MP1584 Frequency and Inductor Selection Guide
According to the MP1584 datasheet, proper selection of the frequency-setting resistor (RFREQ) and output inductor (L1) is essential for achieving optimal efficiency, stability, and ripple performance.
1. Setting the Switching Frequency (RFREQ)
The MP1584 switching frequency is programmed by connecting a resistor from the FREQ pin to ground. The resistor value can be calculated using:
RFREQ (kΩ) = 180000 / ( fS (kHz) )1.1
where fS is the desired switching frequency.
Common Frequency Settings
| Switching Frequency | RFREQ Value |
|---|---|
| 500 kHz | ≈ 200 kΩ |
| 900 kHz | 100 kΩ (datasheet value) |
| 1.0 MHz | ≈ 90-100 kΩ |
| 1.5 MHz (maximum) | ≈ 56 kΩ |
Higher switching frequencies allow the use of smaller inductors and capacitors, but they also increase switching losses and can reduce overall efficiency.
2. Output Voltage Programming
The MP1584 uses a feedback reference voltage of 0.8V. Both the original 5V configuration (210K / 40.2K) and your current 9V configuration (412K / 40.2K) confirm this relationship.
For simplicity, R2 is kept fixed at 40.2KΩ, and only the upper feedback resistor (R1) is changed to set the desired output voltage.
The output voltage is determined by:
VOUT = 0.8 × ( 1 + (R1 / R2) )
where:
- R1 = upper feedback resistor (VOUT to FB)
- R2 = lower feedback resistor (FB to GND), fixed at 40.2KΩ
Recommended Feedback Resistor Values
| Target VOUT | R1 (E96) | R2 | Actual VOUT |
|---|---|---|---|
| 3.3V | 127KΩ | 40.2KΩ | 3.33V |
| 5.0V | 210KΩ | 40.2KΩ | 4.98V |
| 9.0V | 412KΩ | 40.2KΩ | 9.00V |
| 12.0V | 562KΩ | 40.2KΩ | 11.98V |
| 15.0V | 715KΩ | 40.2KΩ | 15.03V |
Minimum Input Voltage and Test Load
Using a 25Ω test load resistor (RL), the expected load current at each output voltage is:
| Output Voltage | Minimum Input Voltage | Load Current (RL = 25Ω) |
|---|---|---|
| 3.3V | 8V* | 0.13A |
| 5.0V | 8V* | 0.20A |
| 9.0V | 12V | 0.36A |
| 12.0V | 15V | 0.48A |
| 15.0V | 18V | 0.60A |
* Although the MP1584 can operate with lower input voltages, an 8V input source is recommended during troubleshooting and validation to provide adequate headroom for regulation.
3. Selecting the Output Inductor (L1)
The required inductor value depends on the input voltage, output voltage, load current, and switching frequency. As a general guideline, the peak-to-peak inductor ripple current (ΔIL) should be approximately 30% of the maximum switch current limit.
Recommended Inductor Values
| Output Voltage | Recommended Inductor |
|---|---|
| 1.8V | 4.7µH |
| 2.5V | 4.7µH to 6.8µH |
| 3.3V | 6.8µH to 10µH (10µH typical) |
| 5.0V | 15µH to 22µH (15µH typical) |
| 12.0V | 22µH to 33µH |
These values are based on the typical application recommendations provided in the MP1584 datasheet.
Important Inductor Selection Considerations
Saturation Current Rating
The inductor's saturation current rating should exceed the calculated peak inductor current:
ILP = ILOAD + ( VOUT / (2 × fS × L1) ) × ( 1 - (VOUT / VIN) )
Choosing an inductor with insufficient saturation current can lead to excessive ripple, reduced efficiency, or converter instability.
DC Resistance (DCR)
Select an inductor with low DC resistance to minimize copper losses and maximize efficiency. The MP1584 datasheet lists several qualified inductors from manufacturers including Würth Elektronik, TDK, and TOKO.
Physical Size and Thermal Performance
Smaller inductors save board space but often have lower current ratings and reduced thermal capacity. Always verify both current handling capability and temperature rise under load.
Quick Reference
- 100KΩ RFREQ → approximately 900 kHz switching frequency.
- 210KΩ / 40.2KΩ → approximately 5.0V output.
- 412KΩ / 40.2KΩ → approximately 9.0V output (current configuration).
- 562KΩ / 40.2KΩ → approximately 12.0V output.
- 715KΩ / 40.2KΩ → approximately 15.0V output.
- For most repairs, a 10µH inductor is appropriate for 3.3V outputs, while 15µH is a common choice for 5V outputs.
This provides a quick way to reconfigure the MP1584 output voltage by changing only R1, while keeping the original 40.2KΩ feedback resistor (R2) in place.
Tutorials: