Multiple linear regulators power handheld devices

Fabien Franc, Staff Applications Engineer, California Micro Devices Corp., Milpitas, Calif.

Wireless handsets, personal digital assistants (PDA) and other handheld devices can't operate everything directly from the battery since each IC requires a specific, steady and accurate supply voltage. This is where the design of a power regulation system becomes necessary. The good news is that there is plenty of linear and switching regulators or a combination of these on the market. The bad news is that there may be some confusion as to which one to choose to make sure it meets the application requirement.

Why use a low dropout regulator (LDO) in a battery-powered product such as a personal digital assistant or a cell phone, when typically switching regulators provide better power efficiency and therefore enable longer run time between battery charges? In fact, LDOs are widely used to generate lower supply voltages in small handheld products in situations with small board space, relative low current and minimum difference between input and output voltages. Also switching regulators introduce ripples because of their switching nature, while LDOs actually reduce the supply noise and attenuate any incoming ripple.

A major factor when designing a power solution into a handheld is its overall cost. This does not favor switching regulators due to the expensive inductor and large capacitor required on the output. LDOs are the preferred solution to power individual chips and guarantee that the regulated voltage remains within their operating range. Typical examples are the digital ASIC cores, RF, ICs, audio CODECs, LCD displays and flash memories. It is now common practice to use multiple LDOs with shutdown pins to allow for having split rails. Various parts of the system can be powered-up and down independently for different power-down modes and therefore reduce the average power consumption, which is so critical in handheld devices.

Selecting the right LDO for the application requires looking carefully at various electrical and mechanical parameters of the device. Although there are tens of parameters, this paper will focus on those that are most critical and relevant to battery-powered applications. These include dropout voltage, quiescent current and shutdown mode, low noise and ripple rejection, and external capacitor requirements.

The dropout voltage defines the minimum acceptable input to output voltage difference for a given load current. For example, an LDO experiences a 100 mV drop under a load of 100 mA, which is equivalent to a 1-ohm on-resistance. This allows operation with a minimum input supply VIN = VOUT + 100 mV for a fixed output voltage with popular values such as 2.8 V, 2.5 V, 1.8 V or 1.5 V. This matters when the battery discharges and therefore the supply drops to a low level well below 3.3 V. In any case, the output voltage can only be less than the input voltage.

Most battery-operated devices are not fully turned-on all the time. The shutdown pin allows you to disable the regulator and reduce the current to near zero (much below 1µA.) The quiescent current, also referred to as the ground current, is the current needed to operate the LDO itself, or wasted current that doesn't go to the load. This is especially relevant under light load conditions (several milliamps) where the quiescent current should remain a small fraction of the load (typically a few tens of microamps).

Some sections of a system like the audio or the RF may be noise sensitive. Here, a low noise LDO is a must. One key specification is the "ripple rejection" which is the ability of the LDO to filter out small amplitude sinusoidal noise coming from the input supply. It is frequency dependent and its value is around a few tens of dB. A 20dB ripple rejection corresponds to an attenuation of 1/10th, and 60dB to 1/1000th. The other parameter is the "output voltage noise" which is the noise measured on the LDO output and is generated by the chip itself. It is specified within a frequency range and is typically a few tens of (VRMS for low noise LDOs.

Just about all LDOs require an output capacitor for stability. In order to lower the cost and size of this capacitor, LDOs are designed to work with a minimum size capacitor. For example, some 150 mA LDOs work fine with just a small 1µF ceramic capacitor. But in order to improve the load transient response, the capacitor value may have to be increased.

LDOs consume power through heat dissipation and a good approximation is (VIN - VOUT) ILOAD. The package size usually sets the maximum power rating but the PC board layout is also important. For example, a SOIC 8-pin package with a power lead-frame can dissipate up to a watt in the best case.

This is how fast an LDO turns-on when it is enabled. This parameter can be critical in some applications that require a quick transition between "idle" mode and "full on" state..

LDO applications

Here are a few examples of specific applications where using an LDO is the right choice. For audio, a noisy supply must be filtered out through an LDO to power the 5V analog input of the audio CODEC. This "application specific" LDO (the CMPWR161) provides a 4.75V-regulated output compatible with the Codec. In battery-powered devices, the 5 V supply typically comes from a switching regulator for better power efficiency. Audio applications are very sensitive to noise and require power supplies to be as clean as possible to minimize coupling into the analog section, which could degrade the analog performance.

An audio-optimized LDO provides a much quieter supply in the audio band from 20Hz to 20kHz. The unique feature of the LDO is its ability to provide high ripple rejection (42dB at 100Hz under 150mA load) with minimum voltage drop across the LDO (VIN - VOUT = 150mV). Most LDOs on the market only specifies the ripple rejection with a VIN - VOUT of 1V or greater.

PC cards (formerly called PCMCIA) and compactFlash cards are compatible with both 5V and 3.3V hosts. The host determines the voltage at which the card operates. Therefore, there is a need to either regulate down a 5V supply to 3.3V, or provide a low resistance connection to a 3.3V supply. A very low dropout 3.3V regulator acts either as a regulator or as a power switch depending on the supply voltage.

Typical applications are in battery-powered products like digital cameras where the power consumption must be low in both operating and shutdown modes. The regulator incorporates a CMOS pass transistor that offers the advantages of very low quiescent current (typically 100µA under full load of 150mA) and very low dropout (150mV at 150mA). An Enable pin is available for shutdown mode to achieve "near-zero" power consumption (typical current of 0.1µA). An internal 500-ohm resistor forces the output to ground when the Enable pin is set to a low level. The part also features current limiting and thermal shutdown. The device is housed in a power MSOP 8-pin package that provides enhanced heat dissipation, and fits both in the PC card and CompactFlash formats because of its 1mm height. Also the part has been designed to operate with a single 1uF ceramic capacitor on the output to reduce the overall cost and space.

Some battery-powered handheld equipment such as digital cameras and MP3 players are connected to PCs via the USB cable for downloading data. For example, MP3 players can download music from the Internet. At that time, it is handy to take the power from the USB 5V or VBUS line (limited to 500mA by the USB specification), instead of the battery. The LDO generates the 3.3V rail needed for the ASIC. The CMPWR160 500mA LDO provides the 3.3V-regulated output and also a reset output signal monitoring the output with automatic detection of invalid level to reset a microprocessor. The monitoring function remains active, even if the USB supply goes away, like in a dual power system.

When a low noise supply is required, a low dropout regulator can be used, with an external bypass capacitor CBYP (typically 10 nanofarads) connected to the internal chip reference. This greatly improves the noise performance.

LDOs provide an effective way to resolve power management issues in battery-powered devices. A significant advantage for the end-user is the broad market offering with various industry-standard package pinouts available for current ratings of 150 mA, 300 mA, 500 mA and 1 A.