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LDOs, buck converters and PMICs

How a battery or adapter is turned into the steady voltages a chip needs, and why the choice of regulator matters for battery life.

Core

Read firstWriting a power budget
LDO, buck converter, quiescent current and PMIC compared An LDO dropping 5 V to 3.3 V burns the difference as heat, so it is at best about 66% efficient. A buck converter stores energy in an inductor and can exceed 90% at moderate loads. A regulator's own quiescent current can outweigh a sleeping board. A PMIC brings several regulators up in a fixed order. LDO regulator Linear: burns the difference as heat 5 V LDO 3.3 V heat At best about Vout / Vin about 66% Buck converter Switching: stores energy in an inductor switch inductor Costs: switching noise and more parts; often poorer efficiency at very light loads. At moderate loads can exceed 90% Quiescent current The regulator's own draw with no load regulator: tens of µA rest of a board sleeping at a few µA Matters most in battery devices that sleep. Not to scale. PMIC Several regulators plus power sequencing and reset control in one chip One PMIC regulator 1 regulator 2 regulator 3 rails up in a fixed order reset control time →
An LDO dropping 5 V to 3.3 V wastes the difference as heat and is at best about 66% efficient; a buck converter can exceed 90%, and a PMIC sequences several rails.

Key points

  1. A linear (LDO) regulator burns the difference between input and output voltage as heat, so its efficiency is at best about Vout/Vin: roughly 66% when dropping 5 V to 3.3 V.1
  2. A switching buck converter stores energy in an inductor and can exceed 90% efficiency at moderate loads, at the cost of switching noise, more parts and often poorer efficiency at very light loads.23
  3. Quiescent current is the regulator's own consumption with no load. It matters most in battery devices that sleep: a regulator drawing tens of microamps can use more than the rest of a board that sleeps at a few.13
  4. A PMIC (power-management IC) combines several regulators with power sequencing and reset control in one chip, and is common on application-processor boards that need many rails brought up in a fixed order.4

Where to go next

Sources

  1. Understanding the Terms and Definitions of LDO Voltage Regulators (SLVA079) · Texas Instruments · Reference
  2. Basic Calculation of a Buck Converter's Power Stage (SLVA477) · Texas Instruments · Reference
  3. TIDA-00393: Adding an LDO for Increased Standby Mode Efficiency (TIDUAA4) · Texas Instruments · Reference
  4. i.MX 8M Mini Hardware Developer's Guide (IMX8MMHDG) Rev. 4.1 · NXP · Reference