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

DN531 - Linear Technology

SiR664DP. M2. SiR664DP. L1. 5.6µH. WÜRTH 7443556560. M3. SiR422DP. M5. SiR422DP. D6. B360A. C19. 0.22µF. 25V. 0603. C20. 0.22µF. 25V. 0603. C21.
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