Figure 1. Host to low-power load voltage drop is greater than the figure given to allow DC voltage drop, it will cause the bus overload.
Battery requirements
1. Single-cell lithium-ion and lithium polymer batteries
Today's lithium battery to its maximum dell laptop battery rated capacity, its voltage is typically between 4.1V to 4.2V. The current market is being sold, the newer, higher capacity battery, its voltage range between 4.3V to 4.4V. Typical prismatic lithium-ion (Li +) and lithium polymer (Li-Poly) battery capacity of 600mAh to 1400mAh.
Pairs of Li + and Li-Poly battery, the preferred charge curve is starting from the constant current charging and continued until the battery voltage reaches the rated voltage. Then, the voltage across the battery charger regulated. These two methods constitute a constant current regulator (CC) constant voltage (CV) charging. Therefore, this type of charger is usually referred to as CCCV charger. CCCV charger into the CV mode, the battery charge current begin to decline. The use of a 0.5C to 1.5C charging rate of a typical charging, then when the battery reaches its full capacity of 80% to 90%, the charger from the CC mode to CV mode. Chargers Once CV charge mode, the battery current is monitored; When the current minimum threshold (a few tens of mA or mA), the charger Laptop Battery to terminate charging. A typical lithium battery charging curve shown in Figure 2.
Figure 2. The use of CCCV of Li + battery charger when the typical curve
Figure 1 shows the USB voltage drop of indicators can be seen that the downstream hub port power supply voltage low-power ports do not have enough margin, it is difficult to charge the battery to 4.2V. Charging path of a small amount of extra resistance there will be disruption to normal charging.
Li + and Li-Poly batteries should be charged under the appropriate temperature. Manufacturer's recommended maximum charging temperature of typically +45 ° C to +55 ° C between the maximum allowable discharge temperature can be higher than 10 ° C or so. Materials used in these batteries, the chemical nature of the very lively, if the battery temperature exceeds +70 ° C, combustion will occur. Li-ion battery charger should have thermal shutdown circuit monitors the battery temperature, if the Latitude D600 battery temperature exceeds the manufacturer's recommended maximum charging temperature, then the termination charge.
2. Nickel-metal hydride batteries (NiMH)
NiMH batteries should be heavier than lithium batteries, their energy density is lower than the lithium batteries. All along, NiMH batteries are cheaper than lithium batteries, but recently the price difference between the two is narrowing. NiMH batteries have a standard size, in most applications can be a direct replacement for alkaline batteries. Each section of the battery nominal voltage of 1.2V, will reach full 1.5V.
Commonly used constant-current source for NiMH batteries. When you reach the full state, exothermic chemical reaction will occur and lead to the battery temperature rises, the battery terminal voltage reduction. Can detect the battery temperature increase rate or a negative voltage change rate, and is used to terminate the charge. The charge termination methods are known as dT / dt and-ΔV. Charging rate is very low, dT / dt and-ΔV less obvious, it is difficult to detect accurately. Overcharge the battery began to enter the state, dT / dt and-ΔV response to starting to Latitude D520 show. At this point if it continues to charge, will damage the battery.
Termination detection in the charge rate is greater than C / 3 than when a low charge rate when the much easier. Temperature rise rate of about 1 ° C / min,-ΔV to respond to a low charge rate than when the more obvious. After fast charge, suggested that a smaller current recharge period of time to completely sufficient battery (supplement charge). Complement the charging end of the period following the introduction of C/20 or C/30 trickle charge current to compensate for the self-discharge effect, so that the battery remained at full status. Figure 3 shows the DS2712 NiMH charger with NiMH batteries (having been a part of the electric charge) for charging the battery voltage curve. In the diagram above a curve of the data is being poured into the battery charge current when the access to, the following data Natiao curve when the measured current is cut off. In the DS2712, this voltage difference is used to distinguish between Latitude D510 NiMH batteries and alkaline batteries. If you detect alkaline batteries, then the DS2712 will not be charging it.
Figure 3. Using DS2712 controller NiMH rechargeable battery
Switching and Linear
USB 2.0 specification allows low-power port to provide the maximum current of 100mA, high-power port to provide the maximum current of 500mA. If using a linear adjustment of the device to adjust the battery charge current, which is the largest available charge current. Linear adjustment device (Figure 4) power dissipation is P = VQ x IBATT. This will result in adjustment of tube heating, you may need to install the radiator to prevent overheating.
Figure 4. Power is equal to the battery charging current multiplied by the adjustment of the voltage across the tube
Corresponds to 5V nominal input voltage, to adjust the device's power consumption and battery type, number and Latitude D505 battery voltage.
Figure 5. Using 5.0V voltage of the NiMH battery charging USB port, the linear adjustment of the power consumption of the device
Nominal input voltage of 5.0V, the linear USB battery charger for NiMH battery power consumption results in Figure 5. Pairs of single-cell charging, the efficiency of a linear charger is only 30%; pairs of two batteries, the efficiency is 60%. With 500mA current on a single battery, the power will be up to 2W. This power is usually need to increase heat sink. Power consumption of 2W, the thermal resistance of +20 ° C / W radiator at +25 ° C ambient temperature will be heated to about +65 ° C, to obtain full capacity performance, but also the need to assist their cooling air flow . Enclosed space at a static air, the temperature will be higher.
Based switching regulator charger to resolve several issues. First of all, when compared with the linear charger can be at a faster rate, larger current on the battery charge (Figure 6). Due to lower power consumption, less heat and thermal management issues are also reduced. At the same time,Latitude D500 due to lower operating temperature, chargers and more reliable.
Figure 6. On a single NiMH battery, the linear charger and switch charger charging at different times
Figure 6 in the calculations and assumptions are based on the following conditions: high-power USB port maximum allowable current (500mA) for about 90% of the charge; switching regulator use of non-synchronous rectification buck converter with a 77% efficiency.
Circuit examples
Figure 7 shows the circuit is used for a single NiMH battery switch-mode buck regulator. It uses DS2712 charge controller regulating charge current and charge termination. Charge controller monitors the temperature, battery voltage and battery current. If the temperature exceeds +45 ° C or below 0 ° C, controller will not charge the Inspiron 600m battery.
Figure 7. USB ports on a single fast-charging NiMH battery schematic
Shown in figure 7, Q1 is the step-down charger switching power transistor; L1 is the filter inductance; D1 is a freewheeling diode, or rectifier. Input capacitance C1 as 10μF, low ESR ceramic filter capacitor. Using tantalum capacitors or other electrolytic capacitor C1 will replace the charger performance degradation. R7 is the current regulator sense amplifier sense resistor. DS2712 reference voltage of 0.125V, and with 24mV hysteresis. Through the provision of closed-loop CSOUT, switch-mode current control. CC1 to Q2 charge control pin down when the gate so that it can Q1's gate drive. Q1 and Q2 are low-Vt (gate - source threshold voltage) of the pMOSFET. CC1 and CSOUT are low, Q2's drain - source voltage will be slightly larger than Vt. The voltage and the forward voltage drop constitutes a CSOUT gate switching voltage of Q1.
CC1 is low, the starting current Inspiron 510m closed-loop control. Figure 8 shows the waveforms when the start switch. The top of the waveform is 0.125Ω (the voltage across sense resistor, the bottom waveform is the Q1 drain to the GND voltage. The beginning, when Q1 opens (CC1 and CSOUT are low), the inductor current is upwards. When the current increased so large that the voltage across the sense resistor to 0.125V when, CSOUT becomes high, the switch off. Since then, the inductor current began to decline, until the voltage across the sense resistor to approximately 0.1V, CSOUT also becomes low power level. as long as CC1 is low, the process continues.
Figure 8. USB NiMH charger to start the waveform
DS2712's internal state machine controls the CC1 work. Charging the beginning, DS2712 first state of the battery tests to ensure that the battery voltage is between 1.0V to 1.65V and confirmed that the temperature in the 0 ° C to +45 ° C between. If the voltage is below 1.0V, DS2712 will be 0.125 of the duty cycle down CC1, slow battery charging, to prevent damage to the Inspiron 500m battery. Once the battery voltage exceeds 1.0V, the state machine to fast charge mode. Fast charge duty cycle of 31/32 hours, or about 97%.
"Skip" within the margins of the battery impedance testing to ensure that no errors into a high-impedance battery charger (such as alkaline batteries) to charge. Detected after-2mV of-ΔV, fast-charge end. If not detected-ΔV, will continue to fast-charge, until the fast charge timer timeout, or to detect over-temperature or over-voltage fault condition (including the impedance of failure) so far. Fast-Charge completed (due to-ΔV, or fast-charge timer timeout), after, DS2712 into the regular complement the charge mode, the duty cycle of 12.5%, duration of fast-charge from time to time established by half. Supplement charge is completed, the charger into the maintenance mode, the duty cycle of 1 / 64, until the battery is removed or re-power.
As shown in Figure 7 using high-power USB port on the charger and 2100mAh 312-0305 NiMH battery, the fast charge time for a little over 2 hours, about 3 hours to complete, including the full complement, including charging the charging process. Lessons from the port current of 420mA. If you need to host enumeration process and the need to enable high-current operation, R9 and ground in series between an open-drain nMOSFET. If the MOSFET turn-off, then the TMR floating empty, DS2712 into a pending state.
Summary
For small consumer electronics devices in terms of battery charging, USB port is an economic and practical power. To fully comply with USB 2.0 specifications, connected to the USB port of the load must be able to host two-way communication. The load must also comply with power management requirements, including low-power mode, and easy to determine when the host needs to port a means of drawing large currents. Although some compatible system to adapt to most of the USB host, but sometimes unexpected results. Only a good understanding of the USB specification requirements and expectations of the load can be regulated when it comes to compatibility between the complexity and load to achieve 312-0306 better balance.
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