The LT3652 input regulation loop linearly reduces the output battery charge current if the input supply voltage falls toward a programmed level. This closed-loop regulation circuit servos the charge current, and thus the load on the input supply, such that the input supply voltage is maintained at or above a programmed level. When powered by a solar panel, the LT3652 implements MPPT operation by simply programming the minimum input voltage level to that panel’s peak power voltage, VMP. The desired peak-power voltage is programmed via a resistor divider.
If during charging, the power required by the LT3652 exceeds the available power from the solar panel, the LT3652 input regulation loop servos the charge current lower. This might occur due to an increase in desired battery charge current or drop in solar panel illumination levels. In either case the regulation loop maintains the solar panel output voltage at the programmed VMP as set by the resistor divider on VIN_REG.
The input regulation loop is a simple and elegant method of forcing peak power operation from a particular solar panel. The input voltage regulation loop also allows optimized operation from other types of poorly regulated sources, where the input supply can collapse during overcurrent conditions.
Integrated, full-featured battery charger
The LT3652 operates at a fixed switching frequency of 1MHz, and provides a constant-current/constant-voltage (CC/CV) charge characteristic. The part is externally resistor-programmable to provide charge current up to 2A, with charge-current accuracy of ±5%. The IC is particularly suitable for the voltage ranges associated with popular and inexpensive “12V system” solar panels, which typically have open-circuit voltages around 25V.
The charger employs a 3.3V float voltage feedback reference, so any desired battery float voltage from 3.3V to 14.4V (or up to 18V with the LT3652HV) can be programmed with a resistor divider. The float-voltage feedback accuracy for the LT3652 is ±0.5%. The wide LT3652 output voltage range accommodates many battery chemistries and configurations, including up to three Li-ion/polymer cells in series, up to four LiFePO4 (lithium iron phosphate) cells in series, and sealed lead acid (SLA) batteries containing up to six cells in series. The LT3652HV, a high-voltage version of the charger, is also available. The LT3652HV operates with input voltages up to 34V and can charge to float voltages of 18V, accommodating 4-cell Li-ion/polymer or 5-cell LiFePO4 battery stacks.
The LT3652 contains a programmable safety timer used to terminate charging after a desired time is reached. Simply attaching a capacitor to the TIMER pin enables the timer. Shorting the TIMER pin to ground configures the LT3652 to terminate charging when charge current falls below 10% of the programmed maximum (C/10), with C/10 detection accuracy of ±2.5%. Using the safety timer for termination allows top-off charging at currents less than C/10. Once charging is terminated, the LT3652 enters a low-current (85µA) standby mode. An auto-recharge feature starts a new charging cycle if the battery voltage falls 2.5% below the programmed float voltage. The LT3652 is packaged in low-profile, 12-lead 3mm × 3mm DFN and MSOP packages.
Energy saving, low quiescent current shutdown
The LT3652 employs a precision-threshold shutdown pin, allowing simple implementation of undervoltage lockout functions using a resistor divider. While in low-current shutdown mode, the LT3652 draws only 15µA from the input supply. The IC also supports temperature-qualified charging by monitoring battery temperature using a single thermistor attached to the part’s NTC pin. The device has two binary coded open-collector status pins that display the operational state of the LT3652 battery charger, /CHRG and /FAULT. These status pins can drive LEDs for visual signaling of charger status, or be used as logic-level signals for control systems.
It's a simple, elegant approach covered by quite a few papers. If it has a downside, it's that you do need to _know_ what the "maximum power voltage" parameter is for your panel. Maybe this can be acquired in production line calibration. Solar panels are rather variable from batch to batch, so you can't just fit and forget this sort of electronics to an arbitrary panel. The extra complexity of a routine that goes looking for the maximum power point does reduce this concern. but for 'emergency' operation, the last few percentage points of conversion efficiency aren't your main concern.
Great article with a host of practical circuit suggestions. I can see using this approach on a handheld battery-based instrument used in the field. A solar charging option is a perfect feature when you can't get back to 'base camp'.