Which is better to choose for voltage regulator: voltage reference or current reference?
Latest update time：2024-11-13
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Overview
This article introduces the shortcomings of the traditional voltage-dividing resistor-regulated voltage regulator circuit and introduces ADI's
current reference regulator
. The article analyzes the application of ADI's step-down switching regulator LT8625S and linear regulator LT3045 through examples, and summarizes
the advantages and design points
of the current reference regulator
.
Generally speaking,
DCDC
switching regulators
have fixed voltage outputs. By adding a resistor divider circuit, we can adjust the output voltage. However, in traditional voltage divider circuits,
the power supply accuracy is affected by the voltage divider resistor
, and the adjustment voltage range is limited, and it cannot be adjusted to 0V.
This article introduces a new ADI technology and sees how to solve these two shortcomings.
1.
Traditional voltage-dividing resistor-regulated voltage stabilization circuit
The following figure is an example of a voltage regulator circuit regulated by a voltage divider resistor:
This is a step-down
regulator circuit
. Power converters usually contain a control loop to maintain a set output voltage.
Figure 1. Traditional voltage regulator circuit (Image source: ADI)
The resistors on the right (RFB1 and RFB2) form a voltage divider circuit. The generated output voltage is adjusted to a specified voltage based on the internal reference voltage VREF. In general, the output voltage cannot be adjusted to a value lower than the internal reference voltage VREF.
If the output
voltage needs to be adjusted down to 0 volts,
a bias positive voltage source
(
as shown
in the red box
in the figure below
)
needs to be added to
the resistor divider circuit
to adjust the output voltage.
The bias positive voltage source can
or
a reference voltage source
.
Figure 2. Conventional voltage regulator circuit with an external bias positive voltage source connected
If this is done,
the accuracy of the resistor divider resistors and the bias positive voltage source will directly affect the accuracy of the resulting supply voltage.
2. ADI's new output voltage regulation solution
As shown in the figure below, the power supply output voltage is directly fed back to the error amplifier inside the chip.
The output voltage is adjusted by a resistor (RSET) connected to the internal power supply. This configuration can adjust the output voltage to 0V.
Figure 3. New architecture of voltage regulator (Image source: ADI)
Another advantage is that less noise is generated at low frequencies below 100kHz.
By adding the CSET capacitor, the low frequency interference of the internal current source can be smoothed and these interferences can be greatly reduced.
Example Analysis 1:
ADI
's
new step-down switching regulator,
the LT8625S,
from its
third-generation Silent Switcher®
family
uses this technology.
Figure 4. ADI's new step-down switching regulator LT8625S
Reference Voltage:
The LT8625S has a precision 100
µA
current
source flowing out of the SET pin and connected to the non-inverting input of the error amplifier. The reference voltage is equal to the SET pin current multiplied by RSET.
We only need
1% RSET resistance value to complete the setting of output voltage.
Below is
the corresponding relationship between output voltage and resistance:
For example, to achieve a 0.8V output, you can connect 8.25kΩ in parallel with 267kΩ to achieve (almost) exactly 8kΩ. Of course, for higher accuracy, you can use 0.1% resistors.
Advantages
of current reference
:
The benefit of using a current reference over a voltage reference used in a conventional regulator is that the regulator always operates in a unity gain configuration,
independent of the programmed output voltage.
This allows
the LT8625S to have loop gain, frequency response, and bandwidth that are independent of the output voltage.
In addition,
output load regulation is tighter
because the gain of the error amplifier is not required to amplify the SET pin voltage to a higher output voltage
.
Design points:
Any leakage current at the SET pin will cause an error in the output voltage. Therefore, if necessary, use
a PCB board made of a high-quality insulating material,
such as Teflon.
The SET pin is a high impedance node, and unwanted signals may couple to the SET pin and cause erratic behavior. This is most noticeable when running with minimum output capacitance at heavy load currents. Bypassing the SET pin to ground with a small capacitor can solve this problem,
100nF is sufficient
, and this is the minimum recommended capacitor value. Larger is better, of course.
For applications requiring higher accuracy or an adjustable output voltage, the SET pin can be actively driven by an external voltage source capable of sinking 100 μA. Connecting a precision voltage reference to the SET pin eliminates any errors in the output voltage due to reference current and SET pin resistor tolerance.
For more information, please see:
Example Analysis 2:
In addition to switching regulators, ADI's linear regulator,
LT3045
, also uses this loop regulation technique. Similarly,
a precision 100
µA
current
source flows from the SET pin and connects to the non-inverting input of the error amplifier.
Figure 5. ADI’s linear regulator, LT3045
Similarly, only
1% RSET resistance value is needed to complete the setting of output voltage.
The following is
the corresponding relationship between output voltage and resistance:
Advantages and design points of current reference:
The advantage of using current reference is that when the output voltage changes, the noise, PSRR and transient performance do not change, and the output load regulation rate is more stringent.
At the same time, the SET pin resistor should use a precision resistor to prevent leakage from affecting the output voltage. In a high humidity environment, the surface coating of the SET pin may be required to provide a moisture barrier.
For more information, please see:
More Products:
LT3045 development board
,
a 500mA ultra-low noise, ultra-high power supply rejection ratio (PSRR) low dropout (LDO) regulator with programmable current limit. Operating in the input voltage range of 3.8V to 20V, the LT3045 can provide a maximum output current of 500mA. In addition to ultra-low noise and ultra-high PSRR, the regulator also provides programmable power good function and programmable current limit, and can also achieve current monitoring by sensing the ILIM pin voltage. Thereby,
helping to shorten the development cycle
Summarize:
|
Traditional voltage-dividing resistor-adjusted voltage stabilization circuit
|
ADI's
new solution for output voltage regulation
|
|
Pros and Cons:
|
Pros and Cons:
|
at last:
The advantages of ADI's new output voltage regulation solution are obvious. It solves the problems of the traditional voltage divider resistor regulation voltage regulation circuit.
The new design architecture produces lower noise levels in the low frequency range. Moreover, the remaining minimum noise does not depend on the set output voltage, and
the voltage can be adjusted in a wider range
.
For more
technical articles
on Voltage Regulators
, please click on the following link. You are also welcome to leave a message at the end of the article for discussion.
refer to:
Editor's words
As shown in the article, current reference regulators are superior to traditional voltage reference regulators in terms of regulation voltage range, noise performance, stability, output load regulation, and design flexibility. Have you used both types of regulators? What experiences or questions do you have about voltage output designs?
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