advertisement Simple, Efficient, All-in-One USB Power Management IC Solution Design Note 1008 John Shannon Introduction Linear Technology offers a variety of devices that simplify converting power from a USB cable, but the LTC®3455 represents the highest level of functional integration yet achieved. The LTC3455 seamlessly manages power flow between an AC adapter, USB cable and Li-ion battery, while complying with USB power standards, all from a 4mm × 4mm QFN package. In addtion, two high efficiency synchronous buck converters generate low voltage rails which most USB-powered peripherals require. The LTC3455 also provides power-on reset signals for the microprocessor, a Hot SwapTM output for powering memory cards as well as an uncommitted gain block suitable for use as a low-battery comparator or an LDO controller. The PCB real estate required for the entire USB power control circuit and two DC/DC converters is only 225mm2. The simplified block diagram of the LTC3455 in Figure 1 shows the major components that allow the LTC3455 to accomplish multiple functions. The DC/DC conversion is WALL ADAPTER VMAX CURRENT SOURCE USB INPUT ~LTC3405 LTC4053 CC/CV Li-Ion BATTERY CHARGER LTC4411 IDEAL DIODE SW1 ~LTC3406 SW2 DN1008 F01 + Li-Ion Figure 1. LTC3455 Simplified Block Diagram 08/04/1008 a relatively straightforward buck converter function. Each of the DC/DC converters takes as its input a voltage called VMAX, which is derived from one of the three available power sources (explained below) and converts it to the desired output voltage. The DC/DC converters provided are high frequency, high efficiency synchronous converters with characteristics that are roughly equivalent to the LTC3405 and the LTC3406. VMAX represents an intermediate voltage bus. Operation with Wall Adapter Present If the wall adapter is present, the intermediate VMAX bus is powered regardless of other available power sources, providing power to both the DC/DC converters and the battery charger. The battery, if also present, is charged via the LTC3455’s constant current, constant voltage, timer terminated, linear charger (similar in features and characteristics to the LTC4053). Operation with No Wall Adapter Present, But Available USB Power If the source of power is a USB input, then the USB power is switched through to VMAX via a current limiting circuit in order to comply with USB current limits. The voltage at VMAX is used to run both DC/DC converters. If the required load power is less than the available USB power, then the remaining power is used to simultaneously charge the battery. As the DC/DC load increases, the battery charge current decreases in such a way as to maintain USB input current compliance. Eventually, as the load power exceeds the available USB power, battery charging ceases and the battery begins to source supplemental power through the internal ideal diode into VMAX. In this mode, some of the load current is provided from the available USB power, with the balance coming from the battery. This intermediate bus power architecture has important benefits at the system level. In contrast with so-called ,LTC and LT are registered trademarks of Linear Technology Corporation. Hot Swap is a trademark of Linear Technology Corporation. “charger fed” systems in which the DC/DC loads are placed in parallel with the battery, this system is capable of running without a battery or with a fully discharged battery. Also, in a charger fed system, power is wasted by first linearly regulating the USB voltage down to the battery voltage. The benefits of the intermediate bus architecture are illustrated in Figure 3. An intermediate POWER INPUTS bus voltage yields faster charging and lower heat dissipation than a charger-fed architecture. Operation when Unplugged When both the adapter and USB input are not present, the LTC3455 powers VMAX through an ideal diode connected to the Li-ion cell. POWER OUTPUTS STATUS OUTPUTS D5 ADAPTER VMAX C1 10µF 1Ω 4.7µF R6 3.32k 10 8 USB 11 1Ω 17 4.7µF 16 1-CELL Li-Ion BATTERY 9 2 C4 1µF R8 1.24k R16 2.49k 3 C6 1µF 25 VMAX USB 20 23 4 RST PBSTAT CHRG HSO HSI WALLFB 14 13 AO SW2 AI VBAT FB2 LTC3455 12 L1 4.7µH R9 237k C3 10µF 18 R10 80.6k PROG TIMER SW1 FB1 GND (EXPOSED PAD) 7 L2 4.7µH 1 R12 100k ON SUSPEND USBHP MODE PWRON ON2 HSON 24 6 5 3.15V Hot Swap OUT 3.15V 600mA 21 22 19 R14 80.6k 15 1.8V 400mA C5 10µF CONTROL INPUTS DN1008 F02 Figure 2. Minimum LTC3455 Implementation CHARGER-FED TOPOLOGY 5V 500mA (2.5W) LTC3455 INTERMEDIATE BUS ARCHITECTURE VUSB VOLTAGE BATTERY CHARGER USB 5V 500mA (2.5W) I≅ PO VUSB I≅ VUSB PASS ELEMENT TO GND 0.5A PO VBAT 1.5W LOAD 1.5W LOAD ICHRG ICHRG VBAT VBAT + DN1008 F03 + VBAT ICHRG PLOSS 4.0 3.6 3.0 2.5 200 200 200 50 0.20W 0.28W 0.40W 0.13W VBAT ICHRG 4.0 0.125 3.6 0.083 3.0 0 2.5 –0.100 PLOSS 0.50W 0.70W 1.00W 1.25W Figure 3. Charge Rate and Dissipation Advantage of Intermediate Bus vs Charger Fed Topologies Data Sheet Download http://www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 2759 dn1008f LT 0804 • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2004