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USB CHARGER ENABLE/SUSPEND TIMER: A Method for USB Battery Charger Start-up and Suspend With One Shot Timer

IP.com Disclosure Number: IPCOM000126661D
Publication Date: 2005-Aug-11
Document File: 3 page(s) / 35K

Publishing Venue

The IP.com Prior Art Database

Abstract

BACKGROUND OF THE INVENTION Portable systems (hereon referred to as handheld or HH) powered by batteries have a problem with supporting USB charging and suspend functions. It will be shown that these two requirements share a similar system state, however the next state is different depending on the circumstances. USB requires the system to enumerate within ~150msec of the USB cable being connected, hereon referred to as “VBUS detection”. If the battery is dead, the HH CPU must get its power from the VBUS (USB connection). If the battery is dead, then all the control signals from it are at LOW state, thus one decides to make the LOW state the enable for the charger, so that with a dead battery, when VBUS is applied, the charger will turn on and power the HH CPU. Note this state, when all signals to the charger are LOW, the designer wants the charger to turn on when power to the charger is present. When USB requires the HH to go into the suspend state, USB specification requires that the total USB supply current not exceed 500uA. With our HH system, and presumably many others, 500uA is not enough current to keep the HH CPU power, and therefore must be powered down. Powering down the HH CPU makes all the control signals to the charger have a LOW state. In this case, we do not want the charger to be on. Note this sate, all the control signals are LOW, and the VBUS is present, but we do not want the charger to start. This is the same state as above, but here we do not want the charger to start, whereas before we did. The conflict of this state and subsequent action is the problem this invention solves. SUMMARY OF THE INVENTION The object of the invention is to resolve the system state between enabling the charger and going into the USB suspend state. One aspect of the invention is to use a one-shot timer to detect when the VBUS input to the system goes above a minimum threshold, for example, 3.0V. Note that USB VBUS limits are 4.35-5.25V. The one-shot timer signal is used to override the charger enable signal. When the one-shot timer detects the VBUS voltage, it enables the charger for ~150ms. During this 150ms, the HH CPU is powered from the charger if the battery was dead. Within the 150ms, the HH CPU will enumerate with the host USB to draw current to charge the battery. Another aspect of the invention is that if the HH CPU fails to turn on the charger through its control signals to the HH CPU, the one-shot timer signal will expire and disable the charger. Thus the HH will not draw power from the USB host. This adds a “watch-dog” timer function so that failed HH CPU does not inadvertently draw current without proper enumeration. Another aspect of the invention is that when the HH CPU is instructed by the host USB to go into suspend mode, the HH CPU will disable the charger. Since the one-shot timer has expired, and no VBUS event occurs, the charger will shut off, even though VBUS is still present. When the charger shuts off, and the battery is dead, there is no power to the host CPU, and all control signals are active LOW. DESCRIPTION OF TYPICAL IMPLEMENTATION U908 has dual functionality. It’s PFI input level is set by the resistor divider R937+R925 and R926 so it matches the U908 internal reference when VBUS drops down to 3.3V (or the value of LBAT voltage set by the U905 and Q908 when operating with low/dead or without battery) causing it’s open drain output (PFO) to go to GND. This would force positive input node of U905 to go to GND and put Q908 in saturation (ON). This set of circuitry creates an Under Voltage Lock Out (UVLO) threshold for the L_BAT boost circuit. This is important as the bq24020 can operate down to 2.5V causing it’s status flag to indicate current delivery to the system even though it really does not (battery internally disconnects below 2.5V). Such condition would cause the system to RST (for battery voltage levels lower than preset min. LBAT value) as the U905/Q908 would try to keep the LBAT voltage, at preset value, by disconnecting the battery and thinking that charger is delivering enough current. The second functionality of U908 is the start-up override on U909-CE, which provides power to the system for minimum of 100mS to initiate and properly enumerate on the USB-BUS. This is accomplished by the keeping it’s RESET open drain output at GND before the VBUS reaches 2.5V and than 100mS after. This keeps the bq24020 enabled during that time. SUMMARY The object of the invention is to resolve the system state between enabling the battery charger and going into the USB suspend state. This invention addresses the problem of supporting the USB Suspend requirement and booting up (or enumerating) with dead batteries. A system using USB to charge the battery has this problem. When the handheld (HH) goes into Suspend, the HH must power down the CPU, and therefore all IO control lines are logically LOW (ground potential). The same condition is true when a HH has a dead battery. The two conditions look the same, yet must be handled differently. To solve the above problem, a one-shot timer is used to detect when the VBUS input to the system goes above a minimum threshold. The timer sends a signal to the charger to enable (regardless of the state of the signal from the CPU), which for a dead battery is LOW. The activated charger powers the CPU for the length of the one-shot timer. The CPU then enumerates with the host USB and enables the charger by setting a control signal. At a later point, the USB may ask the HH to suspend. When this happens, the CPU turns off the charger if the battery is dead. This causes all the CPU control signals to go active LOW. This suspend state must persist indefinitely. If the charger supply cable is removed and re-inserted, the one-shot timer is re-triggered, and the HH CPU is powered. If the HH CPU fails to turn on the charger through its control signals to the HH CPU, the one-shot timer signal will expire and disable the charger. Thus the HH will not draw power from the USB host. This adds a “watch-dog” timer function so that failed HH CPU does not inadvertently draw current without enumeration. Another aspect of the invention is that when the HH CPU is instructed by the host USB to go into suspend mode, the HH CPU will disable the charger. Since the one-shot timer has expired, and no VBUS event occurs, the charger will shut off, even though VBUS is still present. When the charger shuts off and the battery is dead, there is no power to the host CPU, and all control signals are active LOW.

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USB CHARGER ENABLE/SUSPEND TIMER:

A Method for USB

Battery

Charger Start-up and Suspend With One Shot Timer

Disclosed Anonymously

           

BACKGROUND OF THE INVENTION

Portable systems (hereon referred to as handheld or HH) powered by batteries have a problem with supporting USB charging and suspend functions.  It will be shown that these two requirements share a similar system state, however the next state is different depending on the circumstances. 

USB requires the system to enumerate within ~150msec of the USB cable being connected, hereon referred to as “VBUS detection”.  If the battery is dead, the HH CPU must get its power from the VBUS (USB connection).  If the battery is dead, then all the control signals from it are at LOW state, thus one decides to make the LOW state the enable for the charger, so that with a dead battery, when VBUS is applied, the charger will turn on and power the HH CPU.  Note this state, when all signals to the charger are LOW, the designer wants the charger to turn on when power to the charger is present.

When USB requires the HH to go into the suspend state, USB specification requires that the total USB supply current not exceed 500uA.  With our HH system, and presumably many others, 500uA is not enough current to keep the HH CPU power, and therefore must be powered down.  Powering down the HH CPU makes all the control signals to the charger have a LOW state.  In this case, we do not want the charger to be on.  Note this sate, all the control signals are LOW, and the VBUS is present, but we do not want the charger to start.  This is the same state as above, but here we do not want the charger to start, whereas before we did.

The conflict of this state and subsequent action is the problem this invention solves.

SUMMARY OF THE INVENTION

The object of the invention is to resolve the system state between enabling the charger and going into the USB suspend state.

One aspect of the invention is to use a one-shot timer to detect when the VBUS input to the system goes above a minimum threshold, for example, 3.0V.  Note that USB VBUS limits are 4.35-5.25V.  The one-shot timer signal is used to override the charger enable signal.  When the one-shot timer detects the VBUS voltage, it enables the charger for ~150ms.  During this 150ms, the HH CPU is powered from the charger if the battery was dead.  Within the 150ms, the HH CPU will enumerate with the host USB to draw current to charge the battery.

Another aspect of the invention is that if the HH CPU fails to turn on the charger through its control signals to the HH CPU, the one-shot timer signal will expire and disable the charger.  Thus the HH will not draw power from the USB host.  This adds a “watch-dog” timer function so that failed HH CPU does not inadvertently draw current without proper enumeration.

Another aspect of the invention is that when the HH CPU is instructed by the host USB to go into suspend mode, the HH CPU will...