Introduction to DC-DC Conversion EE174 – SJSU Tan Nguyen

OBJECTIVES Introduction of DC-DC Converter Types of DC-DC Converters Linear regulator (LR) Switching mode power supply (SMPS) Advantages and Disadvantages

Batteries are often shown on a schematic diagram as the source of DC voltage but usually the actual DC voltage source is a power supply. DC to DC converters are important portable electronic devices used whenever we want to change DC electrical power efficiently from one voltage level to another. A power converter generates output voltage and current for the load from a given input power source. Depending on the specific application, either a linear regulator (LR) or a switching mode power supply (SMPS) solution to be chosen. Introduction

Car battery 12V must be stepped down to 3-5V DC voltage to run DVD/CD player Laptop computers or cellular phone battery voltage must be stepped down to run several sub-circuits, each with its own voltage level requirement different from that supplied by the battery.circuits Single cell 1.5 V DC must be stepped up to 5V operate an electronic circuitry. A 6V or 9V DC must be stepped up to 500V DC or more, to provide an insulation testing voltage. A 12V DC must be stepped up to +/-40V or so, to run a car hifi amplifier circuitry. A 12V DC must be stepped up to 650V DC or so, as part of a DC-AC sinewave inverter. Typical Application of DC-DC converter

How a Linear Regulator Works In an embedded system, a 15V bus rail is available from the front-end power supply. On the system board, a 10V voltage is needed to power an Op-Amp circuit. The simplest approach to generate the 10V is to use a voltage divider from the 15V bus, as shown below: Does this circuit work well? The answer is usually no. The output 10V is unchanged under the following conditions: V in is stable. The resistor values are unchanged under any condition. The load is not vary under different operating conditions. Linear Regulator

The Basic Linear Regulator A linear regulator uses a voltage-controlled current source to force a fixed voltage to appear at the regulator output terminal. The control circuitry senses the output voltage, and adjusts the current source (as required by the load) to hold the output voltage at the desired value. The design limit of the current source defines the maximum load current the regulator can source and still maintain regulation.

Linear Regulator Operation The pass device (Q1) is made up of an NPN Darlington driven by a PNP transistor. The current flowing out the emitter of the pass transistor (load current I L ) is controlled by Q2 and the voltage error amplifier. The current through the R1, R2 resistive divider is assumed to be negligible compared to the load current. Ideally, V X = V REF  Error Amp = 0  V OUT is constant. When V OUT decreases, VX < VREF  The error amplifier will be high which turns on Q2 and Q1  V IN apply to the circuit that adjusts the output to desired level. When V OUT increases, VX > VREF  The error amplifier will be low which turns off Q2 and Q1  V IN disconnect from the output that adjusts the output to desired level. Q2 VXVX

Linear Regulator Types Standard (NPN Darlington) Regulator Low Dropout or LDO Regulator Quasi LDO Regulator Note: The single most important difference between these three types is the dropout voltage, which is defined as the minimum voltage drop required across the regulator to maintain output voltage regulation.

THE STANDARD (NPN) REGULATOR In order to maintain output regulation, the pass transistor requires a minimum voltage across it given by: V D(MIN) = 2 V BE + V CE Allowing for the -55°C to +150°C temperature range, this minimum V D(MIN) is set at 2.5V to 3V by the manufacturer to guarantee specified performance limits. The actually falls out of regulation will be between 1.5V and 2.2V Example: 1)If V IN = 12V, what is the V OUT_max ? 2)If V IN = 12V, V OUT = 6V, R1 = 1 kΩ, R2 = 2kΩ, what is V REF ? Solutions: 1)V OUT_max = 12 – 3 = 9V 2)R1 = 1 kΩ and R2 = 2kΩ  V X = 4V  V REF = 4V R1 R2 VXVX

THE LOW-DROPOUT (LDO) REGULATOR The minimum voltage drop required across the LDO regulator to maintain regulation is just the voltage across the PNP transistor: V D(MIN) = V CE The maximum specified dropout voltage of an LDO regulator is usually about 0.7V to 0.8V at full current, with typical values around 0.6V. Example: 1)If V IN = 10V, what is the V OUT_max ? 2)If V IN = 10V, V REF = 2.5V and V OUT = 5V, what are values of R1, R2? Solutions: 1)V OUT_max = 10.0 – 0.8 = 9.2V 2)Want V X = V REF = 2.5V so for V OUT = 5V  R1 = R2 = 1 kΩ R1 R2 VXVX

THE QUASI LOW-DROPOUT REGULATOR The minimum voltage drop required across the Quasi-LDO regulator to maintain regulation is given by: V D(MIN) = V BE + V CE The dropout voltage for a quasi-LDO is usually specified at about 1.5V(max). The actual dropout voltage is temperature and load current dependent, but could never be expected to go lower than about 0.9V (25°C) at even the lightest load. R1 R2 VXVX Example: 1)If V IN = 5V, what is the V OUT_max ? 2)If V IN = 9V, V REF = 2V and V OUT = 5V, R1= 3 kΩ, what is value of R2? Solutions: 1)V OUT_max = 5.0 – 1.5 = 3.5V 2)Want V X = 2V for V OUT = 5V and R1 = 3 kΩ  R2 = 2 kΩ

Comparison of the Linear Regulators ≈ 0.8 V (WC) ≈ 1.5 V (WC) ≈ 3 V (WC) Example: Given V IN = 5V, require output V OUT = 3.3V and 2 A max, what is the best choice for the design? Solution: Quasi-LDO

LINEAR REGULATORS The linear regulator is a DC-DC converter to provide a constant voltage output without using switching components. The linear regulator is very popular in many applications for its low cost, low noise and simple to use. It was the basis for the power supply industry until switching mode power supplies became prevalent after the 1960s. Power management suppliers have developed many integrated linear regulators. The linear regulator has limited efficiency and can not boost voltage to make V out > V in.

A typical integrated linear regulator needs only V IN, V OUT, FB and optional GND pins. Figure below shows a typical 3-pin linear regulator, it only needs an input capacitor, output capacitor and two feedback resistors to set the output voltage. ADJUSTABLE LINEAR REGULATORS

LINEAR REGULATORS DRAWBACK A major drawback of using linear regulators can be the excessive power dissipation of its series transistor Q1 operating in a linear mode. Since all the load current must pass through the series transistor, its power dissipation is P Loss = (V IN – V O ) I O. The efficiency of a linear regulator can be estimated by:

The linear regulator can be very efficient only if V O is close to V IN. The linear regulator (LR) has another limitation, which is the minimum voltage difference between V IN and V O. The transistor in the LR must be operated in its linear mode. So it requires a certain minimum voltage drop across the collector to emitter of a bipolar transistor or drain to source of a FET. When V O is too close to V IN, the LR may be unable to regulate output voltage anymore. The linear regulators that can work with low headroom (V IN – V O ) are called low dropout regulators (LDOs). The linear regulator or an LDO can only provide step-down DC/DC conversion. LINEAR REGULATORS DRAWBACK

LINEAR REGULATORS APPLICATIONS There are many applications in which linear regulators provide superior solutions to switching supplies: 1. Simple/low cost solutions. Linear regulator or LDO solutions are simple and easy to use, especially for low power applications with low output current where thermal stress is not critical. No external power inductor is required. 2. Low noise/low ripple applications. For noise-sensitive applications, such as communication and radio devices, minimizing the supply noise is very critical. 3. Fast transient applications. The linear regulator feedback loop is usually internal, so no external compensation is required. 4. Low dropout applications. For applications where output voltage is close to the input voltage, LDOs may be more efficient than an SMPS. We see that price sensitive applications prefer linear regulators over their sampled-time counterparts. The design decision is especially clear cut for makers of: communications equipment small devices battery operated systems low current devices high performance microprocessors with sleep mode (fast transient recovery required)

Regulators Linear regulators are less energy efficient than switching regulators. Why do we continue using them? Depending upon the application, linear regulators have several redeeming features: lower output noise is important for radios and other communications equipment faster response to input and output transients easier to use because they require only filter capacitors for operation generally smaller in size (no magnetics required) less expensive (simpler internal circuitry and no magnetics required) Furthermore, in applications using low input-to-output voltage differentials, the efficiency is not all that bad! For example, in a 5V to 3.3V microprocessor application, linear regulator efficiency approaches 66%. And applications with low current subcircuits may not care that regulator efficiency is less than optimum as the power lost may be negligible overall. LINEAR REGULATORS VS SWITCHING REGULATORS

References: Linear Tecnology - Application Note