L DESIGN FEATURES Power Management IC Combines USB On-The-Go and USB Charging in Compact Easy-to-Use Solution The USB interface was originally designed so that the device providing power (an “A” device) would act as the host and the device receiving power (a “B” device) was the peripheral. The A plug of the USB cable would always connect to the host device and the B plug would connect to the peripheral. The USB On-The-Go (OTG) standard, however, removes that restriction, so that the B device can now become a host and the A device can act as a peripheral. In the USB specification, standard hosts and hubs are limited to providing 500mA to each downstream device, but if a device is designated as a USB charger, it can supply up to 1.5A. USB chargers come in two flavors. A “dedicated charger” is a charger that is not capable of data communication with the attached B device. A ”host/hub charger” is a charger that is capable of data communications with attached B devices. When USB OTG functionality is combined with a USB battery charger in an end-user product, power can flow in both directions, with relatively complicated logic and handshaking steering the flow. To implement a robust solution, an integrated USB battery charger and power manager is a necessity. This article shows how to use the LTC3576 USB power management IC to easily combine USB On-The-Go functionality and battery charger capability into a single portable product. Overview of the LTC3576 The LTC3576 provides the power resources needed to implement a portable device with USB OTG and USB battery charger detection capabilities (see block diagram in Figure 1). The USB input block contains a bidirec by George H. Barbehenn and Sauparna Das VC 26 OVSENS 6 OVGATE 5 OVP 2.25MHz BIDIRECTIONAL PowerPath SWITCHING REGULATOR VBUS 34 WALL DETECT VC CONTROL VBUS 35 27 WALL 28 ACPR 36 SW 2 LDO3V3 3.3V LDO SUSPEND LDO 500µA/2.5mA CLPROG 1 NTCBIAS 3 NTC 4 CHRG 30 BATTERY TEMPERATURE MONITOR CHARGE STATUS 5.1V 1.18V OR 1.15V 33 VOUT – + – + + IDEAL CC/CV CHARGER + + – Introduction – + 0.3V +– – 31 IDGATE 15mV 32 BAT 3.6V 29 PROG 8 VIN1 ENABLE D/A 400mA 2.25MHz BUCK REGULATOR 9 SW1 7 FB1 4 24 VIN2 ENABLE D/A 400mA 2.25MHz BUCK REGULATOR 25 FB2 4 ILIM DECODE LOGIC 23 SW2 16 VIN3 ENABLE D/A 1A 2.25MHz BUCK REGULATOR ILIM0 37 17 SW3 20 FB3 ILIM1 38 21 RST3 4 ENOTG 11 EN1 10 EN2 22 EN3 19 DVCC 12 SDA 14 I2C PORT SCL 13 39 GND Figure 1. The LTC3576 combines USB charging and USB On-The-Go by using bidirectional DC/DC conversion from VBUS to VOUT Linear Technology Magazine • June 2009 DESIGN FEATURES L 3.3V 3.3V VBUS OTG COMPATIBLE DEVICE SUCH AS LTC3576 VBUS D+ D+ D– D– ENOTG < 6.5µF MINI/MICRO A PLUG MINI/MICRO A/B MINI/MICRO A PLUG MINI/MICRO A/B OTG COMPATIBLE BUSS TRANSCEIVER OTG COMPATIBLE DEVICE SUCH AS LTC3576 GND GND < 6.5µF FOR FULL/HIGH SPEED ONLY ID ID ENOTG FOR LOW SPEED ONLY OTG COMPATIBLE BUSS TRANSCEIVER A DEVICE B DEVICE Figure 2. USB On-The-Go system diagram ATTACH PHASE CONNECT PHASE ENUMERATION PHASE PHYSICAL CONNECTION OF DEVICES DETECT VOLTAGE LEVELS ON D+/D– TO DETERMINE DATA SPEED AND POWER LEVELS SOFTWARE HANDSHAKE Figure 3. USB sequence of events at start-up tional switching regulator between VBUS and VOUT. When power is coming from the USB input, this regulator operates as a step-down converter. Using the Bat-Track™ charging technique, the switching regulator sets the voltage at VOUT to VBAT + 0.3V, providing a very efficient charging solution. When operating as an OTG A device, the regulator acts as a step-up converter by taking power from VOUT to produce 5V on VBUS. The LTC3576 also has overvoltage protection and can be used with an external HV Buck regulator to provide VOUT. In OTG mode, the bidirectional switching regulator can take power from the HV buck regulator to supply power to the USB connection. In addition, the LTC3576 provides two 400mA and one 1A step-down switching regulators for generating three independent voltage rails for the portable device. The LTC3576 allows all three step-down switching regulator output voltages to be enabled/disabled and adjusted over a 2:1 range via I2C. All three step-down regulators feature pulse-skipping mode, Burst Mode® operation and LDO mode, which can also be adjusted on-the-fly via I2C. Mode Detection The USB specification allows for a number of different modes of operation for products supporting both the USB OTG specification1 and the battery charger specification2. Figure 2 shows a typical OTG system and Figure 3 shows the sequence of events that occur when the USB cable is plugged in. The product can be a B device and can draw up to 100mA, 500mA, 900mA or 1.5A, depending on the type of A device powering VBUS, as shown in the Table 1. When an OTG device has a micro/ mini-A plug connected to its micro/ mini-AB connector, the OTG device becomes the A device and starts off as the host. The OTG A device supplies power to VBUS, as any other host A device would, when requested by an attached peripheral or OTG B Device. As an A device, the LTC3576 can supply up to 500mA The USB OTG specification provides two means for a B device to signal to the A device that it wants power. The B device may drive the VBUS line above 2.1V, momentarily, or it may signal by driving the D+ or D– signal lines. The D+/D– signaling method could be Table 1. Load power signaling during Attach and Connect Host/Hub IBUS < 500mA Dedicated Charger IBUS < 1.5A Host/Hub Charger IBUS < (LS,FS < 1.5A/HS < 0.9A) Voltage on D– with VDAT_SRC on D+ during Attach 0V 0.5V–0.7V 0.5V–0.7V 1.5kΩ to 3.3V on D– during Connect for Low Speed, measure voltage on D+ — > 2V < 0.8V 1.5kΩ to 3.3V on D+ during Connect for Full/High Speed, measure voltage on D– — > 2V < 0.8V Linear Technology Magazine • June 2009 10 “V” SUFFIX INDICATES A/D INPUT µC VBUSV D– D+ HUBEN IDV FSPUEN IDPUEN GND VBATV VBATVEN 1ACHARGEEN PROGV CLPROGV 10k 10k M6 M7 2.00k 3.01k UNLESS NOTED, RESISTORS: * CAPACTORS: D2, D3: L1: L2, L3: L4: M2, M3, M6: M4, M5, M7: Q1, Q2, Q3: OHMS, 0402 1% 1/16 WATT THREE 1Ω, 5% RESISTORS IN PARALLEL µF, 0402, 10% 25V 1N4148 1098AS-2R0M 1098AS-4R7M LPS4018-3R3MLC NDS0610 2N7002L MMBT3904LT1 100k NTC NTCBIAS PROG CLPROG SCL 4.7k SDA 4.7k DVCC SCL 0.1µF 16V Q2 44.2k SDA Q3 IDAT_SINK EN3 EN2 EN1 ENOTG ILM1 ILM0 OVSENS OVGATE DVCC 2.00k U2B LTC202 D3 D2 VPROCESSOR VDAT_SRC Q1 100k VPROCESSOR 6.2k VBUS VC VBUS GND FB3 SW3 VIN1 FB2 SW2 CC1 R5 1500pF VIN2 7.68k FB1 SW1 VIN1 BAT IDGATE SW VOUT LD03V3 ACPR WALL U1 LTC3576EUFE DVCC 47k U2A LTC202 M5 M4 100k LEAKAGE CURRENT MUST BE <400nA 22µF 6.3V RST3 VBAT M2 15k 15k BATTERY CHARGER HANDSHAKE 1.5k 47k 0.1µF 16V 100k CHRG 100k DVCC 10k M3 VPROCESSOR 3.3V 4.7µF 50V M1 Si2306BDS RST3 100k 47k VPROCESSOR 4.35V TO 5.5V NON-OPERATING FAULT TOLERANCE TO 30V CONTINUOUS CHRG D-V IDAT_SINKEN VDAT_SRCEN SHGND IO D+ D– J1 USBMICRO-AB VBUS 22µF 6.3V L1 2.0µH L2 4.7µH L3 4.7µH 324k 402k 2.2µF 6.3V R23 324k 1.02M 2.2µF 6.3V 324k 1.02M 2.2µF 6.3V 1.8V AT1A 22µF 6.3V 27pF 50V 5% 10µF 6.3V 18pF 50V 5% VPROCESSOR 3.3V AT 400mA 10µF 6.3V 12pF 50V 5% 3.6V AT 400mA LEAKAGE CURRENT MUST BE < 50nA L4 3.3µH 1µF 10V VPROCESSOR VBAT NTC-EXT GND BAT J2 DF3-3P-2DSA 0.337* 100µF 6.3V M8 Si2333DS VOUT L DESIGN FEATURES Figure 4. Portable system with OTG and battery charger support Linear Technology Magazine • June 2009 DESIGN FEATURES L detected by an OTG compatible USB module on the system microcontroller (µC ). The VBUS signaling method could be detected via an A/D on the µC. The LTC3576 bidirectional switching regulator is then enabled as a step-up converter (OTG mode) by setting the appropriate bit in the control registers via I2C. Implementing a System for USB OTG and Battery Charging Figure 4 shows an application for a portable device that supports both USB battery charging and USB OTG. When IDPUEN is low, the ID pin is pulled up via R5, and if IDV is > 3V then it is configured to be a B device. If IDV is < 0.5V then it is configured to be an A device. The components enclosed in the box labeled “battery charger handshake” enable communication of the power capabilities depending on whether the portable device is configured as an A device or a B device. During the Attach phase, if the portable device is a B device, it can apply VDAT_SRC (0.5V~0.7V) to the D+ line, load the D– line with IDAT_SINK (50µA~150µA), and measure the resultant voltage on D– via D–V. If the voltage is 0, the A device is a Host/Hub, if the voltage is VDAT_SRC then the A device is a USB Charger. During the Connect phase, FSPUEN is pulled low to apply 3.3V to D+, indicating a full/high speed device. At the same time the voltage on the D– line is read again via D–V. If it is less than 0.8V, then the A device is a Host/Hub Charger. If the voltage on D–V is above 2V, then the A device is a Dedicated Charger. For OTG functionality, if the portable device is configured as an A device, then it must drive VBUS from VOUT, which in this case is powered from the battery. Since the LTC3576 is capable of supplying 500mA as an A device, the µC asserts HUBEN to indicate it is a Host/Hub. The bidirectional switching regulator in the LTC3576 is enabled by setting the appropriate bit in the control registers via the I2C port. If the B device drawing current from the VBUS line goes idle, then the OTG A device may turn off the VBUS voltage to conserve the battery. When the B device needs the VBUS voltage to be present at some later time, it can request that the A device again drive VBUS by turning the bidirectional switching regulator back on. It can do this by signaling on the D+ or D– lines or by driving the VBUS line to > 2.1V (see Figure 5). The Host A device only needs to respond to one of two SRP signaling methods. However, since not all USB engines respond to the D+/D– signaling, the VBUS line is sensed to check if it is higher than 2.1V via the VBUSV A/D input. When the portable device’s µC detects that the B device is requesting power on VBUS, either by sensing the D+/D– signaling or by sensing that VBUS has been driven higher than 2.1V, it should again turn on the OTG stepup converter in the LTC3576. The PROG (PROGV) and BAT (VBATV) pins allow a Coulomb counter to be implemented in the µC. Reading the BAT voltage requires that the sensing divider be enabled by setting VBATVEN low. This ensures that the sense divider network does not dis- VIH VIL V(D+ or D–) 7.5ms 5V 100ms 4.9s 2.1V 0V VBUS B DEVICE SIGNALING A DEVICE DELIVERING VUSB charge the battery when the battery voltage isn’t being measured. The default battery charge current has been set to 500mA, but can be increased to 1A by asserting the 1AchargeEN signal. This turns on M7, halving the PROG resistance and increasing the charge current. The input current limit will need to be set to 10X mode (1A) using the I2C port. The optional network of C14 and R27/R28/R29 suppresses ripple on the BAT pin (and consequently on the VBUS pin) if there is no battery present. This ripple can be in the tens of mV. While this will not damage anything, it may be desirable to suppress this signal. The CLPROG (CLPROGV) and CHRG signals are often useful for housekeeping tasks in the µC. The LTC3576 has an overvoltage protection function that controls M1, and protects the system from excessive voltages on the USB (J1) connector. Because the A/D is configured to monitor VBUS, it must also be protected by D1 from excessive voltages. The LDO3V3 regulator is configured to power the µC in low power mode (<20mA). When the µC needs to leave low power mode it first enables Buck Regulator 2, which will provide up to 400mA. Conclusion The LTC3576 is a versatile PMIC consisting of a bidirectional power manager, overvoltage protection, three step-down switching regulators and a controller for an external high voltage step-down switching regulator. In conjunction with a few support components, the LTC3576 allows the implementation of a complete power management system for portable devices that support both USB OTG and USB battery charging. L Bibliography 1 ”On-The-Go Supplement to the USB Specification”, Revision 1.3 2 “Battery Charging Specification”, Revision 1.0 3 www.usb,org/developers/docs Figure 5. Session Request Protocol timing reference1 Linear Technology Magazine • June 2009 11