“Low dropout (LDO) linear regulators have been widely used in noise-sensitive applications for decades. However, as the latest precision sensors, high-speed and high-resolution data converters (ADCs and DACs), and frequency synthesizers (PLL/VCOs) continue to challenge traditional LDO regulators to produce ultra-low output noise and ultra-high With high power supply ripple rejection (PSRR), noise requirements become increasingly difficult to meet.
Low dropout (LDO) linear regulators have been widely used in noise-sensitive applications for decades. However, as the latest precision sensors, high-speed and high-resolution data converters (ADCs and DACs), and frequency synthesizers (PLL/VCOs) continue to challenge traditional LDO regulators to produce ultra-low output noise and ultra-high With high power supply ripple rejection (PSRR), noise requirements become increasingly difficult to meet.
For example, when powering a sensor, power supply noise directly affects the accuracy of the measurements. Switching regulators are often used in power distribution systems to achieve higher overall system efficiency. To build low-noise power supplies, LDO regulators typically post-regulate the output of relatively noisy switching converters without the need for bulky output filter capacitors. High frequency PSRR performance of LDO regulators becomes critical.
ADI’s LT3042 is the industry’s first linear regulator with only 0.8μV rms output noise and 79dB PSR at 1MHz. Two similar devices, the LT3045 and LT3045-1, offer higher ratings and additional features. All of these devices are positive LDO regulators. When the system has bipolar devices such as op amps or ADCs, a negative LDO regulator must be used in the polar power supply design. The LT3094 is the first negative LDO regulator with ultra-low output noise and ultra-high PSRR. Table 1 lists the key features of the LT3094 and related devices.
Table 1. LT3094 and Low Noise LDO Features
The LT3094 features a precision current source reference followed by a high performance output buffer. The negative output voltage can be programmed by a −100µA precision current source flowing through a single resistor. This current-reference-based architecture provides a wide output voltage range (0V to −19.5V) with nearly constant output noise, PSRR, and load regulation, independent of the set output voltage. Figure 1 shows a typical application and the demo board is shown in Figure 2. The overall solution size is only about 10mm x 10mm.
Figure 1. −3.3 V output low noise solution.
Figure 2. Demonstration circuit shows a −3.3 V tiny solution.
The LT3094 features ultra-low output noise of 0.8µVrms from 10Hz to 100kHz and an ultra-high PSRR of 74dB at 1MHz. In addition, the LT3094 features programmable current limit, programmable power good threshold, fast start-up function and programmable input-to-output voltage control (VIOC). When the LT3094 post-regulates the switching converter, if the LDO regulator output voltage is variable, the voltage across the LDO regulator will be held constant by the VIOC function.
The LT3094 protects against device damage through internal protection features including internal current limit with foldback, thermal limit, reverse current and reverse voltage protection.
Direct Paralleling for Higher Currents
The LT3094 can be easily paralleled to increase output current. Figure 3 shows a solution to achieve 1A output current using two LT3094s in parallel. To parallelize the two devices, connect the SET pins together and place a SET resistor R between the SET pin and groundSET. flow through RSETThe current is 200µA, which is twice the amount of SET current in a single device. In order to obtain good current sharing characteristics, each output of the LT3094 uses a small current resistance of 20mΩ.
Figure 3. Schematic of two LT3094s in parallel.
Figure 4 shows the thermal performance of the circuit in Figure 3 with an input voltage of −5V and an output voltage of −3.3V operating at a load current of 1A. The temperature of each device rose to approximately 50°C, indicating an even distribution of heat. For higher output current and lower output noise, there is no limit to the number of devices that can be paralleled.
Figure 4. Thermal image of two LT3094s in parallel.
Positive and Negative Dual Power Supplies with Variable Output Voltage
Power supplies are typically configured with switching converters post-regulated by LDO regulators to achieve low output noise and high system efficiency. To maintain an appropriate trade-off between power dissipation and PSRR, the optimal voltage difference between the input and output of the LDO regulator is about −1V. Maintaining this voltage difference in a variable output voltage system is complicated, but the LT3094 has a tracking function VIOC that keeps the voltage constant across the LDO regulator even if the output voltage varies.
Figure 5 is a dual-supply schematic using the LT8582, LT3045-1, and LT3094. The LT8582 is a dual-channel PWM DC/DC converter with built-in switches capable of generating positive and negative outputs from a single input. The first channel of the LT8582 is configured as a SEPIC for generating the positive output and the second channel is an inverting converter for generating the negative supply rail.In the negative rail, the voltage across the LT3094 is controlled by the VIOC voltage
Figure 5. Adjustable dual output positive/negative power supply with high ripple rejection and low temperature operation.
where VFBX2is 0mV, IFBXis 83.3µA. put R2set to 14.7kΩ, then for variable output voltage the VIOCThe voltage is set to 1.23V. Resistor R1At 133kΩ, limiting the input voltage of the LT3094 to 16.5V, the calculation is as follows:
A thermal image of the circuit operating at 12V input is shown in Figure 6. The temperature rise of the LT3094 remains constant when the output voltage varies from ±3.3V to ±12V. Table 2 lists the voltages and currents for all three devices. Figure 7 shows the ±5V supply transient response at 12V input.
Figure 6. Thermal image of dual power supply at 12 V input.
Figure 7. Dual-supply transient response at 12 V input, ±5 V output.
Table 2. Circuit Performance of Dual Output Positive/Negative Power Supplies at 12 V Input, ±500 mA Load
In Figure 5, no additional capacitors are placed at the input of the LT3094 other than the output capacitor of the LT8582. Normally, the input capacitor will reduce the output ripple, but this is not the case with the LT3094. If the LT3094 has an input capacitance, the switching current of the switching converter will flow through the input capacitance, resulting in electromagnetic coupling of the switching converter to the output of the LT3094. Output noise will increase, resulting in lower PSRR. If the switching regulator is located within two inches of the LT3094, for best PSRR performance, we recommend that no capacitors be placed on the input of the LT3094.
The LT3094 is a negative LDO regulator with ultralow noise and ultrahigh PSRR. It features a current reference-based architecture that makes noise and PSRR performance independent of output voltage, and multiple LT3094s can be easily paralleled to increase load current and reduce output noise. When the LT3094 is used to post-regulate switching converters, the VIOC feature minimizes power dissipation in the LDO regulator, making it ideal for variable output voltage applications.
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