New architecture for LDOs
08 February 2008
The low drop out linear regulator is regaining popularity. Recent advances have given a new life to the LDO.
Power supply designers are benefiting from feature improvements such as lower dropout voltages and simplified paralleling techniques. Furthermore, reverse battery and reverse current protection shield the IC and the surrounding system, which serve to increase overall reliability.
Design challenges
Surface mount board design has evolved with the advent of sophisticated manufacturing techniques, multi-layer PCBs, smaller and thinner discrete components and thinner IC packages. However, power supply output current is often limited by the ability of a surface mount IC to dissipate power (about 2W). At higher currents, a traditional linear regulator requires a heat sink, precluding an all surface mount solution. One alternative is a high performance switching regulator which increases complexity, cost and noise. Another alternative is to use multiple load-sharing linear regulators in parallel. This increases available output current and spreads the dissipated power over a larger area in a surface mount system.
New high performance digital circuits require less than 1.2V and there is no reason to believe that these voltages will not continue to decrease. Traditional linear regulators use a 1.2V reference that is boosted to generate a regulated output equal to or above 1.2V. A new generation of linear regulators produce voltages below 1.2V (see Table 1) and Linear Technology’s LT3080’s current source architecture allows it to produce output voltages down to 0V.
New architecture
Paralleling linear regulators on a PC board spreads the heat, increases maximum output current, and helps maintain low peak board temperatures. Traditionally, this has required an external op amp and several resistors. That is no longer the case with the 1.1A low-noise LDO LT3080 (see figure 1).
This new current reference-based architecture also makes it possible to set VOUT with a single resistor down to 0V. With the ability to provide zero output, the LT3080 can control powering-down parts of the system. The trimmed 10μA ±1 per cent current reference is available at the set pin. Connecting a single resistor from set to ground generates a voltage that becomes the reference point for the error amplifier. The reference voltage is a straight multiplication of the set pin current and the value of the resistor. Any output voltage can be obtained from zero up to the maximum defined by the input power supply. A minimum load current of 1mA is required to maintain regulation regardless of output voltage. The input voltage capability is 1.2V to 36V (40V max). The dropout voltage is 300mV (two supply operation) at full load, limiting power dissipation and increasing overall system efficiency. Output noise is 40µVRMS over a 10Hz to 100kHz bandwidth.
Protection features include current limiting with foldback and thermal shutdown. The IC’s direct paralleling and wide VIN and VOUT capability, tight line and load regulation, high ripple rejection and low external parts count ensure it is suited to multiple-rail systems. The LT3080 is offered in a variety of thermally-enhanced surface-mount compatible packages that are able to dissipate 1W to 2W without a heatsink. In addition, the device is housed in a TO-220 power package for mounting to heatsinks for higher power dissipation operations.
One possibly damaging scenario in battery-powered systems occurs when an end user incorrectly connects a battery. If the IC is exposed to a reverse voltage supply, large currents flow to ground through parasitic junctions in the die; potentially destroying fragile junctions in the IC. An on-chip solution not only protects the IC and the load, but also eliminates problems incurred by adding external components.
Linear regulators can be destroyed if they are forced to source excessive current. This type of protection circuitry kicks in under short circuit or excessive load conditions in which VOUT < VIN. In a short circuit condition, not only is the pass transistor sourcing excessive current, but the voltage across it is at a maximum (since VOUT is at ground, the voltage across the transistor is VIN). Linear regulators typically use one of two types of short circuit protection on chip; constant current limit, or current limit with foldback. The addition of foldback, or safe operating area (SOA) protection, decreases the current limit as the input voltage increases to keep the power transistor in its SOA.
With thermal shutdown, the part is actually shut off and the die must cool down by the amount of hysteresis built into the thermal shutdown circuitry. Once the part has cooled down, it is re-started. If the fault or overload exists, the part heats up to the thermal shutdown temperature and turns off. Therefore, the part thermally oscillates at a low frequency and duty cycle depending on the thermal shutdown temperature, the amount of hysteresis, and the associated thermal time constants. Linear Technology’s higher current (≥ 500mA) LDOs utilise this type of protection.
The LT1965 LDO
The LT1965 is a low noise, low voltage 1.1A PNP power device LDO with high power density. It features low dropout voltage of 300mV at full load, with VIN capability of 1.8V to 20V and low adjustable output from 1.2V to 19.5V. Output tolerance is tightly regulated to within ±3 per cent over line, load and temperature. The device’s low quiescent current of 500µA (operating) and less than 1µA (shutdown) make it suited to applications requiring high output drive capability with low current consumption.
The LT1965 regulator optimises stability and transient response with ceramic output capacitors as small as 10µF. These tiny external capacitors can be used without any series resistance as is common with other regulators. The LT1965 incurs no damage if its output is pulled below ground. If the input is left open circuit or grounded, the output can be pulled below ground by 22V. For the adjustable version, the output acts like an open circuit and no current flows from the output. However, current flows in (but is limited by) the resistor divider that sets the output voltage. If the input is powered by a voltage source, the output sources current equal to its current limit capability and the LT1965 protects itself by thermal limiting. In this case, grounding the SHDN (shutdown now) pin turns off the device and prevents the output from sourcing current.
Furthermore, no damage occurs even if the adjacent pin is pulled above or below ground by 9V. If the input is left open circuit or grounded, the adjacent pin performs like an open circuit when pulled below ground and like a large resistor (typically 5k up to 3V on the adjacent pin and then 1.5k up to 9V) in series with a diode when pulled above ground. In situations where the adjacent pin connects to a resistor divider that would pull the adjacent pin above its 9V clamp voltage, the adjacent pin input current must be limited to less than 5mA. For example, a resistor divider is used to provide a regulated 1.5V output from the 1.2V reference when the output is forced to 20V. The top resistor of the resistor divider must be chosen to limit the current into the adjacent pin to less than 5mA when the adjacent pin is at 9V. The 11V difference between the ‘out’ and adjacent pins divided by the 5mA maximum current into the adjacent pin yields a minimum top resistor value of 2.2k.
In circuits where a back-up battery is required, several different input/output conditions can occur. The output voltage may be held up while the input is either pulled to ground, pulled to an intermediate voltage, or is left open circuit. Current flow back into the output follows a curve. If the LT1965’s ‘in’ pin is forced below the ‘out’ pin or the ‘out’ pin is pulled above the ‘in’ pin, input current typically drops to less than 2µA. This occurs if the LT1965 input is connected to a discharged (low voltage) battery and either a back-up battery or a second regulator holds up the output.
STEVE KNOTH is product marketing engineer, Linear Technology.
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