55V High Efficiency Buck-Boost Power Manager and Multi-Chemistry Battery Charger Design Note 531 Charlie Zhao Introduction Today, battery chargers are expected to easily support a variety of battery chemistries and accept a range of voltage inputs, including wide-ranging solar panels. It is increasingly common for input voltage ranges to span above and below the output battery voltage, requiring both step-down and step-up capability (buck-boost topology). The LTC4020 buck-boost power manager and multi-chemistry battery-charging controller can take wide-ranging 4.5V to 55V inputs and produce output voltages up to 55V. Its buck-boost DC/DC controller supports battery and system voltages above, below, or equal to the input voltage. The charger is easily optimized for a variety of battery chemistries. For instance, it can follow a constantcurrent/constant-voltage (CC/CV) charge algorithm, with either C/10 or timed termination for lithium-based battery systems, a constant-current (CC) characteristic with timed termination, or an optimized 4-step, 3-stage lead-acid charge profile. 6.3A Charger for 25.2V Battery Float Voltage Figure 1 shows a 15V to 55V input, 25.2V/6.3A buckboost battery charger, featuring a high efficiency 4-switch (M2–M5) synchronous buck-boost DC/DC L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and PowerPath is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. C1, 0.1µF 100V, 0603 VIN 15V TO 55V C2 56µF 63V 63HVH56M + + R38 5.1Ω 0805 R1 100k C23 C16 4.7µF 100V 1210 C3 56µF 63V 63HVH56M GND PVIN R2 510k SENSVIN SENSTOP LTC4020 INTVCC STAT2 STAT1 BST1 BST2 SHDN VIN_REG R4 51k C40 0.22µF 0603 R5 OPT C21 10µF 6.3V 0805 R9 226k 0603 1% R10 24.9k 0.1% R11 24.9k 1% C12 0.033µF SENSGND M4 SiR664DP RCBRB1 0.002 Ω 1W 2512 BG2 TG2 CSP VFBMIN VFBMAX VFB R19 20Ω CSN FBG R14 10k C4 100pF 0603 C5 0.22µF 0603 NTC CSOUT MODE TIMER RT R13 100k SGND VBAT VC ITH SGND PGND D3 RED D4 GRN C20 0.22µF 25V 0603 D6 B360A WÜRTH 7443556560 C11 4.7µF 6.3V 0603 M3 SiR422DP R18 100Ω C27 10µF 35V 1210 C18 10µF 35V 1210 M5 SiR422DP R20 100Ω C14 10µF 35V 1210 R15 36k 0603 C6 6.8nF 0603 RCBAT1 0.008Ω 1W 2512 BAT 25.2V FLOAT 6.3A MAX R16 47k 0603 GND C5 47pF 0603 dn531 F01 Figure 1. 15V to 55V Input, 25.2V/6.3A Buck-Boost Battery Charger 10/14/531 + C28 10µF 35V 1210 M1 Si7145DP BGATE MODE L1 5.6µH M2 SiR664DP TG1 VOUT D2 SBR0560S1 R26 3k BG1 R8 226k 0603 0.1% R27 3k C19 0.22µF 25V 0603 SENSBOT VBAT D1 SBR0560S1 SW1 SW2 RNG/SS ILIMIT C39 0.22µF 0603 RCBRT1 0.002Ω 1W 2512 C15 220µF 35V 35HVH220M C17 220µF 35V 35HVH220M VOUT 21V TO 28V 8A MAX + GND converter requiring only one inductor (L1). The proprietary average current mode architecture uses two sense resistors (RCBRT1 and RCBRB1) to monitor the inductor current. In this buck-boost solution, when VIN is higher than VOUT, the converter operates in buck (step-down) mode; when VIN is lower than VOUT, the converter works at boost (step-up) mode. When VIN is close to VOUT, the converter operates in 4-switch buck-boost mode. 100 90 EFFICIENCY (%) 80 70 60 15VIN, 25.2VOUT 24VIN, 25.2VOUT 55VIN, 25.2VOUT 30 0.1 1 LOAD CURRENT (A) The LTC4020 automatically configures the PowerPath FET as an ideal diode when the battery charger is not in a charging cycle—namely, the buck-boost converter is operating exclusively for the system load. This allows the battery to remain disconnected from the converter output in normal operation. If, however, the system load current exceeds the buck-boost converter’s capacity, additional power can be efficiently drawn from the battery through this ideal diode. Additional Features The LTC4020 supports timer-based charging algorithms—a capacitor from the TIMER pin to ground programs the end of the cycle time. 50 40 Figure 1) as a linear regulator, allowing the buck-boost converter output to rise above the battery voltage, while still providing charge current to the battery. This function is called PowerPath instant-on, when the PowerPath FET acts as a high impedance current source, providing charge current to the battery. 10 dn531 F02 Figure 2. Efficiency vs Load Current IOUT (VOUT = 25.2V) of the Converter in Figure 1 The converter operates at a programmable constant switching frequency within the range of 50kHz to 500kHz, set with a resistor (R13 = 100k, 250kHz). The solution shown in Figure 1 can supply up to 8A to the system load (VOUT = 25.2V). Figure 2 shows full load efficiency (IOUT = 8A, VIN = 24V) can reach above 98%. The LTC4020 uses an external feedback resistor divider from the BAT pin to program battery voltages via the VFB 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. Battery charge current is monitored through a sense resistor (RCBAT1). Maximum average battery charge current is easily programmed by selecting the value of RCBAT1. Dynamic current limit adjustment is possible through the RNG/SS pin. Instant-On and Ideal Diode Functions with PowerPath FET For a heavily discharged battery, the LTC4020 can automatically configure the PowerPath FET (M1 in Data Sheet Download www.linear.com/LTC4020 Linear Technology Corporation The LTC4020 features battery temperature monitoring and control. By connecting an NTC (negative temperature coefficient) thermistor to the NTC pin, and by placing the thermistor close to the battery pack (or other desired monitoring location), if the NTC pin voltage is out of range (above 1.35V or below 0.3V), the LTC4020 triggers an NTC fault and halts battery charging. The LTC4020’s VIN_REG pin allows input voltage regulation. This pin can be used to program the peak power voltage for a solar panel, or help maintain a minimum voltage for other high impedance input supplies. The LTC4020 has two open collector outputs, STAT1 and STAT2, to report charger status and fault conditions. These two pins are binary coded. Conclusion The LTC4020 is a versatile high voltage, high efficiency buck-boost power manager and multi-chemistry battery charger, supporting input voltages above, below or equal to the output battery or system voltages. Its low profile (0.75mm) thermally enhanced 38-pin 5mm × 7mm QFN package is suitable for portable industrial and medical equipment, solar-powered systems, military communications equipment, and 12V to 24V embedded automotive systems. For applications help, call (408) 432-1900, Ext. 3258 dn531f LT/AP 1014 111K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2014