CDMA handset design challenge: 11 separate power supplies

Consumers don't want to think about the batteries in their cell phones. They want to talk for hours, and go for weeks between charges. When they finally have to charge the battery they want to be able to continue to use the phone, and they want the charging function to be completed quickly. The battery should be capable of being charged in the home or in the car, making the situation even more complex. All this requires low power operation and sophisticated power management.

A typical CDMA handset can have as many as 11 separate power supplies. Each of these supplies requires an independent low dropout (LDO) regulator. Not all subsections of the cell phone operate simultaneously, so unused subsections must be powered down to maximize efficiency. For example, the phone can't ring while a call is in process, so the vibrator circuits can be shut down. Similarly, when the phone is in standby mode the audio circuitry is not needed and can, therefore, be shut down. For super high efficiency, special LDOs that are optimized for light loads can be used as backups for the primary LDOs, further reducing power consumption and increasing battery life in standby mode.

In addition to powering the cell phone and charging the battery, the power management system can provide several important peripheral functions including keypad interface, real-time clock (RTC), general-purpose inputs and outputs, under-voltage detection, and stay-alive timer. All of these functions must be highly integrated in order to reduce the size and cost of the cell phone. Efficient power management can make the difference between the sale of a phone from one manufacturer and the sale of a competing model from a second manufacturer.

Cell phones must turn on in response to various stimuli and turn off in the absence of stimuli. This is one of the functions of the power management system. Obviously, the phone must turn on when the user presses the power switch. Typically it would turn on in an "active load" standby mode. Baseband and RF functions would be enabled as the phone searches for an available cell on the wireless network, and the display would be illuminated. If the user receives a call the ring circuits will be activated. If the call is answered the audio circuits will also be enabled. If no calls are initiated or received for a preset length of time the phone might switch into a "mid-load" standby mode; the RF circuitry and display illumination would be disabled. In "light load" standby mode power would be further reduced by putting the baseband circuitry into a very low power mode. The circuitry would have little to do other than monitor the keypad for activity, check the real-time clock for alarms, or wait for an incoming call. These functions would trigger the phone to return to mid-load standby, active load standby, ring, or talk modes. In light-load standby mode the battery may be able to last for two or three weeks.

If the alarm time that has been programmed on the real-time clock is reached while the phone is off, the phone will switch itself on and the alarm will sound. If the user is on an airplane, in a hospital, or in an automated factory environment the use of a cell phone may cause interference, or may be prohibited. Thus, the phone will power-up in the mid-load standby state, with the RF circuitry disabled. The user will be queried as to whether or not to activate the phone for calls.

Connecting an external charger can also turn the handset on. The turn-on mode will depend upon the battery voltage. If the battery has been deeply discharged, and its voltage below an under-voltage lockout threshold, the LDOs will remain off. If the battery voltage is sufficient they will turn on.

LDOs for everything?

Each of the low dropout regulators in a cell phone power management system must be optimized for its own special function. For example, the backup LDOs must consume very low quiescent current under light load conditions; the analog baseband and audio LDOs must reject the ripple from the RF power amplifier, consume low quiescent current under light load conditions, and provide high output current during talk mode; the noise on the output of the transmit oscillator LDO must be very low in order to maintain a stable clock frequency; and the real-time clock LDO must consume low quiescent current and provide reverse current protection when the main battery is removed and the coin cell powers the RTC. Some of the LDOs may be programmable or adjustable to accommodate different types of memory or analog peripherals. In addition, each of the LDO outputs must be properly sequenced, ensuring that each output turns on and off at the right times to prevent latch up, excess current draw, or logic errors.

The battery charger found in a cell phone power management IC must automatically operate in one of several modes. These modes include pre-charge, low current charge, and full current charge, depending on the voltages measured on the charger, battery, and LDO outputs. Some chargers may be capable of charging both Li-Ion and NiMH batteries; others are optimized for a single battery type.

The charger is detected, and charging is allowed, when the charger is plugged in and turned on, and the charge voltage is greater than the battery voltage. If the battery voltage is less than the deep discharge lockout value a small pre-charge current is turned on to safely bring the battery voltage back up. Once this occurs, the coin cell LDO is turned on. The pre-charge current is turned off, the low current charge mode is enabled, and all of the baseband LDOs are turned on. The baseband circuitry determines the correct battery voltage, and the full charge current is enabled. Charging is terminated once the nominal battery voltage is reached.

Cell phone power management is much more than turning on the radio to make a call, or turning on the charger to recharge the batteries. It includes optimized regulators to efficiently power each subsystem in the cell phone when it is needed, a charger that can automatically tailor the charging profile to maximize battery lifetime, a real-time clock with alarm functions, and system and user interfaces. All this must be contained in a small package, must operate over a wide temperature range, must be configurable to operate with many types of cell phones, and must be available at low cost. By reducing cost, supporting more features, and maximizing talk and standby times, the right power management system can help cell phones sell themselves.