Applying dedicated power supplies
07 February 2008
Different applications require different design parameters. Size, efficiency, flexibility and external component count all have to be considered to suit the end purpose
Small size is important for integrated circuits for smartphones. In navigation systems, however, due to the large display and form factor, space is not as critical. Navigation systems are designed to operate for only a few hours at a time. Clipped to the car dashboard, they are powered by a 12V adapter connected to the car´s battery. The adapter typically contains a pre-regulator to provide 5V DC. The input voltage is typically used to charge a Li-Ion battery. Battery chargers can be with or without power path.
For chargers without power path, the battery is connected directly to the load and the current provided by the charger is split between the load and the battery. If the application is turned off and there is no load current, the full current provided by the charger charges the battery. When the application is turned on, the charge current is reduced and part of the current powers the application. However, the charge current into the battery cannot be predicted and only the total output current of the charger for battery and application is known.
Topologies
In the second charger topology, the battery is separated from the load by a switch. If there is no input voltage to the charger, the switch is closed and the battery is connected to the output, powering the application. When an external power supply is connected, the switch from the battery to the power output is opened and a second switch from the input of the charger to the power output is closed. The input voltage is either directly connected to the output or, is pre-regulated to some 100mV above the battery voltage or to a fixed voltage. A second circuit independently charges the battery. Chargers with a power path provide options to limit the input current, the current from the car adapter or, from a USB bus. The charge current can be set independently. The battery’s charge current does not depend on the load, charge termination is precise and, if externally powered, the output voltage can be equal to the input voltage.
Depending on what type of charger is used, the input voltage range differs. The minimum operating voltage is typically defined by the minimum voltage of the Li-Ion cell, which may be as low as 3V with standard Li-Ion cells. The maximum voltage depends on the charger. For those without a power path, the maximum voltage equals the maximum battery voltage, which is typically 4.2V. With a power path active, the voltage may rise to more than 5V. It desirable to have a power supply with a good efficiency over the full input voltage range. This is crucial if there are LDOs (low dropout linear regulators) integrated on the power supply chip, as their efficiency mainly depends on the voltage across the pass element, defined by the difference of input and output voltage.
For an integrated circuit, solutions should not have a battery charger integrated into a power management unit (PMU). The charger can be adapted to the input sources available and the battery used with more flexibility. The charger can be located close to the battery or input connectors, while the PMU can be placed close to the processor to be powered.
Integration options
There are also solutions integrating power for the display and display backlight as well as blocks like audio amplifiers and audio codecs. A device integrating several blocks becomes a customer-specific device, which is difficult to adapt without compromising on certain parameters.
As an example, the TPS65024x PMUs contain three step-down converters dedicated for the I/O, memory and core voltage of a handheld device. In addition there are three LDOs for voltage rails that require a supply voltage with very low ripple or low current. LDO1 and LDO2 can provide an output current of 200mA, while LDO3 is dedicated for a voltage rail (Vdd_alive) that needs to be turned on even when the application processor is in sleep mode. The output current capability is 30mA and the supply current for LDO3 is 10uA only, keeping the current from the battery in sleep mode as small as possible.
All devices are optimised for low quiescent supply current, the current needed without providing any current to the output, but still maintaining the output voltage. This parameter is critical for applications that are operated in standby mode over a long period. A low quiescent supply current improves standby time and is also an important parameter when it comes to efficiency at very low output current of a DC/DC converter.
High efficiency
The efficiency of a DC/DC converter e.g. a step-down converter, is influenced by three factors. At a high output current, the efficiency is mainly determined by the resistance of the internal power switches, so a low resistance is important. In step-down converters, operated in fixed frequency pulse width modulation mode (PWM), the duty cycle depends on the input to output voltage ratio. For low output voltages, the internal low side switch (NMOS) is turned on for much longer than the high side switch (PMOS), for high output voltages, the high side switch is turned on most of the time. It makes sense to adapt the size, and hence the resistance, of the switches to the output voltage of the converter to which it is targeted.
For an output current in the range of 10mA to 200mA, the resistance of the switches does not account for the majority of the losses any more; the gate charge for the power switches and inductor losses determine the efficiency. Adapting the switching frequency to the output current is the key technique to maintain high efficiency in this operating range, called pulse frequency mode (PFM). PFM provides a constant portion of energy to the output, resulting in a higher switching frequency at high output current and a low switching frequency and therefore low switching losses at low output current. At a very low output current on the converters, the constant loss caused by the quiescent supply current determines the efficiency.
The TPS650240 is optimised for Samsung application processors, which require a core voltage of 1V in low power mode and 1.3V in normal operating mode. To minimise the amount of external components, step-down converter 1 has a fixed 3.3V or 2.8V output dedicated to the I/O voltage. Converter 2 is for the memory voltage of 2.5V or 1.8V. The output voltage on converter 3 can be switched between 1V and 1.3V, depending on the status of a digital input called DEFDCDC3. Therefore, no external components are needed to set the voltage for the two step-down converters. To maintain flexibility, an external voltage divider can be connected to set the output voltage in a range of 0.6V to the input voltage (Vbat) for converter 1 and converter 2.
Two of the three LDOs come with a separate input voltage pin, allowing them to be powered by any input voltage in the range of 1.5V to 6.5V. LDO3 is powered internally from the input voltage pin Vcc. In addition, there is a voltage comparator, which can be used to detect if the voltage falls below a certain threshold and to warn the application processor.
THOMAS SCAHEFFNER is systems engineer for Power Management unit, Texas Instruments Europe
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