Part Number: TPS7A49
Other Parts Discussed in Thread: TPS7A30
Hello!
Help calculate resistance R1 and R2 for TPS7A4901 and TPS7A3001. I did not fully understand the instruction.
My options:
Input Voltage 25V
Output Voltage 18V
Load Current 100mA
Please show me an example calculation
Best regards,
Sergey
Hi Sergey,
Both the TPS7A49 and TPS7A30 regulators have feedback networks that operate the same way. TPS7A30 simply uses negative voltages instead of positive voltages. As long as we have enough current flowing through the resistor divider (Ifb<<Idivider) we can calculate the output voltage using the following equation:
Vout = Vfb x [1 + (R1 / R2)]
For TPS7A49 Vfb is 1.185 V (typ) as seen in the Electrical Characteristics table. In order to ensure that Ifb << Idivider, we recommend that the divider current be at least 5 uA for this device:
5 uA < Vfb / R2 or when re-arranged R2 < Vfb / 5 uA
R2 < 1.185 V / 5 uA
R2 < 237 k
Lets choose a 100 k resistor for R2. We can now re-arrange the equation above to solve for R1 and plug in your application conditions:
Vout = Vfb x (1 + R1 / R2) => R1 = R2 x [(Vout / Vfb) - 1]
R1 = 100 k x [(18 V / 1.185 V) - 1]
R1 = 1.42 M
We can check our calculation by plugging in our resistor divider into the original equation:
Vout = Vfb x [1 + (R1 / R2)]
Vout = 1.185 V x [1 + (1.42 M / 100 k)]
Vout = 18 V
You will note that the feedback voltage for TPS7A30 is -1.179 V (typ). In order to make the feedback pin current negligible, we recommend that the current through the feedback resistors be greater than 5 uA. This means that the current through R2 should be greater than 5 uA as shown in equation 2. Once we know R2, we can calculate R1 also shown in equation 2.
You will note that we give recommendations for a -18 V rail in Table 2. If we plug in these recommendations, you will note that the output is -18 V:
Vout = Vfb x [1 + (R1 / R2)]
Vout = -1.179 V x [1 + (143k / 10k)]
Vout = -18 V
Very Respectfully,
Ryan
