ST AN2865 APPLICATION NOTE

AN2865

Application note

STUSBCD01B configuration and operation

Introduction

With the ever-increasing number of mobile devices adopting the USB bus as the standard communication port and source for recharging, a standardization of the characteristics of the charging devices and methods to detect them is required to optimize the performance of the charging process and reduce the risks of damaging standard USB ports.

The new USB battery charging specification provides rules and guidelines to follow when designing new USB architectures capable of battery charging and when defining new charging host ports. The specification also extends the range of current which can be drawn from a USB port.

In order to be able to distinguish between this new class of USB ports, standard USB host ports and dedicated chargers, the new specification also defines detection methods which must be used to determine the right amount of current the portable device can draw from the USB bus. This also guarantees backward compatibility with standard USB ports.

The STUSBCD01B is a USB charging detection IC developed on the base of the USB battery charging specification which can be easily added in new platforms to provide them with charging detection capability.

September 2009

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Contents	AN2865

Contents

1	STUSBCD01B description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 3
2	Application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		4
3	Interface and control pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
	3.1	Status/method pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	3.2	Detect pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	3.3	Default method pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
4	Current sink detection method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		8
5	Dedicated charger detection method . . . . . . . . . . . . . . . . . . . . . . . . . .		11
6	Software detection and hardware detection . . . . . . . . . . . . . . . . . . . . .		14
7	References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
8	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		16

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AN2865	STUSBCD01B description

1 STUSBCD01B description

The STUSBCD01B is the ideal solution for all mobile products using the USB bus for battery charging. It can be used in all USB architectures (low-, fullor high-speed) where the transceiver or the battery charger does not have smart charger detection. The STUSBCD01B implements two different detection methods to distinguish between dedicated chargers, charging host ports and standard host ports.

The device can be fully controlled through digital inputs (software detection mode) and is also able to perform the charger detection automatically when the battery voltage is too low to allow the application controller to be operative (hardware detection mode). A VBAT- referred open-drain output (detect) is available for direct control over the USB charging controller.

The STUSBCD01B also provides a clamping circuit which can be used to protect each USB IC connected to the USB VBUS against overvoltage. The device requires few external components and its power consumption is extremely low.

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Application circuit	AN2865

2 Application circuit

The STUSBCD01B requires only five external components:

●two capacitors to bypass the power supplies to ground

●one resistor for the VBUS clamping circuit

●two series resistors on the DP/DM data lines

Figure 1 shows the typical application circuit for the STUSBCD01B.

Figure 1. STUSBCD01B application circuit

VIO

VI/O

CONTROLLER

	USB CHARGING		USB PHY
		VBAT	(Transceiver)
	CONTROLLER		DM DP VBUS

C1	C2

100nF	1V8V	DETECT	100nF

VIO	VBAT
SHUTDOWN			USB
			Receptacle
STATUS/METHOD	A	B	VBUS
/OE	VBUS	R3 = 470Ω
DEF. METHOD			D+
	DP
		R1 = 470Ω
			D-
STUSBCD01	DM
		R2 = 470Ω
			GND
GND

The STUSBCD01B can operate in two different modes: hardware detection mode, which does not require external control, and software detection mode. The user can choose between two different detection methods: dedicated method and current sink method. More details on each operating mode are provided in the following paragraphs. The operating mode is defined by the status of the digital I/Os, VBAT voltage, VIO voltage and default method input. See Table 1 for a summary of all operating conditions.

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AN2865							Application circuit
Table 1.	STUSBCD01B operating modes
VBAT	VIO	VBUS	Shutdown		Status/method	Default method	Operating mode
					pin	pin
< 2.2 V	-	-	-		-	-	Power down
> 2.2 V	Not	Not	-		-	-	Standby (no SW control)
	present	present
> 2.2 V	Not	Present	-		-	VBAT	Active, HW detection,
	present						current sink method
> 2.2 V	Not	Present	-		-	GND	Active, HW detection,
	present						dedicated method
> 2.2 V	Present	Present	VIO		-	-	Standby (SW control)
> 2.2 V	Present	Not	-		-	-	Standby (SW control)
		present
> 2.2 V	Present	Present	GND		VIO(1)	-	Active, SW detection,
							current sink method
> 2.2 V	Present	Present	GND		GND(1)	-	Active, SW detection,
							dedicated method

1.The level of the Status/Method pin is read and latched on the falling edge of the shutdown input signal. When detection is finished, this pin becomes output.

SW = Software: HW = Hardware; “-” = Don't Care

The external resistors are very important to guarantee proper operation:

–The R1 and R2 series resistors are needed to mask the DP/DM pins’ parasitic capacitance which is seen on the bus during high-speed USB communication.

Removing these resistors might lead to degradation of USB high-speed signal quality and eye pattern failure. A value of 470 Ω is suggested in order to have optimal performance;

–The R3 resistor is required for the VBUS clamping feature. If a value of 470 Ω is used, the VBUS pin voltage (node A) never exceeds 6 V when USB VBUS voltages up to 10 V are applied (node B). Every device needing overvoltage protection must be

connected to the VBUS pin of the STUSBCD01B as shown in Figure 1: STUSBCD01B application circuit (node A). Bus-powered devices cannot take advantage of this

clamping feature because high currents drawn from the USB VBUS voltage would cause a voltage drop over the R3 resistor. If this voltage drop is too high, the device's

VBUS comparators would read a false VBUS level which might lead to malfunctioning. It is therefore strongly recommended to connect bus-powered USB devices directly to the USB receptacle's VBUS line (node B).

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Interface and control pins	AN2865

3 Interface and control pins

The STUSBCD01B is controlled and communicates with the controller using 5 I/Os. While shutdown and OE are standard VIO referred CMOS inputs, the remaining pins (Status/Method, default method and detect) have different characteristics.

3.1Status/method pin

This pin is either input or output depending on the operating conditions. It is input before the start of the detection process (used to set the detection method) and is output at the end of the detection process (it outputs the result of the detection). The application designer should program the application controller so that it sets the level of this pin (VIO or GND) before the detection starts and maintains it during the falling edge of the shutdown signal (when the value is internally latched). The STUSBCD01B then outputs the detection result (at the end of detection) on this pin and therefore the application should read it after the maximum

detection time has passed (see parameters TVBUS_DET_CS and TVBUS_DET_DC on the STUSBCD01B datasheet). The output structure is not a standard CMOS output but consists

of a weak pull-up or a weak pull-down (~10 kΩ) connected to the pin depending on the detection result as shown in figure 2.

Figure 2. Status/method pin I/O

VIO

Closed at detection end if charger detected

STATUS/

METHOD

Closed at detection end if charger not detected

GND

3.2Detect pin

This pin is an open-drain output which can be used as a VBAT referred signal for the detection result. It is always enabled in hardware detection mode, while in software detection mode it is enabled/disabled by the OE (active low) digital input. When enabled, the open-drain structure (see Figure 3) uses a PMOS transistor to pull the pin high (VBAT) when the detection is successful, otherwise an internal pull-down resistor (~ 300 kΩ) keeps the output low.

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