Overview
DC-DC can be understood as converting one DC level to another (conversion between different DC supply levels). By this definition, any device that performs such conversion could be called a DC-DC converter, including LDOs. However, in common usage DC-DC generally refers to devices that perform DC-to-DC conversion using switching techniques.
LDO Characteristics
LDO stands for low dropout. Low-dropout linear regulators have several notable advantages: low cost, low output noise, and low quiescent current. They require few external components, typically only one or two bypass capacitors. Modern LDOs can reach specifications such as 30 μV output noise, 60 dB PSRR, 6 μA quiescent current, and only 100 mV dropout. This performance is mainly due to the use of P-channel MOSFETs as the pass element rather than PNP transistors used in traditional linear regulators. P-channel MOSFETs are voltage-driven and do not require drive current, which significantly reduces the regulator's own current consumption. In circuits using PNP transistors, the input-to-output voltage drop cannot be too small without risking transistor saturation and loss of output capability. In contrast, the voltage drop across a P-channel MOSFET is approximately the product of output current and on-resistance. Because MOSFETs can have very low on-resistance, the voltage drop can be very low.
When to Choose an LDO
If the input and output voltages are very close, an LDO is often the best choice because it can achieve high efficiency in that condition. For example, converting a lithium-ion battery voltage to a 3V output commonly uses an LDO. Although some battery energy may be unused in the end, an LDO can still provide long operating time while keeping noise low.
Switching DC-DC Converters
If the input and output voltages are not close, a switching DC-DC converter should be considered. LDO input current is essentially equal to output current, so with a large voltage drop the power dissipated in an LDO becomes large and efficiency suffers.
DC-DC converters include boost, buck, buck-boost, and inverting topologies. Their advantages are high efficiency, the ability to supply large output currents, and low standby current. As integration has increased, many modern DC-DC converters require only a few external inductors and filter capacitors. However, switching regulators produce larger output ripple and switching noise, and they tend to have higher cost.
Recent Developments
Advances in semiconductor technology have reduced the cost and size of surface-mount inductors, capacitors, and highly integrated power-control chips. Low Rds(on) MOSFETs enable high power output without the need for external high-power FETs. For example, with a 3V input, integrated NFETs can provide a 5V/2A output. For low-to-medium power applications, compact and low-cost packages are available. Increasing switching frequency to around 1 MHz can reduce cost and allow the use of smaller inductors and capacitors. New devices also add features such as soft-start, current limiting, and selectable PFM or PWM modes.
Summary
For boost (step-up) applications, a switching DC-DC converter is required. For step-down applications, the choice between a DC-DC converter and an LDO depends on trade-offs among cost, efficiency, noise, and performance.
LDO vs DC-DC: Direct Comparison
- Efficiency: DC-DC converters are generally much more efficient than LDOs due to their switching operation.
- Topology: DC-DC includes boost, buck, and buck-boost options (some also categorize charge pumps here). LDOs provide only step-down regulation.
- Noise: Because of switching, DC-DC converters produce significantly more power-supply noise than LDOs. Pay attention to PSRR when the supply must be clean for sensitive analog circuits; in such cases an LDO may be chosen despite lower efficiency.
- External components and layout: LDOs typically require simpler and smaller external components and occupy less board area. DC-DC converters usually require inductors, diodes, large capacitors, and sometimes MOSFETs. Design considerations for DC-DC include inductor maximum current, diode reverse recovery, and capacitor ESR, making the external component selection and layout more complex and space-consuming compared with LDO solutions.
