“Low dropout (LDO) linear regulators are often used to provide clean power to processor cores and communication circuits. In these applications, LDO regulators are exclusively used due to the stringent performance requirements of processors and power amplifiers on power supply output noise and load transient response. These circuits typically require an LDO regulator that can meet each IC’s current rating and supply rail requirements to minimize solution size.
By Andy Radosevich Senior Applications Engineer at ADI
Low dropout (LDO) linear regulators are often used to provide clean power to processor cores and communication circuits. In these applications, LDO regulators are exclusively used due to the stringent performance requirements of processors and power amplifiers on power supply output noise and load transient response. These circuits typically require an LDO regulator that can meet each IC’s current rating and supply rail requirements to minimize solution size.
LDO regulators often require hardware modifications to adjust the output voltage, but if the specifications are constantly changing, changing the board and components can increase development time. In such applications, LDO regulators with software programmable output voltages can save time and cost.
However, software control of the LDO regulator output can only solve part of the problem. LDO regulators are often used as post-regulators to switching regulators. From an LDO regulator perspective, switching regulators are typically used to pre-regulate input power before it reaches a linear regulator. Ideally, the output of the switching regulator should have just the right amount of headroom (above the dropout of the LDO regulator) to allow the LDO regulator to operate in the most efficient region and optimize transient response. In order to maintain the proper input voltage of the LDO regulator, the output of the switching regulator must be adjusted according to the output of the LDO regulator. Again, this is best achieved without expensive hardware modifications.
The LT3072 dual 2.5 A linear regulator meets the challenging demands of digital IC power supplies, enabling hardware-independent output voltage regulation even when the LT3072 uses a pre-regulator input supply. The LT3072 features UltraFast™ transient response and a low dropout voltage of 80 mV, making it easy to generate precisely regulated supply voltages under fast load changes.
The low 12 µV rms output noise and ultrafast transient response of the LT3072 can be achieved with only 10 µF (1 µF + 2.2 µF + 6.8 µF) output capacitors. Low noise is essential for communication or sensor circuits to maintain their high performance.
The LT3072 integrates two fully independent 2.5 A LDO regulators in a single package. The LT3072’s 0.6 V to 2.5 V output voltage range is wide enough to power a variety of digital IC supply rails. Several tri-state pins are set on the LT3072 to program the output voltage of each channel, which can be easily accomplished with jumpers, microcontrollers, or power system management (PSM) ICs.
Programmable dual output with low noise and ultrafast transient response
Figure 1 shows the LT3072 in a stand-alone circuit for a digital IC load with stringent power requirements. An important part of strict power supply specifications is the ability to respond quickly to load transients, as shown in the ultrafast transient response curve of the LT3072 in Figure 2.
Each output value is set by three tri-state pins: VO1B2, VO1B1, VO1B0, VO2B2, VO2B1, and VO2B0. Each tri-state pin is set by grounding, floating, or applying voltage to it. In this way, the output can be programmed from 0.6 V to 2.5 V.
In addition to setting the nominal programming voltage, the programmed output voltage can be adjusted by ±10% through the input margin setting. The corresponding input voltage can be as low as 200 mV, or slightly above 2.5 V and 0.6 V output voltage to optimize transient response performance margin. The output voltage status is indicated by the PWRGD pin, and some pins are used for analog monitoring of the output current, and a ±7% accuracy output current limit can also be set. There are also pins for analog monitoring of die temperature.
Figure 1. The ultrafast load transient response, 12 µV rms output noise, and 80 mV dropout characteristics of the dual 2.5 A LT3072 meet the demands of digital ICs with stringent power requirements. The schematic shows that OUT1 and OUT2 of the tri-state VO1B2C0 and VO2B2C0 pins are fixed at 2.5 V and 0.6 V, respectively, but the output voltage can be changed simply by changing the state on these pins, without the need for time-consuming and expensive hardware modifications. Realize software control of LT3072.
Figure 2. The single-output ultrafast transient response of the LT3072 shows microsecond settling time using only 10 µF (1 µF + 2.2 µF + 6.8 µF) output capacitors. The middle trace shows that additional capacitance can be used to limit the offset amplitude, but the settling time is slightly longer.
Dynamic Control of Pre-Regulator
The LT3072 dynamically controls the switching outputs in front of it. This allows immediate adjustment of the LDO regulator’s output voltage while maintaining its input voltage at a level that maintains high efficiency and fast load transient response.
The pre-regulator power supply for the LT3072 in Figure 3 uses the LT8616, a 42 V, dual 1.5 A/2.5 A synchronous monolithic step-down regulator. This setup accepts a single 3.6 V to 42 V system input voltage range. In this solution, OUT1 of the LT3072 has a programmable output range of 0.6 V to 1.8 V. The OUT1 channel uses VIOC to control the corresponding LT8616 output, allowing the LDO regulator to operate within the conversion range for optimum efficiency and transient response. OUT1 can be dynamically adjusted from 0.6 V to 1.8 V using the VO1B2-1 pin.
The current limit of the OUT1 linear regulator channel is set to 1.8 A, slightly higher than the 1.5 A maximum output current of channel 1 of the LT8616. OUT2 is fixed at 0.6 V and can be 2.5 A with a 3 A current limit.
The LT3008-3.3 provides bias current to the LT3072. The PG2 (power good) pin of the LT8616 provides a small delay before the LT3072 starts up. Figure 4 shows the LT3072 dynamically controlling the switching channel at the input of a prescaled LDO regulator.
Figure 3. IN1 and IN2 of the LT3072 are preregulated by the dual, step-down LT8616. The connection between VOIC1 of the LT3072 and TR/SS1 of the LT8616 allows the LT3072 to dynamically pre-regulate its IN1 input for excellent efficiency and load transient performance, while allowing the LT3072’s output voltage to be adjusted without changing the hardware.
Figure 4. Dynamic testing of the circuit shown in Figure 3. The traces show how a software change to tri-state pins VO1B2 and VO1B1 (VO1B0 is grounded) enables adjustment of the LT3072’s OUT1 voltage. In turn, the LT3072 dynamically controls the LT8616 Channel 1 output, which preconditions the IN1 input of the LDO regulator. In this way, the IN1 voltage of the LDO regulator is maintained at a fixed dropout voltage above the LDO regulator OUT1, enabling high efficiency and high load transient performance, all without any hardware changes.
The LT3072 dual-channel LDO regulator for digital IC power supplies features two low-noise channels and ultra-fast load transient response. The two output voltages can be programmed by setting several tri-state pins without the use of resistors. When the input supply to the LT3072 is a preregulator, this input supply can be controlled using the LT3072 VIOC function, allowing dynamic changes when programming the output voltage without affecting transient response performance or efficiency.
About the Author
Andy Radosevich is a senior applications engineer at Analog Devices, Inc., supporting Power by LinearTM non-isolated DC-DC switches, linear voltage and current regulators. Andy holds a master’s degree in electrical engineering with a specialization in power electronics from San Jose State University. He also has a background in motion control and light to medium manufacturing processes. Andy is an avid traveler by public transportation and often takes the bus to Silicon Valley engineering events.
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