POWER ELECTRONICS LIBRARY PART 2. SEMICONDUCTORS CHAPTER 7: VOLTAGE REGULATOR irtually all power supplies employ semi- conductors to provide a regulatedIC output s voltage. If the supply has an ac input, it is rectified to be a dc voltage. A power convert- Pass VIN VOUT er IC accepts the dc input and produces a Transistor dc output or controls external power output semiconductor switches to produce a dc output. It is a V CIN voltage regulator when its output voltage is fed back to Error COUT a circuit that causes the voltage remains constant. If the Amplifier output voltage tends to rise or fall, the feedback causes Ground the output to remain the same. The power converter can operate either as a switch- LDO Bandgap Bypass mode or linear circuit. In a linear configuration, the Voltage Reference controlling transistor always dissipates power, which can be minimized by using low dropout regulators (LDOs) CBYPASS that regulate properly even when there is a relatively low voltage differential between their input and output. LDO ICs have simpler circuits than their switch-mode cousins 7-1. In the basic LDO, one input to the differential and produce less noise (no switching), but are limited by error amplifier, set by resistors R1 and R2, monitors their current-handling and power dissipation capability. a percentage of the output voltage. The other error Some LDO ICs are specified at about 200mA and others amplifier input is a stable voltage reference (VREF). If the can handle up to about 1A. output voltage increases relative to VREF, the differential Efficiency of the LDO ICs may be 40-60%, whereas error amplifier changes the pass-transistor’s output to the switch-mode ICs can exhibit up to 95% efficiency. maintain a constant output load voltage (VOUT). Switch-mode topologies are the primary approach for embedded systems, but LDOs also find use in some and wireless applications. LDOs with an on-chip power applications. MOSFET or bipolar transistor typically provide outputs in the 50 to 500mA range. Low Dropout (LDO) Linear Regulator An LDO voltage regulator operates in the linear LDO linear regulators are usually employed in sys- region with the topology shown in Fig. 7-1. As a basic tems that require a low-noise power source instead of a voltage regulator, its main components are a series pass switching regulator that might upset the system. LDOs transistor (bipolar transistor or MOSFET), differential also find use in applications where the regulator must error amplifier, and precise voltage reference. maintain regulation with small differences between the Key operational factors for an LDO are its dropout input supply voltage and output load voltage, such as voltage, power-supply rejection ratio (PSRR), and output battery-powered systems. Their low dropout voltage and noise. Low dropout refers to the difference between the low quiescent current make them a good fit for portable input and output voltages that allow the IC to regulate ☞LEARN MORE @ electronicdesign.com/powermanagement | 31 POWER ELECTRONICS LIBRARY CHAPTER 7: VOLTAGE REGULATOR ICs voltage, maximum load current, minimum VIN IN LT3042 5 V ±5% dropout voltage, quiescent current, power 4.7 µF 100 µA dissipation, and shutdown current. EN/UV – Controlling the LDO’s frequency com- V + OUT pensation loop to include the load capacitor 200k OUT 3.3 V I (MAX) reduces sensitivity to the capacitor’s ESR PG OUT OUTS 200 mA (equivalent series resistance), which allows SET GND ILIM PGFB 4.7 µF a stable LDO with good quality capacitors 450k of any type. In addition, output capacitor placement should be as close as possible 499Ω to the output. 4.7 µF 33.2k 50k Additional features in some LDOs are: • An enable input that allows external con- trol of LDO turn-on and turn-off. 7-2. The LT3042 is an LDO that uses a unique architecture to minimize • Soft-start that limits inrush current and noise effects and optimize Power Supply Ripple Rejection (PSRR). controls output voltage rise time during power-up. the output load voltage. That is, an LDO can regulate the • A bypass pin that allows an external capacitor to re- output load voltage until its input and output approach duce reference voltage noise. each other at the dropout voltage. Ideally, the dropout • An error output that indicates if the output is going out voltage should be as low as possible to minimize power of regulation. dissipation and maximize efficiency. Typically, dropout • Thermal shutdown that turns the LDO off if its tempera- is considered to be reached when the output voltage ture exceeds the specified amount. has dropped to 100mV below its nominal value. The • Overcurrent protection (OCP) that limits the LDO’s load current and pass transistor temperature affect the output current and power dissipation. dropout voltage. An LDO’s internal voltage reference is a potential LT3042 noise source, usually specified as microvolts RMS over The LT3042 from Linear Technology is a low dropout a specific bandwidth, such as 30 µV RMS from 1 to 100 (LDO) linear regulator that uses a unique architecture kHz. This low-level noise causes fewer problems than to minimize noise effects and optimize Power-Supply the switching transients and harmonics from a switch- Ripple Rejection (PSRR). mode converter. In Fig. 7-1, the LDO has a (voltage-ref- PSRR describes how well a circuit rejects ripple, erence) bypass pin to filter reference voltage noise with injected at its input. The ripple can be either from the a capacitor to ground. Adding the datasheet-specified input supply such as a 50Hz/60Hz supply ripple, switch- input, output, and bypass capacitors usually results in a ing ripple from a DC/DC converter, or ripple due to the non-problematic noise level. sharing of an input supply with other circuits. Among their operational considerations are the type For LDOs, PSRR is a function of the regulated out- and range of the applied input voltage, required output put voltage ripple compared to the input voltage ripple over a given frequency range (typically10Hz to 1MHz), VIN IN LT3080 expressed in decibels (dB). It can be an important factor 1.2 V TO 36 V when an LDO powers analog circuits because a low VCONTROL PSRR may allow output ripple to affect other circuits. + Low-ESR output capacitors and added reference voltage bypass capacitors improve the PSRR perfor- 1 µF – OUT mance. Battery-based systems should employ LDOs V OUT that maintain high PSRR at low battery voltages. SET 2.2 µF The LT3042 shown in the simplified schematic of Fig. RSET 7-2 is an LDO that reduces noise and increases PSRR. V = R • 10 µA OUT SET Rather than a voltage reference used by most tradition- al linear regulators, the LT3042 uses a current refer- 7-3. The LT3080 can program output voltage to any level ence that operates with a typical noise current level of between zero and 36V. 20pA/√Hz (6nARMS over a 10Hz to 100kHz bandwidth). ☞LEARN MORE @ electronicdesign.com/powermanagement | 32 POWER ELECTRONICS LIBRARY CHAPTER 7: VOLTAGE REGULATOR ICs ON ESR, unlike other regulators. 10% OFF The LT3080 is especially well suited to applications needing multiple rails. Its architec- ON Voltage 50% OFF ture adjusts down to zero with a single resistor Reference handling modern low-voltage digital ICs as well ON OFF as allowing easy parallel operation and thermal + 90% + PWM Out management without heat sinks. Adjusting to – Driver “zero” output allows shutting off the powered Error – Amp PWM circuitry and when the input is pre-regulated— Comp. such as a 5V or 3.3V input supply—external Oscillator resistors can help spread the heat. (Ramp) Output A precision “0” TC 10μA internal current voltage source connects to the non-inverting input of its Feedback Compensation power operational amplifier, which provides a Network low-impedance buffered output to the voltage on the non-inverting input. A single resistor from 7-4. A PWM controller produces square waves of different widths the non-inverting input to ground sets the output dependent on the out voltage feedback. voltage; setting this resistor to zero produces zero output. Any output voltage can be ob- The current source is followed by a high performance tained from zero up to the maximum defined by the input rail-to-rail voltage buffer, allowing it to be easily paral- power supply. leled to further reduce noise, increase output current Use of a true current source allows the regulator to and spread heat on a PCB. Paralleling multiple LT3042s exhibit gain and frequency response independent of the further reduces noise by a factor of √N, where N is the positive input impedance. Older adjustable regulators number of parallel circuits. change their loop gain with output voltage and change bandwidth when bypassing their adjustment pin. For the LT3080 LT3080, the loop gain is unchanged by changing the Linear Technology’s LT3080 is a unique, 1.1A LDO output voltage or bypassing. Output regulation is not that you can paralleled to increase output current or fixed at a percentage of the output voltage but is a fixed spread heat in surface-mounted boards (Fig. 7-3). fraction of millivolts. Use of a true current source allows This IC brings out the collector of the pass transistor to all the gain in the buffer amplifier to provide regulation allow low dropout operation—down to 350 mV—when and none of that gain is needed to boost the reference used with multiple supplies. Protection features include to a higher output voltage. short-circuit and safe operating area protection, as well The IC can operate in two modes. One is the as thermal shutdown.