Jurgen Neuhausler looks at selecting suitable output capacitors for dc-dc converters in portable battery powered applications.

In modern portable µC or DSP applications like PDAs, mobile phones or digital media players, available space for the circuits is always limited. The trend is moving towards thinner, lighter and smaller.
On the other hand, micro-controllers or DSPs are getting more and more power hungry and switch quickly between different power save modes and full loading. Because of this, the demand for small power supply solutions with high efficiency, which can handle fast load transients is increasing.
Although LDOs are cheap and small they are often replaced by switching dc-dc converters. The reasons are due either to a smaller amount of battery cells, therefore requiring boost converter solutions, or the power losses of the LDO cannot be handled thermally, or the battery runtime requires converters with higher efficiency. So the designer of the power supply solution is faced with more complex switching dc-dc converter circuits. What to consider when selecting the output capacitor, and why different dc-dc converter topologies have different requirements are explained here.
The different requirements for step down converters and boost converters are discussed in detail and an example calculation of the results for a 3.3V 600mA system is also given.
Like LDOs, step down converters always need an input voltage, which is higher than the regulated output voltage. So this topology is the best choice in replacing LDOs, if higher efficiency is required. It is a very popular solution in generating 3.3V from a single cell Li-Ion, or three NiMH or alkaline cells. A schematic principle is shown in Fig. 1.
Because of the nature of switching converters, passive electronic components are required to store energy and supply the load during switching cycles.
A step down converter needs one inductor and one capacitor.
During steady state operation, energy is stored in the inductor while the main switch S is closed and the inductor supplies the load while the main switch is open. While being charged, the inductor supplies the load as well. The capacitor is needed to compensate for the difference in inductor current and load current.
Using smaller inductances results in higher inductor ripple current, which means that a higher output capacitance is required to achieve the same output voltage ripple. Because EMI problems increase with the inductor ripple current, it is good to keep it low. Lower inductor ripple currents also help to increase the efficiency of the converter.
The RMS currents in the components get lower. Reasonable values are lower than 20 per cent of the highest average inductor current under worst case conditions.

The example circuit uses a Li-Ion cell as the input source, which is very popular in portable applications. The worst case for inductor ripple current considerations is a completely charged battery with 4.2V typical output voltage. In the example this results in the use of a 6.8µH inductor, which is one of the recommended inductor values that can be used with the TPS62020 synchronous step down converter.
The operating frequency is 1.2MHz. To achieve a 1 per cent output voltage ripple in steady state operation, the required output capacitance would be almost negligible. The calculation results in 680nF.
During load transients the inductor again plays a significant role. Due to its basic characteristic of limiting current changes, it limits input current increase during the load transient. The current in the inductor can only be increased when voltage is applied. The higher the voltage, then the higher the current transient. The highest voltage available in a step down converter is the input voltage, which is applied at the inductor during normal operation as well. So the fastest current transient can be achieved by turning on the switch S as long as needed to ramp up the current to a value necessary for the new output current.
Since the inductor current is lower than needed to supply the load during the transient, the output capacitor needs to compensate for the difference. In general, the calculated 680nF to achieve the desired ripple during steady state operation is too small to handle this difference at a reasonable voltage drop.
For a load step from 300mA to 600mA at 1 per cent additional voltage drop, a required output capacitance of 22µF can be calculated. The battery voltage is assumed to be at 3.8V, a nominal output voltage of a Li Ion cell during discharging.
In case the battery voltage is always lower than the nominal output voltage, a boost converter must be used. For 3.3V voltage rails this is typically required when using single or dual cell alkaline batteries or similar numbers of rechargeable NiCd or NiMH cells. A schematic principle is shown in Fig. 2.
Like the step down converter, the boost converter requires passive electronic components to store energy and supply the load during switching cycles. As shown in Fig. 2, a boost converter needs one inductor and one capacitor as well as the step down converter.
During steady state operation energy is stored in the inductor while the main switch S is closed. The inductor supplies the load and charges the output capacitor while the main switch is open. In contrast to the step down converter, the capacitor is needed to supply the complete load during the inductor's charging phase.
The considerations regarding EMI and efficiency are similar to those for the step down converter. Due to the lower input voltage, however, the RMS inductor currents are higher than those of the step down converter at the same load.

The example calculation uses two alkaline cells as the input source. The worst case consideration for ripple is the lowest possible input voltage. With two alkaline cells, this is typically 1.8V. In the example, the calculation results in the need for a 6.8µH inductor. This is a typical value used with the TPS61090 synchronous boost converter.
The operating frequency is 600kHz. To achieve a one per cent output voltage ripple during steady state operation, the required output capacitance can be calculated to be in the range of 4.7µF.
In the same way as in the step down converter, the load transient determines the capacitance, which is really needed. At load transients a specific characteristic of the boost converter plays a significant role. As with the step down converter the current transients are limited by the inductance.
Additionally, during charging of the inductor, no impact on the output voltage can be seen. At this time the complete load current is supplied only by the output capacitor, which is discharging into the load. Whether, or not the current change in the inductor fits the new output current requirements can only be detected after the discharge phase.

In a standard boost converter the load current is usually not sensed. So the only measurement, which can indicate whether the increased inductor current fits the new load requirements or not, is the output voltage change after the discharge phase of the inductor. Permanently turning on the switch for several operating cycles as is usually done in modern step down converters, is not suitable in a boost topology. The current transient has to be handled by the standard control loop by simply increasing the duty cycle.
The calculation example uses the same load step as applied in the step down converter allowing the same additional 1 per cent voltage drop. As in that case, a typical operating input voltage is used for the calculation. For a two cell NiMH battery, this is 2.4V.
Assuming an average duty cycle change of 20 per cent (47 per cent instead of 27 per cent) during the load transient, the required output capacitance of 77µF can be calculated.
Compared with an LDO there are physical, topology-related limitations to the minimum output capacitance of the inductive switching dc-dc converter. Basically this is the inductor itself, but especially in a boost converter, additional topology-related limitations restricts the fastest possible current transient.
In step down converters, standard ceramic capacitors can be economically used even to handle large load transients. But in boost converters, the required capacitance is significantly higher to achieve the same quality of output voltage. In this capacitance range, low ESR tantalum capacitors are the more economical choice. To compensate for the higher ripple caused by the ESR of the tantalum capacitor, it is better to use even larger values.
For the calculation example this could be a 100µF type. At lower input voltages in both types of converter, the capacitance requirements further increase, since the applied voltage needed to charge the inductor gets lower. With the new step down (TPS62020) and boost (TPS61090) converters, the kind of loads described are already considered when optimising the control loop and selecting suitable values for recommended capacitors and inductors.
As the calculations show, the switching frequency of the converter is not a parameter to calculate the output capacitance during load transients. Higher switching frequency only has a benefit if the inductor value can be reduced. This reduces the time for ramping up the current and lowers the capacitance requirements at the output. Unfortunately increasing switching frequency lowers conversion efficiency as well.

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Jurgen Neuhausler is senior systems engineer - Low Power dc-dc converter, with Texas Instruments, Freising, Germany. www.ti.com