Input and Output Parameters
Control and MOSFET Parameters
Please enter all required values.
RESULTS
Input Parameters Specification
Input Voltage RangeVin max, nominal and min are used for LM25085 buck regulator timing, duty cycle and capacitor design.
Output ConditionsVout, output current and ripple define inductor, output capacitor and current limit selection.
MOSFET TimingOn time, off time and gate charge affect delay, switching frequency and LM25085 internal power loss.
Feedback and SenseRf1 and current sense resistor calculate feedback resistor, current limit and sense resistor dissipation.
Practical Operational Examples
LM25085 Buck Supply
Use this calculator for step-down switching regulator design with external MOSFET, diode, inductor and output capacitor.
Component Selection
The results help select RT, Radj, L, Cin, Cout, Rf2, R3, C1 and estimate diode, IC and sense resistor power loss.
Ripple Design
Max output ripple input is used to estimate output capacitance for switching regulator ripple control.
Current Limit Check
Rsen and Radj values help compare calculated peak current limit with desired load current margin.
Diagrams & Theory
The LM25085 switching regulator uses an external MOSFET buck stage, diode clamp, inductor, input/output capacitors and feedback divider to regulate output voltage.
Formulas & Mathematical Logic
Standard value selection uses: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2 decades up to 1000.
Delay time: td = (50 + Qoff - Qon) × 1e-9.
Duty cycle: DutyCycle = Vout / Vin × 100.
RT = Vout × (Vin - 1.56) / (1.45e-7 × Vin × Fs) - td × (Vin - 1.56) / 1.45e-7 - 1.4.
Tonmin and Tonmax are calculated from RT, Vinmax, Vinmin and td, then displayed in ns.
Inductor ripple uses Irip = 2 × Imin, or 20% of Imax when that is larger. L is selected to the next standard value.
Cout = Irip / (8 × Vrip × Fs). Cin = Imax × Tonmax / 0.5.
Rf2 = Rf1 / ((Vout / 1.25) - 1). Power losses calculate diode, LM25085 IC and sense resistor dissipation.
Step-by-Step Example
Example: Vin max = 24 V, Vin nominal = 12 V, Vin min = 9 V, Vout = 5 V, Iout max = 2 A, Iout min = 0.2 A, Fs = 300 kHz.
Step 1: Duty cycle is calculated from Vout divided by nominal input voltage.
Step 2: MOSFET on/off time and gate charge are used to calculate delay and switching loss related values.
Step 3: RT is calculated from Vout, Vin, switching frequency and delay time.
Step 4: Inductor ripple current is chosen from Imin or 20% of Imax, then the next standard inductor value is selected.
Step 5: Output capacitor, input capacitor, feedback resistor and current limit values are calculated and displayed.
How to Use This Calculator
Enter maximum, nominal and minimum input voltage for the LM25085 buck regulator design.
Enter output voltage, diode forward voltage, maximum/minimum output current and ripple limit.
Enter current sense resistor, feedback Rf1, desired switching frequency and MOSFET timing values.
Click Calculate to get duty cycle, timing, losses, inductor, capacitors, feedback and current-limit values.
Compare results with datasheet recommendations, standard component values and real hardware measurements.
About This Calculator
Design key LM25085 buck regulator components faster.
The CalcBoy LM25085 Switching Regulator Design Calculator estimates timing, current limit, inductor, capacitor, feedback and power loss values for LM25085 buck converter circuits.
The LM25085 is used in non-synchronous buck regulator designs where an external MOSFET, diode, inductor, input capacitor and output capacitor set the main power stage behavior. Choosing these values manually can take time because switching frequency, MOSFET delay, output ripple, current limit and feedback network all interact.
This calculator is useful for early design checks, component selection, circuit repair comparison, datasheet-based design review and power supply prototyping. It helps estimate duty cycle, minimum and maximum on-time, diode loss, IC dissipation, sense resistor power, inductor value, peak current limit, output capacitor, input capacitor, feedback resistor, RT, Radj, Cadj and compensation-related values.
Use the result as a design starting point. Final switching regulator design should also consider PCB layout, MOSFET SOA, diode recovery, thermal rise, current ripple, capacitor ESR, inductor saturation current, load transient response, EMI and manufacturer datasheet limits.
Best UseLM25085 buck regulator component estimation.
Supported OutputsDuty cycle, timing, losses, L, Cin, Cout, feedback and current limit values.
Helpful ForSMPS design, buck converter repair, prototype supply planning and datasheet checks.
Design ReminderVerify thermal performance, layout and ripple on real hardware.
Tip: For switching regulators, PCB layout can make or break the design. Keep the high-current loop short and place input capacitor close to the MOSFET and diode path.
Frequently Asked Questions
What does this LM25085 calculator estimate?
It estimates LM25085 buck regulator timing, duty cycle, inductor, capacitors, feedback resistor, current limit and power dissipation values.
Can this replace the LM25085 datasheet?
No. Use this as a fast calculation tool, then verify every design value with the LM25085 datasheet and application notes.
Why are Vin max and Vin min required?
They affect Ton min, Ton max, current ripple, timing and component stress across the regulator operating range.
What is Rf2?
Rf2 is the lower or calculated feedback resistor derived from Rf1 and the desired output voltage.
Why does the calculator select standard values?
The source logic rounds some components such as inductor and capacitor values to common standard value steps.
Should I test the final converter?
Yes. Always measure switching waveform, output ripple, temperature, startup behavior and load transient response on the real PCB.
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