Power Electronics

Universal AC Wall Adapters Will Support Future GSM Cell Phones

Scheduled for initiation in 2012, a universal charging solution will provide for the charging of all cell phones, worldwide. This will require the appropriate battery charger ICs.

LEADING MOBILE operators and manufacturers have said they are committed to implementing a cross-industry standard — called the Universal Charging Solution (UCS) — that will create a universal charger for future GSM cell phones. The aim is to ensure the mobile industry adopts a common format for mobile-phone charger connections and energy-efficient chargers, which could result in an estimated 50% reduction in standby energy consumption and the potential elimination of up to 51,000 tons of duplicate chargers. The industry group agreed that by January 1, 2012, the majority of all new cell phones will support a universal charging connector and the majority of chargers shipped will meet the high-efficiency targets set out by the Open Mobile Terminal Platform (OMTP), the industry body that developed the technical requirements behind UCS.

A universal charger will allow users to use the same charger for future cell phones, as well as offer the ability to charge their phone anywhere from any available charger (Fig. 1). UCS chargers will also include a four-star or higher efficiency rating, which is up to three times more energy-efficient than an unrated charger. The initial group of companies who have joined the UCS initiative include 3 Group, AT&T, KTF, LG, mobilkom austria, Motorola, Nokia, Orange, Qualcomm, Samsung, Sony Ericsson, Telecom Italia, Telefónica, Telenor, Telstra, T-Mobile, and Vodafone.

GSM is a digital mobile telephony system that is widely used in Europe and other parts of the world. The other competing network technology is CDMA, which is used in the U.S. Still to be decided is whether CDMA cell-phone systems will also follow UCS approach.

Brian Chu, applications engineer for Microchip Technology, points out that one of the technical challenges in developing a UCS is the charger IC's ability to recognize whether its input is via a USB port or an ac adapter, or if there is a short on the input. Another potential problem in charging can occur if the USB port is not protected against an overcurrent condition.

Implementation of the UCS requires an appropriate battery-charger IC that will be integrated within the cell phone. This battery-charger IC subsystem should have the following characteristics:

  1. Minimum parts count
  2. Minimum weight increase for the cell phone
  3. Efficient operation
  4. Ability to charge a single Li-Ion battery
  5. Ability to charge the battery via an ac adapter or PC USB port
  6. Distinguish whether the input is an ac adapter, USB port, or is shorted
  7. “Quiet” operation that won't interfere with critical analog circuits in the cell phone (preferably a linear topology)
  8. Low supply current during standby mode to extend battery life

A BATTERY CHARGER IC FOR UCS

An example of a battery charger IC capable of meeting the new UCS cell-phone standard is Microchip Technology's MCP73871. It is a fully-integrated linear solution for system load sharing and Li-Ion/Li-Polymer battery-charge management, with an ac-dc wall adapter or USB-port power source. It can also provide autonomous power-source selection between the input and battery. Its small physical size and small number of required external components makes it ideally suited for UCS applications.

The IC powers the cell phone via output terminals while independently charging the associated battery. This reduces the charge and discharge cycles on the battery, allows proper charge termination, and also enables the cell phone to run with an absent or deeply depleted battery pack.

This IC automatically obtains power for the cell phone from a single-cell Li-Ion battery or the input power source. The MCP73871 (Fig. 2) specifically adheres to the current-drain limits in the USB specification. It operates from an input supply voltage of VREG +0.3 V to 6 V, where VREG is its regulated output voltage.

Battery charging current reduces if the voltage on the IC's IN pin drops to a preset value, determined by the threshold established at its voltage proportional charge control (VPCC) input. This can occur due to a limited amount of input current or input source impedance.

Battery charging current also drops if the IC reaches its input current threshold. The input current-limit control (ICLC) tries to reach a steady-state condition where the cell-phone load has priority and the battery is charged with the remaining current.

No active control limits the current to the cell phone. Therefore, if the system demands more current than the input can provide or the input ICLC is reached, an ideal diode becomes forward-biased and the battery can supplement input current to the cell-phone load.

The ICLC sustains the cell phone as its highest priority. This is done by reducing the non-critical charge current while adhering to the current limits governed by the USB specification or the maximum ac-dc adapter current supported. Further demand from the system is supported by the battery, if possible.

The IC's input-source-type selection (SEL) pin selects the input power source for the input current-limit control feature. With the SEL input High, the MCP73871 provides a typical 1.65 A to system power and charges the Li-Ion battery from a regular 5-V wall adapter. The MCP73871 limits the input current up to 1.8 A. When SEL is active Low, the input source provides system power and the Li-Ion battery is charged from a USB port input while adhering to the current limits governed by the USB specification.

With an ac-dc wall adapter providing cell-phone power, an external resistor sets the magnitude of 1 A maximum charge current while supporting up to 1.8 A total current for the cell-phone load and battery charge current.

An internal undervoltage lockout (UVLO) circuit monitors the input voltage and keeps the charger in shutdown mode until the input supply rises above the UVLO threshold. If a battery is present when the input power is applied, the input supply must raise approximately 100 mV above the battery voltage before the MCP73871 becomes operational. The UVLO circuit places the IC in shutdown mode if the input supply falls to approximately 100 mV of the battery voltage.

The UVLO circuit is always active. Any time the input supply is below the UVLO threshold, or approximately 100 mV of the voltage at the VBAT pin, the MCP73871 is placed in a shutdown mode. During any UVLO condition, the battery reverse discharge current is less than 2 µA.

The MCP73871 also protects against a faulted or shorted input. Without this protection, a faulted or shorted input would discharge the battery pack through the body diode of the internal pass transistor.

The MCP73871 employs a constant current/constant voltage (CC/CV) charge algorithm with a selectable charge termination point. Its constant voltage regulation includes four available options: 4.10 V, 4.20 V, 4.35 V, or 4.40 V, which accommodates new and emerging battery-charging requirements.

The MCP73871 also limits the charge current based on die temperature during high-power or high-ambient conditions. This thermal regulation optimizes the charge cycle time while maintaining device reliability.

The maximum constant charge current is set by placing a resistor from PROG1 to VSS. PROG1 sets the maximum constant charge current for both the ac-dc adapter and USB port. However, the actual charge current is based on input source type and system load requirements.

The MCP73871 USB-port current-regulation-set input (PROG2) is a digital input selection. A logic Low selects a 1-unit load-input current from the USB port (100 mA); a logic High selects a 5-unit load-input current from the USB port (500 mA).

The MCP73871 continuously monitors battery temperature during a charge cycle by measuring the voltage between its THERM and VSS pins. An internal 50-µA current source provides the bias for most common 10-kΩ negative-temperature-coefficient (NTC) thermistors.

The MCP73871 compares the voltage at the THERM pin to factory-set thresholds of 1.24 V and 0.25 V, typically. If the IC detects a voltage outside the thresholds during a charge cycle, the MCP73871 immediately suspends the charge cycle. The charge cycle resumes when the voltage at the THERM pin returns to the normal range. You can set the charge temperature window by placing fixed-value resistors in series-parallel with a thermistor.

The charge cycle terminates when, during constant voltage mode, the average charge current diminishes below a threshold established with the value of a resistor connected from PROG3 to VSS or internal timer has expired. A 1-msec filter time on the termination comparator ensures that transient load conditions do not result in premature charge cycle termination. The timer period is factory set and can be disabled.

The MCP73871 continuously monitors the voltage at the VBAT pin in the charge complete mode. If the voltage drops below the recharge threshold, another charge cycle begins and current is once again supplied to the battery or load. The recharge threshold is factory set.

Both STAT1 and STAT2 are open-drain logic outputs for connecting to an LED for charge status indication. STAT1 also serves as a low-battery-output (LBO) indicator, reminding the user when the Li-Ion battery voltage level is low. The LBO feature activates when the system is running from the Li-Ion batteries and can be used as an indication to the user via a lit LED that an input source other than the battery is supplying power.

A power-good (PG) open-drain logic output indicates power-supply operation. The PG output is low whenever the input to the MCP73871 is above the UVLO threshold and greater than the battery voltage. You can use the PG output as an indication to the user via an illuminated LED that an input source other than the battery is supplying power.

If the voltage at the VBAT pin is less than the preconditioning threshold, the MCP73871 enters a preconditioning mode. The preconditioning threshold is factory set. In this mode, the MCP73871 supplies 10% of the fast charge current (established with the value of the resistor connected to the PROG1 pin) to the battery. When the voltage at the VBAT pin rises above the preconditioning threshold, the MCP73871 enters the constant current (fast charge) mode.

The MCP73871 limits the charge current based on the die temperature. Thermal regulation optimizes the charge-cycle time while maintaining device reliability. The MCP73871 suspends charge if the die temperature exceeds 150°C. Charging will resume when the die temperature has cooled by approximately 10°C.

Thermal shutdown is a secondary safety feature in the event that there is a failure within the thermal regulation circuitry. The MCP73871 is fully specified over the ambient temperature range of -40° to 85°C.

For optimum voltage regulation, place the battery pack as close as possible to the device's VBAT and VSS pins to minimize voltage drops along the high-current-carrying PCB traces. If the PCB layout is used as a heat sink, adding many vias in the heat-sink pad can help conduct more heat to the PCB backplane and reduce maximum junction temperature. Fig. 3 shows an evaluation module for the MCP73871.

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