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HP 360LX and 340LX Palmtop PCs - Power Management

Introduction
The power supply system in the HP Palmtop PC is designed to accept three input power sources. The palmtop PC also contains a built-in charging circuit to charge NiCd batteries installed in the unit. The charging control algorithm is controlled by software.
The input sources are 2xAA size alkaline or NiCd batteries installed in the unit, an external DC input supply through the DC jack and 1xLi CR2032 size battery installed in the unit to be used as a backup power source. With an external DC power source, the maximum power available to the system is 3W. For new alkaline batteries, power up to 3W is possible. For nearly depleted batteries, power available to the system can be as low as 1W. Therefore for batteries that are near the depleted state, power hungry devices such as PCMCIA card, CF card and backlight have to be disabled to conserve the battery life.
At any one time, only one input source is used to power the system. They work in the following order:
  1. External power source
  2. Main power source
  3. Backup power source
This means that when the external power source is supplied to the system, the supply sources from both the main and backup batteries will be cut off. In the absence of the external power source, the main batteries will provide power to the system. In the case when the main batteries are either removed from the system or are weak, the backup battery will come into operation. However, the backup battery can only function to keep the system in suspend mode and it does not enable the users to operate the system. These controls will all be hardware controlled.
DC Adapter Input Supply
Only an external 5 V HP adapter is allowed to be used to power the system. All other internal input sources will be cut off using hardware when the external adapter is powering the system.
Main battery
The main battery consists of 2 AA size alkaline or rechargeable NiCd batteries. However, the system is designed to optimize for the use of alkaline batteries with NiCd as a secondary battery source. The housing for the main battery is designed so that when the batteries are inserted the reverse way, there will not be any contact made to the circuit.
Alkaline battery
The capacity of a typical alkaline battery is spec to be more than 1800 mAh. Set the type of battery used to "Alkaline" when using an alkaline battery as the main supply source.
NiCd battery
The system can support charging for 2 AA size rechargeable NiCd battery. The recommended capacity of the battery is between 600 mAh to 1000 mAh. Unlike alkaline, NiCd batteries have a very flat discharge curve. The fully charged combined voltage is slightly greater than 2.7 V (NiCd Fully Charged) and the end of charge voltage is around 1.8 V. The first 10 percent and the final 10 percent of the discharge curve are very steep. The 10 percent discharge voltage is around 2.6 V and the final 10 percent discharge voltage is around 2.4 V. From 10 percent discharge to 90 percent discharge, the voltage drops linearly from 2.6 V to 2.4 V.
NiMH battery
The HP Palmtop PC is designed to work with NiMH batteries. Below is the power applet screen. Notice that the tabs for Battery Status and Type are no longer used. Instead all the information will be on one screen. The supported NiMH capacity range is 1000 mAh to 1400 mAh.
It is important to note that the default Battery Type is Alkaline.
Figure : AC Power and NiMH battery - charging enabled
Charging
The system can support both quick charge and trickle charge when the battery type chosen is NiCd and the charging icon is enabled. In our application, the quick charge current is designed to be 180 mA. However, in order to support higher capacity batteries at 1000 mAh, the maximum charging time is set to be 8 hours. This will ensure that a higher capacity battery will be fully charged.
The quick charging termination is set when any of the following conditions is true :
  1. The battery has reached a maximum voltage at 3.2 V. (NiCd Battery Overvoltage)
  2. The system registers a battery voltage drop of more than 25 mV.
  3. The drop in voltage can be detected by comparing the current voltage measured as compared to the maximum voltage that the system has measured since current was started.
  4. The charging time has reached a maximum time of 8 hours.
The quick charging termination algorithm is monitored by the system software. After the quick charge is terminated, the system will provide trickle charging to the battery as long as the ACIN remain high and charging is not deactivated. For trickle charging, a charge rate of 0.025 C will be adequate to counter self-discharge.
Backup battery
The system uses one Li CR2032 battery for backup purposes. This backup battery will be used to maintain the system when the main batteries are removed from the system or when the main batteries are weak. The typical capacity of the backup battery is 180 mAh. The backup battery comes into operation when there is no ACIN signal and the VLB signal is active low. The Very Low Battery (VLB) signal is set to be low when the main battery voltage drops below 1.4 V. This signal is reset when the main batteries' voltages rise above 1.8 V.
If the backup battery is low, the device may function abnormally. The abnormalities may be related to the display, calibration, settings, sound, or other features. For example, the image on the screen will blink on and off or mix colors on the screen and/or the stylus may frequently need to be calibrated. Often the abnormality will occur with more frequency as the battery loses more power. The backup battery should be replaced AT LEAST every 6 months: sooner if the device is operated often with a low main battery.
Voltage management
The system’s worst current consumption occurs when the system is either accessing the DRAM or the CompactFlash card while the PCMCIA is also being accessed. First battery low warning to data loss is 5 days.
Battery Stages
Alkaline
NiCd
Main Battery Low Warning
2.20V
2.30V
Main Battery Very Low Warning*
2.05V
2.10V
Main Battery NMI Voltage**
1.78V
1.78V
Main Battery Cutoff***
1.50V
1.50V
* At main battery very low warning, the system will issue the main battery very low warning message then shutdown the system immediately.
** At NMI, the system will go into critical suspend within 120Ts.
*** Backup battery will kick in to replace the main battery.

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