Maxim > Design=20 support > App = notes=20 > Battery=20 Management > APP 3607
Maxim > Design = support >=20 App = notes=20 > Interface=20 Circuits > APP 3607

Sep 23, = 2005

Keywords: USB, USB Charger, Li+ USB = charger,=20 Lithium Ion USB charger, NiMH USB charger, USB battery = charger, charging batteries from USB, charging from = USB,=20 charger, battery charger
Related=20 parts

Download,=20 PDF format (117kB)

[?]

APPLICATION NOTE 3607

Charging batteries from USB

Abstract: USB=20 offers great opportunities as a power source for all = types of=20 low power electronics, many of which are battery = operated. The=20 widespread availability of USB presents unique = opportunities=20 as well as challenges for battery-charging designs. = This=20 article describes how to interface a simple battery = charger to=20 a USB power source. This review of USB power bus=20 characteristics include an overview of NiMH and Li+ = battery=20 technologies, charging methods, and charge termination = techniques, and a complete circuit example for = smart-charging=20 NiMH cells from a USB port.

Introduction

The Universal=20 Serial Bus (USB) port is a bidirectional=20 data port with power and ground. Peripherals of all = types can=20 be connected to the USB, including external drives, = memory=20 devices, keyboards, mice, wireless=20 interfaces, video and still cameras, MP3 players, and=20 countless other electronics. Many of these devices are = battery=20 powered, some with internal batteries. The widespread=20 availability of USB presents unique opportunities, as = well as=20 challenges, for battery-charging designs. This paper = describes=20 how to interface a simple battery charger to a USB = power source.=20 This review of USB power bus characteristics includes = voltage,=20 current limits, inrush=20 current, connectors, and cabling. An overview of = nickel=20 metal hydride (NiMH) and lithium battery = technologies,=20 charging methods, and charge-termination techniques is = given.=20 A complete example circuit for smart-charging NiMH = cells from=20 a USB port is presented, along with charging data. =

USB characteristics

The USB bus can provide = power for=20 low-power electronics. It is isolated from power mains = and is=20 relatively well regulated. However, there are = limitations on=20 available current and potential interactions between = the load=20 and the host or power source.

The USB port = consists of=20 a 90=CE=A9 bidirectional differential=20 shielded twisted pair, V_BUS (+5V power), = and=20 ground. These four wires are shielded with an inner = shield of=20 solid aluminum and a stranded outer shield. Copies of = the USB=20 2.0 specification are available free of charge from = the USB = organization.=20 Full compliance with the specification requires = bidirectional=20 communication between the device and the host through = a=20 function controller. The specification defines a unit = load as=20 100mA (max). The maximum current that any device is = allowed to=20 draw is five unit loads.

USB ports are = classified as=20 either low-power ports, which supply up to one unit = load, or=20 high-power ports which supply up to five unit loads. = When=20 devices are first connected to the USB port, an = enumeration=20 process identifies the device to determine its load=20 requirements. During this time, the device is only = allowed to=20 draw one unit load from the host. After the = enumeration=20 process, higher powered devices are permitted to draw = higher=20 current if the power-management software in the host = allows=20 it.

Some host systems (including downstream = USB hubs)=20 have current limiting either through fuses or active = current=20 sensors. If a USB device presents a high current (over = one=20 unit) load to the USB port without enumerating, it can = cause a=20 detectable overcurrent condition that could shut=20 down one or more of the USB ports in use. Many=20 commercially available USB devices, including = stand-alone=20 battery chargers, draw over 100mA without a function=20 controller to handle the enumeration process; they run = the=20 risk of causing problems for the host under the wrong=20 circumstances. For instance, if a device drawing 500mA = is=20 plugged into a bus-powered USB hub, it could overload = both the=20 hub port and the host port if it is not properly = enumerated.=20

Further complications arise when the host = operating=20 system is using advanced=20 power management, especially for notebook = computers, and=20 is expecting the port current to be extremely low. In = some=20 power-saving modes, the computer issues suspend = commands to=20 USB devices, which are then expected to go to a = low-power=20 mode. It is always a good idea to include a function=20 controller to communicate with the host even with = low-power=20 devices.

The USB 2.0 specification is quite = thorough=20 and specifies power quality, connector construction, = cable=20 materials, allowable voltage drops, and inrush = current.=20 Low-current and high-current ports have different=20 power-quality specifications. These are determined = primarily=20 by the voltage drop in the connectors and cabling = between the=20 host and the load, including voltage drop across a = USB-powered=20 hub. A host, such as a computer or self-powered USB = hub, has=20 high-current ports capable of supporting up to 500mA. = Lower=20 current ports are found on passive, bus-powered USB = hubs.=20 Table 1 gives the allowable tolerances for the = voltage=20 at the pins on the upstream (source) side of the USB = port for=20 high- and low-current ports.

Table 1. USB = 2.0=20 specification power-quality standards=20

Parameter	Requirement
DC voltage, high-power port*	4.75V to 5.25V
DC voltage, low-power port*	4.40V to 5.25V
Maximum quiescent=20 current (low power, suspend mode)	500=C2=B5A
Maximum quiescent current (high power, suspend = mode)	2500=C2=B5A
Maximum allowable Input capacitance (load = side)	10=C2=B5F
Minimum required output capacitance (host = side)	120=C2=B5F =C2=B120%
Maximum allowable inrush charge Into load	50=C2=B5C

*These = specifications apply=20 to the pins of the host or hub port connector on the = upstream=20 side. Additional I x R drops due to cables and = connectors must=20 be counted separately.

In hosts that are = compatible=20 with the USB 2.0 specification, the upstream side of a = high-power port is provided with 120=C2=B5F of low-ESR = capacitance.=20 The input capacitance of attached USB devices is = limited to=20 10=C2=B5F, and the total allowable charge drawn from = the host (or=20 powered hub) during an initial load connection is = 50=C2=B5C. Thus,=20 when a new device is connected to a USB port, the = transient=20 voltage drop at the upstream port is less than half a = volt. If=20 more capacitance is required for correct operation of = the=20 load, it must be provided with an inrush current = limiter to=20 charge the larger capacitance at no greater than = 100mA.=20

The allowable DC voltage drops for a USB port = having a=20 bus-powered USB hub with low-powered functions = attached are=20 shown in Figure 1. A high-power load connected = to an=20 unpowered hub has larger voltage drops than shown in = Figure 1=20 and can overload the bus.

Figure=20 1. Drops larger than these allowable DC voltage drops = from=20 host to low-power load can overload the = bus.

Battery-charging requirements

Single-cell lithium=20 ion and lithium polymer

Present-day lithium=20 chemistries are typically 4.1V to 4.2V when the cells = are=20 charged to their maximum-rated capacity. Newer, higher = capacity cells are being marketed with voltages in the = 4.3V to=20 4.4V range. Typical prismatic lithium ion (Li+) and = lithium=20 polymer (Li-Poly) have capacities of 600mAh to = 1400mAh.=20

The preferred charge profile for both Li+ and = Li-Poly=20 cells is to start the charge with a constant charge = current=20 until the cell voltage reaches the rated voltage. When = this=20 occurs, the charger then regulates the voltage across = the=20 cell. These two regulation states are called = constant-current=20 (CC) and constant-voltage (CV) charging; therefore, = this type=20 of charger is usually referred to as a CCCV charger. = When the=20 CCCV charger is in CV mode, the current into the cell = begins=20 to drop. For typical charge rates, between 0.5C and = 1.5C, the=20 transition between CC and CV mode occurs when the cell = has=20 accepted about 80% to 90% of its full-charge capacity. = Once=20 the charger is in CV charging mode, it then monitors = the cell=20 current; when it reaches a low threshold (milliamps or = tens of=20 milliamps), the charger terminates charge. A typical = charge=20 profile for a lithium chemistry battery is shown in = Figure=20 2.

Figure=20 2. Typical results are shown for a Li+ battery charged = using a=20 CCCV charger.

The USB voltage drops shown = in Figure=20 1 show that a low-powered port, on the downstream side = of a=20 port-powered hub, would barely have enough headroom to = charge=20 a cell to 4.2V. Small amounts of additional resistance=20 in the charge path could prevent proper charge. =

Li+=20 and Li-Poly cells should be charged at moderate = temperatures.=20 Manufacturers' recommended maximum charging = temperatures are=20 typically in the +45=C2=B0C to +55=C2=B0C range, with = allowable=20 discharge temperatures about 10=C2=B0C higher. The = materials used=20 in these cells are highly reactive and can ignite if = the cell=20 temperature exceeds +70=C2=B0C. Chargers for lithium = chemistry=20 cells should be designed with a thermal cutoff circuit = that=20 monitors cell temperature and terminates charging if = the cell=20 temperature exceeds manufacturer-recommended maximum = charging=20 temperatures.

Nickel metal hydride cells

NiMH cells are = heavier and=20 have less energy density than lithium-based cells.=20 Historically, they have been less expensive than = lithium, but=20 the price gap has recently been shrinking. NiMH cells = are=20 available in standard sizes and are a direct = replacement for=20 alkaline cells in most applications. Nominally, each = cell is=20 1.2V, and when fully charged they are as high as=20 1.5V.

NiMH batteries are usually charged with a = constant-current source. As they reach the fully = charged=20 state, an exothermic chemical reaction occurs that = causes the=20 battery temperature to increase and the terminal = voltage to=20 decrease. Either the rate of rise of the battery = temperature=20 or a negative voltage change can be detected and used = to=20 terminate the charge. These termination methods are = known as=20 dT/dt and -=CE=94V, respectively. At very low charge = rates, the=20 dT/dt and -=CE=94V effects become less pronounced and = can be=20 difficult to detect accurately. The dT/dt and -=CE=94V = responses=20 begin as the cell starts to become overcharged. = Continued=20 charging beyond this point can damage the=20 cell.

Termination detection at charge rates = above C/3=20 is much easier than at low charge rates. The = temperature rise=20 is about 1=C2=B0C/min, and the -=CE=94V response is = more pronounced than=20 at lower rates. After fast-charge terminates, an = additional=20 charging period at a reduced current is recommended to = top off=20 the cell (top-off charge). When the top-off charge = cycle is=20 done, a trickle-charge current of C/20 or C/30 = counters the=20 effects of self-discharge and maintains the battery in = a fully=20 charged state. A graph showing the cell voltage during = a=20 charging cycle of a partially charged NiMH cell using = a DS2712=20 NiMH charger is shown in Figure 3. In this = graph, the=20 upper curve is data taken while current is being = driven into=20 the battery; the lower curve shows data taken with the = charge=20 current source turned off. In the DS2712, this = difference in=20 voltage is used to distinguish between NiMH cells and = alkaline=20 cells. If an alkaline cell is detected, the DS2712 = will not=20 charge it.

Figure=20 3. A DS2712 charge controller is used to charge a NiMH = cell.

Switching vs. linear

The USB 2.0 specification = allows=20 up to 100mA from a low-power port and 500mA from a = high-power=20 port. If a linear pass element is used to regulate = charge=20 current to the battery, these are the maximum = allowable charge=20 currents. The power dissipated in the linear pass = element=20 (Figure 4) is P =3D V_Q x = I_BATT.=20 This causes power dissipation in the pass transistor = and can=20 require the use of a heat=20 sink to prevent overheating.

Figure=20 4. Power dissipation is the battery current times the = voltage=20 across the pass transistor.

For a nominal = input=20 voltage of 5V, the pass element dissipates a varying = amount of=20 power based on the type and number of cells, as well = as the=20 battery voltage.

Figure=20 5. Power is dissipated in a linear pass element, with = NiMH=20 cells being dissipated from a USB port at 5.0V input=20 voltage.

Figure 5 shows the = calculated power=20 dissipation for a linear USB charger with NiMH cells = at a=20 nominal input voltage of 5.0V. With single-cell = charging, a=20 linear charger is only about 30% efficient; a two-cell = charger=20 is 60% efficient. Charging a single cell at 500mA = results in=20 up to 2W of power dissipation. This amount of power = typically=20 requires a heatsink.=20 At 2W of dissipation, a +20=C2=B0C/W heatsink heats up = to about=20 +65=C2=B0C from an ambient=20 temperature of +25=C2=B0C, and it needs exposure = to free air=20 for full performance. Inside an enclosure with still = air, it=20 will get much hotter.

Several problems are = solved by=20 using a switching-regulator-based charger. First, the = cells=20 can be charged faster and at a higher current than = with a=20 linear charger (Figure 6). Because less power = is lost=20 as heat, the thermal=20 management problem is reduced. Also, the charger = is more=20 reliable, as it runs cooler.

Figure=20 6. Charge times differ for a linear charger vs. a = switching=20 charger when charging a single-cell NiMH=20 battery.

The calculated values in Figure 6 = are=20 based on charging from a high-power USB port at about = 90% of=20 the 500mA maximum allowable current. The switching=20 regulator in this example is assumed to be a=20 nonsynchronous buck converter with 77% = efficiency.

Circuit example

The circuit shown in Figure = 7=20 is a switch-mode buck=20 regulator for charging a single NiMH cell. It uses = a=20 DS2712 charge controller to regulate charging current = and=20 terminate charge. The charge controller monitors = temperature,=20 battery voltage, and battery current. If the = temperature is=20 above +45=C2=B0C or below 0=C2=B0C, the controller = does not begin=20 charging the battery.

Figure=20 7. Schematic shows a single-cell NiMH charging from a = USB=20 port.

In Figure 7, Q1 is the switching = power=20 transistor for the step-down charger; L1 is the = smoothing=20 inductor; and D1 is the freewheeling, or catch, diode.=20 Input C1 is a 10=C2=B5F, very low-ESR ceramic filter = capacitor.=20 Substituting tantalum or other electrolytic capacitors = for C1=20 can adversely influence charger performance. R7 is the = current-sense resistor for the sense amplifier=20 of the current regulator. The reference voltage for = the DS2712=20 is 0.125V with an hysteretic component of 24mV. = Closed-loop,=20 switch-mode current control is provided through CSOUT. = Gate=20 drive to Q1 is enabled when the gate of Q2 is pulled = low by=20 the charge control pin, CC1. Both Q1 and Q2 are low-Vt = (gate-source threshold voltage) pMOSFETs. When CC1 and = CSOUT=20 are both low, the drain to source voltage of Q2 is = slightly=20 over one Vt. This voltage, plus the forward voltage = drop of=20 CSOUT, establishes the available switching-gate drive = voltage=20 for Q1. When CC1 is low, it enables closed-loop = control of the=20 current. Startup switching waveforms are shown in = Figure=20 8. The top waveform is the voltage across the = 0.125=CE=A9=20 current-sense resistor, and the bottom waveform is the = Q1=20 drain to GND voltage. Initially, current ramps up in = the=20 inductor while Q1 is ON (CC1 and CSOUT both low). When = the=20 current reaches 0.125V, CSOUT goes high. Also, current = ramps=20 down until the voltage on the current-sense resistor = reaches=20 about 0.1V, whereupon CSOUT goes low again. This = process=20 continues as long as CC1 is low.

Figure=20 8. Startup waveforms are shown for a USB NiMH=20 charger.

The DS2712's internal state = machine=20 controls the gating action of CC1. At the beginning of = charge,=20 the DS2712 performs cell qualification tests to make = sure that=20 the cell voltage is between 1.0V to 1.65V, and to = verify that=20 the temperature is between 0=C2=B0C and +45=C2=B0C. If = the voltage is=20 below 1.0V, the DS2712 gates CC1 low at a duty factor = of=20 0.125, which slow-charges the cell to prevent damage. = Once the=20 cell voltage exceeds 1.0V, the state machine = transitions into=20 fast-charge. The fast-charge duty factor is 31/32, or = about=20 97%. The "skipped" pulse is used to perform impedance=20 tests on the cell to make sure that a high-impedance = cell,=20 such as an alkaline cell, has not been installed in = the=20 charger. Fast-charge continues until a =CE=94V of -2mV = is detected.=20 If no -=CE=94V is detected, fast-charge continues = until the=20 fast-charge timer expires or until an overtemperature = or=20 overvoltage fault condition (including impedance = failure) is=20 detected. When the fast-charge is done (either due to = -=CE=94V or=20 expiration of the fast-charge timer) the DS2712 enters = its=20 timed topoff mode, with a duty factor of 12.5% and a = duration=20 of half of the programmed fast-charge timeout. After = topoff is=20 complete, the charger goes into maintenance mode with = a duty=20 factor of 1/64, and remains in maintenance until the = cell is=20 removed or power is cycled.

The charger shown = in=20 Figure 7 fast-charges a 2100mAh NiMH cell from a = high-power=20 USB port in a little over two hours, with a full = top-off=20 charge achieved in about three hours. Current drawn = from the=20 port is 420mA. If enumeration with the host and = high-current=20 enable is required, an open-drain=20 NMOSFET can be inserted in series between R9 and = ground. If=20 the MOSFET=20 is off, TMR floats and the DS2712 is in its suspend=20 state.

Summary

The USB port is an economical and = practical=20 power source for charging batteries for small consumer = electronics. To be fully compliant with the USB 2.0=20 specification, loads connected to the USB port must be = capable=20 of bidirectional communication with the host. Loads = must also=20 comply with power-management requirements, including = low-power=20 modes and a means of allowing the host to determine = when high=20 power is drawn from a port. While partially compliant = systems=20 may operate compatibly with most USB hosts, they = occasionally=20 give unexpected results. A good understanding of USB=20 requirements and expectations of the load are needed = to make=20 the right tradeoff between full compliance and load=20 complexity.

A similar version of this article = appeared=20 in the January, 2006 issue of Battery Power = Products=20 Technology magazine.

Related parts =

DS2711	Loose Cell = NiMH=20 Chargers	Free=20 samples
DS2712	Loose Cell = NiMH=20 Chargers	Free=20 samples
DS2715	NiMH Battery = Pack=20 Charge Controller	Free=20 samples=20

Automatic updates
Would you = like to be=20 automatically notified when new application notes are=20 published in your areas of interest? S= ign=20 up for EE-Mail=E2=84=A2.

Download,=20 PDF format (117kB)
APP 3607: Sep = 23, 2005=20
AN3607, AN 3607, = APP3607,=20 Appnote3607, Appnote 3607=20 =