How do I calculate the appropriate resistor to set the output voltage of the regulator?
Let's start with the diagram below.
The voltage regulator is regulated by a feedback circuit. The voltage regulator includes a voltage reference (specified as VREF in Figure 1) to which the output voltage is scaled. The scale factor depends on the values of the feedback resistors r1 and r2.
Figure 1 includes the iB, which is the bias current of the error amplifier negative (-input. First, let's assume that the bias current is zero.
When the output voltage is regulated to an appropriate value, current i1 will flow through resistors R2 and R1 and generate a voltage equal to the VREF voltage at node A. In the absence of bias current (ib), the current flowing through R2 and R1 is exactly the same.
Since the voltage at node A is equal to the VREF voltage, the amount of current flowing through R1 will be equal to VREF R1. The same current flows through R2. The voltage across R2 will be vout vref. Therefore, two equations can be written to define the value of the current i1.
i1 = vref/r1 (1)
i1 = vout /(r1 + r2) (2)
Once r1, vref, and vout are defined, you can set these two equations to be equal to each other and derive the equations to calculate r2. The derivation process of the formula will be covered later, but let's first discuss a simple way to quickly calculate the resistance value without having to figure out the formula.
The voltage regulator's datasheet will define the VREF (sometimes this has other names, such as vadj or ref). For the following example, let's assume vref = 0.6 volts.
Once we know the vref, we can select r1 to generate the amount of current we want to flow through it. For example, let's say we want 0. to flow through a resistor divider25 mA (250 microamps) current. To calculate r1, divide the vref by 025 ma。This will result in r1 having a value of 2400 ohms.
The same current (250 ua) will flow through R2. When the loop is properly regulated, the voltage across R2 will be vout vref. In this example, we choose VOUT as 5 volts. Then the voltage at both ends of R2 will be 5 06 = 4.4 volts. The same 250 UA current flows through this resistor, so it has a value of 44/250 ua = 17.6 k ohms.
In some applications, such as battery-powered circuits, one may want to maximize the resistor value to reduce the amount of current flowing in the resistor divider. How do you determine how much resistor you can use?
This is usually determined by the magnitude of the bias current ib.
When a bias current is present (in this case, we assume that the bias current flows into the pin error amplifier) it will flow out of the corrected voltage (vout) and therefore through resistor R2. This will cause the output voltage to be higher than expected. An easy way to estimate how much resistance R2 can use is to first define an acceptable percentage error. The value of the resistor r2 can then be calculated using the following formula:
r2 = (%error/100) (vout/ib) (3)
For example, let's use the example where vout is equal to 5v, and you determine that the acceptable amount of error caused by the bias current can be 2%. If the current ib is 100 na, then the r2 value is.
r2 = (2/100) (5/100na) = 1 m (4)
Once the value of R2 is obtained, the amount of current required to achieve regulation can be calculated. Do this without including the bias current. Let's use the example above, where vout = 5 v and vref = 0.6 v。Hence the voltage at both ends of R2 is 44 volts. Based on the acceptable error caused by the bias current, we calculated that R2 is equal to 1m. The current through R2 will be 44/1m = 4.4 ua。The same current will flow through R1. The voltage across R1 will be vref, which is 06 volts. So r1 can pass vref 44 ua = 136.3k to calculate. People will use the closest resistance value. If a tolerance of 1% is acceptable, the value is 137k.
Some IC regulators sometimes specify the size of the R2 resistor. For example, Renesas has a voltage regulator that recommends that the R2 resistor be set to 300K. The reason for this is that the device has an internal compensation capacitor and the RC combination is best set to a specific frequency. So, in this case, the value of r2 is fixed. The value of r1 can be calculated using the example from the previous paragraph.
Derive the equation to determine the resistance value:
Current flowing through r1:
i1 = vref/r1 (5)
Current flowing through R1 and R2:
i1 = vout /(r1 + r2) (6)
Let the two equations (5) and (6) be equal:
vref/ r1 = vout/(r1 + r2) (7)
Use the equation to separate r1 and r2:
vref(r1 + r2) = vout(r1) (8)
vref(r1) +vref(r2) = vout(r1) (9)
vref(r2) = vout(r1) -vref( r1) (10)
The final equation for R1 is shown based on R1, Vout, and Vref:
r2 = (r1)((vout/vref) -1) ）11）
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