Abstract:
A portable information handling system (IHS) includes an external module which derives its power from the internal unregulated DC power rail of the IHS. An IHS power subsystem includes a multiple threshold current protection circuit which continuously monitors power usage by the external module. The multiple threshold current limit protection circuit dynamically adjusts the current limit depending on whether the IHS is powered by an AC source or a DC battery source.

Description:
BACKGROUND 
   The disclosures herein relate generally to information handling systems and more particularly to a portable information handling system providing power to an external module and employing a current limit protection circuit for the external module. 
   As the value and use of information continue to increase, individuals and businesses seek additional ways to process and store information. One option available to users is information handling systems. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems. 
   Portable information handling systems (IHS&#39;s) often include external modules such as CD Read, CD Read/Write, DVD Drives and floppy disk drives, for example. The external modules typically derive their power from the unregulated DC power circuitry within the IHS, thereby placing a power drain on the resources of the IHS power subsystem. It is desirable that the IHS provide protection from an over-current state by placing a limit on the amount of power which the external modules draw. 
   Prior information handling systems have relied on current interrupting devices such as constant current fuses to limit the amount of power to external modules and to protect the critical power levels within the IHS. This method has been shown to be inadequate do to the large variation in current drain associated with the wide variety of external modules and the associated unpredictable power usages of these external modules. In addition, the current drain associated with each external module is affected by the voltage of the supply within the IHS. The power source of the IHS may be the AC mains or a battery powered DC source. One AC source that is used for IHS power in a fixed location is an AC adapter which provides an unregulated DC source to the internal main DC unregulated power rail of the IHS. Typical DC power sources that are used for an IHS to operate in a portable mode are nickel metal hydride batteries and lithium ion batteries. When an IHS is powered by batteries, variations in voltage occur at the main DC unregulated power rail due to the variable voltage associated with DC batteries conditional upon the level of charge. 
   One additional problem with the above constant current fuse approach is that selecting a single current value for the current limit does not allow the IHS to function over a full range of AC adapter output DC voltages and battery DC voltages as well as the wide range of external module power requirements. The portable IHS is left vulnerable to excessive current drain by the external module and subsequent external module faults (shorts) which could potentially cause system shutdowns and data loss. 
   What is needed is an information handling system which is capable of supplying power to an external module while accommodating a wide range of AC adapter output DC voltages and battery DC voltages. 
   SUMMARY 
   Accordingly, in one embodiment, a method of operating an information handling system (IHS) is provided that includes sensing whether the IHS is drawing power from a DC power source or an AC power source. The method also includes interrupting current to an external module of the IHS if, when the IHS is drawing power from a DC power source, the current to the external module exceeds a first current limit. The method further includes interrupting current to the external module if, when the IHS is drawing power from an AC power source, the current to the external module exceeds a second current limit. 
   In another embodiment, an information handling system (IHS) is disclosed which includes a main subsystem including a processor and a memory coupled to the processor. The IHS also includes an external module. A power subsystem is coupled to the main subsystem and the external module. The power subsystem supplies DC current to the main subsystem and the external module. Moreover, the power subsystem interrupts DC current to the external module if, when the IHS is drawing power from a DC power source, the current to the external module exceeds a first current limit. The power subsystem also interrupts DC current to the external module if, when the IHS is drawing power from an AC power source, the current to the external module exceeds a second current limit. 
   A principal advantage of the embodiment disclosed herein is that the main DC system unregulated power of the IHS is protected over a wider range of external module power uses. Moreover, the current limit level of the external modules need not be unduly restricted due to the differences associated with the primary power source, either AC or DC battery, within the IHS. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an embodiment of the disclosed information handling system (IHS) including an IHS main subsystem and an IHS power subsystem having a multiple threshold current protection circuit coupled to an external module. 
       FIG. 2  is a hardware block diagram of an embodiment of the multiple threshold current protection circuit used in the IHS of  FIG. 1 . 
       FIG. 3  is a power demand graph resulting when a constant current fuse is used in the IHS of  FIG. 1  instead of the disclosed technology of  FIG. 2 . 
       FIG. 4  is a power demand graph of the IHS of  FIG. 1  when the disclosed technology of  FIG. 2  is employed. 
       FIG. 5  is a flow chart showing hardware and software states associated with the multiple threshold current protection circuit employed in the IHS of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram of the disclosed information handling system  100  which solves the above-described problems. Information handling system  100  is an example of one system in which the disclosed technology is practiced. For purposes of this disclosure, an information handling system may include instrumentalities operable to compute, classify, process, transmit, receive, retrieve, originate, switch, store, display, manifest, detect, record, reproduce, handle, or utilize any form of information, intelligence, or data for business, scientific, control, or other purposes. For example, an information handling system may be a personal computer, a network storage device, or any other suitable device and may vary in size, shape, performance, functionality, and price. The information handling system may include random access memory (RAM), one or more processing resources such as a central processing unit (CPU) or hardware or software control logic, ROM, and/or other types of nonvolatile memory. Additional components of the information handling system may include one or more disk drives, one or more network ports for communicating with external devices as well as various input and output (I/O) devices, such as a keyboard, a mouse, and a video display. The information handling system may also include one or more buses operable to transmit communications between the various hardware components. 
   As seen in  FIG. 1 , information handling system (IHS)  100  includes an IHS main subsystem  102  coupled to an IHS power subsystem  105 . IHS main subsystem  102  includes a processor  110  such as an Intel Pentium series processor or one of many other processors currently available. An Intel Hub Architecture (IHA) chipset  115  provides information handling main subsystem  102  with graphics/memory controller hub functions and I/O functions. More specifically, IHA chipset  115  acts as a controller which communicates with a graphics controller  120  coupled thereto. A display  125  is coupled to the graphics controller  120 . IHA chipset  115  further acts as a controller for main memory  130  which is coupled thereto. IHA chipset  115  also acts as an I/O controller hub (ICH) which performs I/O functions. Input devices  135  such as a mouse, keyboard, and tablet, are also coupled to chipset  115  at the option of the user. An expansion bus  140 , such as a Peripheral Component Interconnect (PCI) bus or a PCI Express (PCIE) bus for example, is coupled to chipset  115  as shown. Expansion bus  140  includes one or more expansion slots (not shown) for receiving expansion cards which provide IHS main subsystem  102  with additional functionality. A local area network (LAN) controller  142 , alternatively called a network interface controller (NIC), is coupled to IHA chipset  115 . System basic input-output system (BIOS)  145  is also coupled to IHA chipset  115  as shown. A media drive controller  147  such as an integrated drive electronics (IDE) controller is coupled to IHA chipset  115  so that devices such as media drive  150  can be connected to processor  110  and other components of the system. Devices that can be coupled to media drive controller  147  include hard disk drives, CD-ROM drives, DVD drives and other fixed or removable media drives. 
   A power management controller (PMC)  151  is part of an IHS power subsystem  105  that is coupled to IHA chipset  115  to provide communication between processor  110  and IHS power subsystem  105 . A microcontroller is typically employed as power management controller  151 . A nonvolatile memory  152 , such as FLASH memory for example, is coupled to power management controller  151  to provide control software therefor. Power management controller  151  communicates through a system management bus (SMBus)  155  to a DC power regulation circuit  160  coupled thereto. Battery  162  is coupled to DC power regulation circuit  160  which operates in conjunction with battery  162  to provide regulated main DC power at power lines  165  to IHS power subsystem  105  and IHS main subsystem  102 . In one embodiment, these main DC power lines  165  provide IHS  100  with regulated DC voltages such as 5.0 volts and 3.3 volts. DC power regulation circuit  160  includes charge and discharge circuitry (not shown separately) for battery  162 . 
   Main DC unregulated power is provided to main DC unregulated power output  170  located at the output of DC power regulation circuit  160 . Main DC unregulated power output  170  receives energy either from DC power source battery  162  or external AC power source  172  which is coupled to AC power adapter  175 . AC adapter  175  converts AC from AC source  172  to unregulated DC in one embodiment. The resultant unregulated DC power is transmitted through a protection diode  180  to provide unregulated DC power to output  170 . Main DC unregulated power output  170  is coupled via power cut-off switch  182  to multiple threshold current protection circuit  185  which will be discussed later in greater detail. A power FET can be used as cut-off switch  182 . External module  190  is coupled to multiple threshold current protection circuit  185  and is supplied DC power thereby. Examples of typical external modules  190  include CD ROM drives, CD-R/W drives, DVD drives, floppy drives as well as other power consuming devices which provide additional functionality to IHS. 
     FIG. 2  shows a detailed representation of one multiple threshold current protection circuit  185  that can be employed in IHS  100 . Power management controller (PMC)  151  (shown earlier in  FIG. 1 ) supplies the following signals to multiple threshold current protection circuit  185 . PMC  151  supplies an AC/BATTERY signal to an AC/BATTERY input  205  and a RESET signal to a RESET input  210 . The AC/BATTERY signal indicates whether the IHS is currently being power by AC power adapter  175  or DC battery  162 . The RESET signal instructs protection circuit  185  when to reset as explained later in more detail. A FAULT FLAG signal is generated by multiple threshold current protection circuit  185  and is supplied to PMC  151  by FAULT FLAG output  215 . 
   When IHS  100  commences operation, power management controller (PMC)  151  initiates a reset of multiple threshold current protection circuit  185  via the RESET input signal supplied to RESET input  210 . The RESET input signal at RESET input  210  is fed to the reset input (R) of a set/reset latch  220 . The output (D) of set/reset latch  220  drives cut-off switch  182  closed which couples the main DC unregulated power output  170  through switch  182  and resistor  225  to provide external module  190  with unregulated DC power. Cut-off switch  182  is implemented as an FET power switch. When cut-off switch  182  is closed, main DC unregulated power output  170  is coupled to external module  190 . However, when a fault or over-current condition occurs, as discussed later, cut-off switch  182  is opened to disconnect external module  190  from main DC unregulated power output  170 . 
   The scenario wherein cut-off switch  182  is closed after IHS initiation is presently considered. The voltage drop across resistor  225  is proportional to the current driving external module  190  and thus gives an indication of that current. The voltage drop across resistor  225  is sensed by coupling one terminal of the resistor to the negative input of a current amplifier  230  and the remaining terminal of the resistor to the positive input of current amplifier  230 . The output of current amplifier  230  is a voltage directly proportional to the current drain of external module  190  and is coupled to a low pass filter first formed by resistor  235  and capacitor  240  as shown. The output of resistor  235  is a voltage reference directly proportional to the current draw of external module  190  and is made less susceptible to short transient signal noise by the low pass filter just described. 
   The output of resistor  235  is coupled to the positive input of a comparator  245 . The output of threshold switch  250  is coupled to the negative input of comparator  245 . In this embodiment, threshold switch  250  is capable of selecting two different reference threshold voltages for comparator  245 . Comparator  245  compares the voltage developed as a result of the current draw of external module  190  (as supplied to its positive terminal) and the reference voltage at the output of threshold switch  250  (as supplied to its negative terminal). The voltage supplied to switch  250  can be either reference voltage VREF 1  which is supplied to switch terminal  255  or reference voltage VREF 2  which is supplied to switch terminal  260 . One or the other of these two reference voltages is selected depending on whether IHS  100  is being supplied power by battery  162  or AC power adapter  175  as indicated by the AC/BATTERY signal supplied to threshold switch  250 . It is noted that reference voltage VREF 1  is associated with a current limit REF 1  and reference voltage VREF 2  is associated with a different current limit REF 2 . Thus, when IHS  100  is supplied by AC power, then VREF 1  with its associated REF 1  current limit is selected, and when IHS  100  is supplied by DC battery power, then VREF 2  with its associated REF 2  current limit is selected. 
   More particularly, when IHS  100  is powered by AC power adapter  175 , power management controller (PMC)  151  generates a high on the AC/BATTERY input signal at input  205 . This action drives reference threshold switch  250  to select VREF 1  as the input to switch  250 . VREF 1  is the reference voltage to be used when IHS  100  is powered by AC power adapter  175 . In contrast, when IHS  100  is powered by DC battery  162 , PMC  151  generates a low AC/BATTERY signal at input  205 . This action causes switch  250  to select VREF 2  signal as its source. VREF 2  is the reference voltage to be used when IHS  100  is powered by DC battery  162 . VREF 1  and VREF 2  are different reference voltages appropriate for AC power and DC battery power respectively. In one embodiment, VREF 2  is 1.7 volts and VREF 2  is 3 volts although other threshold voltages can be employed specific to the particular implementation. In one embodiment, if op amp  230  exhibits 1 volt/1 amp gain, then current limit REF 1  (for AC) is 1.7 amp and current limit REF 2  (for DC) is 3.0 amp. The output of reference threshold switch  250  is coupled to the negative input of comparator  245 . With a selected reference voltage established at the negative input of comparator  245 , multiple threshold current protection circuit  185  is capable of comparing the current draw of external module  190  with a selected current limit (corresponding to current limited threshold voltages VREF 1  or VREF 2 , whichever is selected). Comparator  245  determines if the selected current limit has been exceeded by the current draw of external module  190 . If the selected limit has been exceeded, then switch  182  is opened to disconnect external module  190  from the IHS. 
   In more detail, when the current draw of external module  190  exceeds the selected current limit as determined by VREF 1  or VREF 2 , comparator  245  transmits a signal to the set input (S) of latch  220  which sets the output of latch  220  low thus driving the FAULT FLAG signal at FAULT FLAG output  215  low. Because FAULT FLAG output  215  is coupled to PMC  151 , the low FAULT FLAG signal is supplied to PMC  151  to inform the PMC that an over-current fault condition has occurred. In addition the output of latch  220  drives cut-oft switch  182  open, thereby removing the power source to external module  190  and protecting the main DC unregulated power output  170  within the IHS  100 . The capability of providing a different current threshold or current limit for AC and DC power sources, respectively, is a significant feature of this embodiment. 
     FIG. 3  is a power demand graph showing the behavior of IHS  100  of  FIG. 2  without the presence of multiple threshold protection circuit  185 . More particularly,  FIG. 3  is a power demand graph of an IHS wherein a constant current fuse (not shown) is employed in series with external module  190  to limit current to the external module without the benefit of protection circuit  185 . The current drain by external module  190  is shown on the vertical axis. The voltage at the main DC unregulated power output or rail  170  is shown on the horizontal axis. In this particular example, the main DC unregulated power output or rail  170  is rated at 50 Watts derived from a power adapter  175 . 
   The power demand graph of  FIG. 3  includes two vertical voltage regions  300  and  305 . Voltage region  300  represents voltages ranging from 8 to 16 volts DC, namely the working voltage range of main DC unregulated power output  170  while the IHS is powered by DC battery source  162 . Voltage region  305  represents voltages ranging from 19 to 21 volts DC, namely the working voltage range of main DC unregulated power output  170  while the IHS is powered by AC power adapter  175 . 
   The power demand graph of  FIG. 3  includes three unique cross-hatched areas associated with each of voltage ranges  300  and  305  of the unregulated DC output  170 . Cross-hatched region  300 A represents the current and voltage relationships wherein external module  190  is in its normal operating range below its maximum allowed power draw. 
   Cross-hatched region  305 A represents the current and voltage (or power) relationships wherein external module  190  is operating below power use region  305 B for those times when IHS  100  is powered by AC power adapter  175 . This is a safe state for the IHS to operate. Power use region  305 B represents a fault condition for external module  190  in which maximum allowed power draw is exceeded, but the IHS is still operational. In this case, a fault condition in the external module can be detected without risking IHS shutdown due to power starvation. 
   Cross-hatched region  300 B represents a normal power use region of typical power draw by external module  190  during normal operation when supply voltage is derived from DC battery source  162 . 
   Cross-hatched region  300 C represents the current and voltage relationships wherein the power source (battery or AC adapter) is driven beyond its ability to provide adequate power to the IHS and external module. Region  300 C is a fault condition in which overload or brownout can cause IHS shutdown or malfunction. 
   Cross-hatched region  305 C represents the current and voltage relationships where IHS  100  is again being driven beyond its ability to provide adequate current to external module  190 , except that now IHS  100  is powered by AC power adapter  175 . 
   As seen in the graph in  FIG. 3  a constant current fuse selected to accommodate a safe working region within the voltage range of 8 to 16 volts (i.e. within region  300 B) when the IHS is powered by DC battery  162  does permit a safe working current limit within the range of 19 to 21 volts (i.e. within region  305 B) when the IHS is powered by AC power adapter  175 . However, when constant current line  310  is drawn to represent the current limit associated with a constant current fuse as shown in  FIG. 3 , it passes through a safe region  300 B which corresponds to safe operation when powered by the DC battery, but unfortunately also passes through unsafe region  305 C when powered by the AC adapter. The selection of a current limit associated across all working voltages of areas  300 B and  305 B is not possible. This demonstrates the limitation of previous constant current fuse protection designs. 
     FIG. 4  above is a power demand graph similar to the graph of  FIG. 3 . However, two unique current limit values, REF 1  and REF 2  are employed for respective normal current operating regions  400 B and  405 B. In other words, one current limit value REF 1  is employed while the IHS is operating on AC power from AC adapter  175  and another current limit value REF 2  is employed when the IHS is operating on DC power from battery  162 . Current limits REF 1  and REF 2  are associated with voltages VREF 1  and VREF 2 , respectively. By having different threshold current limit values for AC and DC power regions, a safe working current range is maintained at main DC unregulated power output  170  for all power draws of external module  190 . It is noted that when comparing the power demand graphs of  FIG. 4  and  FIG. 3 , regions  400 A,  400 B and  400 C of  FIG. 4  correspond to regions  300 A,  300 B and  300 C of  FIG. 3 . Also, regions  405 A,  405 B and  405 C of  FIG. 4  correspond to regions  305 A,  305 B and  305 C of  FIG. 3 . Voltage ranges  400  and  405  of  FIG. 4  correspond to voltage ranges  300  and  305  of  FIG. 3 . 
     FIG. 5  is a flow chart which represents the process flow of the IHS  100 , PMC  151  and multiple threshold current protection circuit  185  in  FIG. 2 . It will be recalled that PMC  151  provides protection circuit  185  with an AC/BATTERY input signal which indicates whether the IHS is presently being powered by an AC power source or a DC battery power source. PMC  151  also provides protection circuit  185  with a RESET signal which resets protection circuit  185  when the IHS is first powered up or restarted. It will also be recalled that protection circuit  185  provides a FAULT FLAG output signal back to PMC  151 . The FAULT FLAG output signal is generated by protection circuit  185  when external module  190  is drawing too much current or power. 
   IHS  100  is first turned on or restarted to initiate process flow as indicated by start block  300 . Protection circuit  185  is then reset by PMC  151  providing a RESET signal to the reset input (R) of latch  220 . A test is then conducted at decision block  510  to determine if protection circuit  185  has been reset. If protection circuit  185  has not yet been reset, then decision block  510  continues testing until such a reset does occur. Once PMC  151  requests a reset, such as upon starting or restarting IHS  100 , the reset of latch  220  occurs and the FAULT FLAG is cleared as per block  520 . Immediately upon clearing any previous faults, cut-off switch  182  is closed as per block  530 . This action provides external module  190  and multiple threshold current protection circuit  185  with power from main DC unregulated power output  170 . 
   A test is then conducted by protection circuit  185  at decision block  540  to determine if IHS  100  is powered by an AC or DC source. If IHS  100  is powered by AC power adapter  175 , then the current limit of protection circuit  185  is set to REF 1  as per block  550 . This is achieved by connecting switch terminal  255  to a VREF 1  voltage reference which corresponds to the current limit REF 1 . However, if it is found at decision block  540  that IHS  100  is being supplied power by a DC battery power source, then the current limit of protection circuit  185  is set to REF 2  as per block  560 . This is achieved by connecting switch terminal  260  to a VREF 2  voltage reference which corresponds to the current limit REF 2 . 
   Assuming that the source of power for IHS  100  is AC power adapter  175  and that the REF 1  current limit is selected, a test is conducted at decision block  570  to determine if the current draw associated with external module  190  is greater than the selected current limit REF 1 . IHS  100  is now in a state where external module  190  is drawing power from IHS power subsystem  105  and protection circuit  185  is monitoring the current draw of external module  190  to determine if the current draw is too high. If the current draw of the external module is not greater than the predetermined current limit REF 1 , then process flow will loop back to decision block  540 . The loop thus formed will continue testing until the current draw of the external module exceeds the predetermined current limit REF 1  at block  570 . When this occurs, a fault condition exists which causes the FAULT FLAG signal to be set as per block  580  and cut-off switch  182  to be opened as per block  590 . Thus, when this over-current fault condition occurs, power is cut off to external module  190 . External module  190  is in effect disconnected from the IHS. Following this fault condition the system returns to a wait state at decision block  510  and waits until PMC  151  initiates a RESET and starts the process once more. 
   Assume however now that instead of being supplied power by AC power adapter  172 , IHS  100  is in fact being supplied power by a DC battery source such that the REF 2  current limit is selected. In this case a test is conducted at decision block  570  to determine if the current draw associated with external module  190  is greater than the selected current limit REF 2 . IHS  100  is now in a state where external module  190  is drawing power from IHS power subsystem  105  and protection circuit  185  is monitoring the current draw of external module  190  to determine if the current draw is too high. If the current draw of the external module is not greater than the predetermined current limit REF 2 , then process flow will loop back to decision block  540 . The loop thus formed will continue testing until the current draw of the external module exceeds the predetermined current limit REF 2  at block  570 . When this occurs, a fault condition exists which causes the FAULT FLAG signal to be set as per block  580  and cut-off switch  182  to be opened as per block  590 . Thus, as before, when an over-current fault condition exists, power is cut off to external module  190 . External module  190  is again disconnected. Following this fault condition the system again returns to a wait state at decision block  510  and waits until PMC  151  initiates a RESET and starts the process once more. 
   An information handling system and method of operating the system are thus disclosed which are capable of providing DC power to external modules. The disclosed IHS dynamically determines the power source as being either AC or DC. The IHS dynamically selects different current threshold limits for the external module dependent on whether the power source is use is AC or DC. The disclosed IHS disconnects the power to the external module or modules when an over-current or fault condition appears. 
   Although illustrative embodiments have been shown and described, a wide range of modification, change and substitution is contemplated in the foregoing disclosure and in some instances, some features of an embodiment may be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in manner consistent with the scope of the embodiments disclosed herein.