How to choose a powerful, wide input range battery charger?

The charger can be easily optimized for a variety of battery chemistries. For example, it can follow a constant current/constant voltage (CV/CC) charging algorithm with C/10 or timed termination (for nickel-based battery systems); a constant current (CC) characteristic with timed termination; or an optimized 4-step, 3-stage lead-acid battery charging profile.

Figure 1 shows a 15V to 55V input, 25.2V/6.3A buck-boost battery charger with a high-efficiency four-switch (M2~M5) synchronous buck-boost DC/DC converter that requires only one inductor (L1). The proprietary average current mode architecture uses two sense resistors (R _CBRT1 and R _CBRB1 ) to monitor the inductor current. In this buck-boost solution, when V _IN is higher than V _OUT , the converter operates in buck mode; when V _IN is lower than V _OUT , the converter operates in boost mode. When V _IN is close to V _OUT , the converter operates in four-switch buck-boost mode.

Figure 1: 15V to 55V Input, 25.2V/6.3A Buck-Boost Battery Charger

Figure 2: Efficiency vs. load current I _OUT for the converter in Figure 1 (V _OUT = 25.2V)

The converter operates at a programmable constant switching frequency between 50kHz and 500kHz, set with a single resistor (R13 = 100k, 250kHz). This solution, shown in Figure 1, is capable of delivering up to 8A to the system load (VOUT ₌ 25.2V). As shown in Figure 2, full load efficiency (IOUT ₌ 8A, _VIN = 24V) is over 98%.

The LTC4020 uses an external feedback resistor divider from the BAT pin to set the battery voltage through the V _FB pin. The PowerPath™ FET (M1) is on during normal battery charging, forming a low impedance connection between the battery and the buck-boost converter output when possible. The battery charge current is monitored through a sense resistor (R _CBAT1 ). The maximum average battery charge current can be easily set by selecting the value of R _CBAT1 . Dynamic current limit adjustment can be achieved through the RNG/SS pin.

For a severely discharged battery, the LTC4020 can automatically configure the PowerPath FET (M1 in Figure 1) as a linear regulator, allowing the buck-boost converter output to rise above the battery voltage while still providing charging current to the battery. This feature is called PowerPath instant-on, and the PowerPath FET acts as a high impedance current source, responsible for providing charging current to the battery.

When the battery charger is not in a charge cycle (i.e., the buck-boost converter is operating exclusively for the system load), the LTC4020 automatically configures the PowerPath FET as an ideal diode. This allows the battery to remain disconnected from the converter output during normal operation. However, if the system load current exceeds the buck-boost converter's ability to supply, additional power can be effectively drawn from the battery through the ideal diode.

• The LTC4020 supports a timer-based charging algorithm where a capacitor connected between the TIMER pin and ground programs the completion of the charge cycle.

• The LTC4020 features battery temperature monitoring and control. By connecting an NTC (negative temperature coefficient) thermistor to the NTC pin and placing the thermistor near the battery pack (or other monitoring location required), if the NTC pin voltage is out of range (above 1.35V or below 0.3V), the LTC4020 will trigger an NTC fault and stop battery charging.

• The LTC4020 has two open-collector outputs (STAT1 and STAT2) to report charger status and fault conditions , and these two pins are binary coded.

The LTC4020 is a universal high voltage, high efficiency buck-boost power manager and multi-chemistry battery charger that supports input voltages above, below or equal to the output to power the battery or system. Its low profile (only 0.75mm high) thermally enhanced 38-pin 5mm x 7mm QFN package is suitable for portable industrial and medical equipment, solar-powered systems, military communications equipment, and 12V to 24V embedded automotive systems.