to 250kΩ/3, which is about 83kΩ.

Due to reference voltage tolerance, it is possible for one of these ICs to power up before the others during auto-recharge. In this case, the battery automatically recharges at a maximum current of 2A. If the battery continues to discharge due to a load >2A, the second charger is engaged. The larger discharge current will cause the third charger IC to also be engaged, allowing it to generate the full 6A system charge current. The /CHRG pins of all LT3652s are tied together to enable the input isolation FET, which presents a low impedance, regardless of the order in which the ICs auto-restart.

The three LT3652s share the NTC and status functions, while each IC uses a dedicated NTC thermistor. The open collector status pins of these ICs are shorted together so that the /CHRG status indicator will light up when any or all chargers are enabled. Similarly, an NTC fault in any IC will cause the /FAULT status indicator to light up. The NTC functions of each LT3652 are subordinate to each other, and this subordination is achieved through a diode connected between the common /FAULT pin and the common VIN_REG pin of all three ICs. If any IC has an NTC fault, the diode pulls the VIN_REG pin down below the VIN_REG threshold, which will disable all output charging currents until the temperature fault condition is removed.

Conclusion
The LT3652 is a versatile platform for simple and efficient solar-powered battery charger solutions for a wide variety of battery chemistries and configurations. The LT3652 is equally well suited for applications powered by conventional means at home, providing a small, efficient charging solution for a wide range of battery chemistries and battery stack voltages.

These solar powered charger solutions maintain near 100% utilization of the solar panel, thereby reducing solution cost by minimizing panel area. The compact size of the IC coupled with minimal external component requirements allows for the construction of a standalone charger system that is both tiny and inexpensive, providing a simple and efficient solution for making portable electronics truly independent of the grid.