What Is the Default Output Voltage of the LMR43620MSC5RPERQ1? Can It Be Adjusted?
The LMR43620MSC5RPERQ1 is a 2A automotive synchronous buck converter from Texas Instruments, housed in a 2mm × 2mm HotRod QFN package and operating from an input as high as 36V. A common question during schematic review is straightforward: what voltage does this part produce by default, and can that voltage be changed on the same board? The short answer is that the default output is 5V, and the same device can be configured for an adjustable output using an external resistor divider.
1. The Default State: 5V Fixed Output
The suffix code of the LMR43620MSC5RPERQ1 encodes its factory configuration. The “C5” designates a 5V fixed-output variant within the LMR436x0-Q1 family. In this default mode, the VOUT/FB pin is connected directly to the output node, and the device regulates the output to 5V without any external feedback resistors. The converter senses the resistance of the feedback path during startup and determines whether fixed or adjustable operation is required. For a 5V rail, this means fewer external components and a simpler layout.
It is important to distinguish this variant from its 3.3V counterpart, the LMR43620MSC3RPERQ1, which is factory-configured for 3.3V fixed output. Choosing the correct orderable part number is the first step in avoiding an unnecessary resistor network on the board.
2. Adjustable Configuration: Using the FB Divider
The same LMR43620MSC5RPERQ1 can be reconfigured for a different output voltage by connecting an external feedback resistor divider between the output node, the FB pin, and system ground. The converter holds the FB pin at an internal reference voltage of 1V, and the output voltage is set by the ratio of the top resistor (RFBT) and bottom resistor (RFBB).
The recommended range for the parallel combination of RFBT and RFBB is 5kΩ to 10kΩ. A larger resistance reduces quiescent current but increases noise sensitivity; a smaller resistance improves noise immunity at the cost of slightly higher bias current. For a 3.3V output, typical 1% resistor values are 33.2kΩ for RFBT and 14.3kΩ for RFBB. For a 2.5V output, 24.9kΩ and 16.5kΩ are commonly used.
The adjustable output voltage range spans from 1V up to 0.95 × VIN. This upper limit is determined by the converter’s duty-cycle limit rather than by the feedback network. At very low input voltages, the device may reduce switching frequency to maintain regulation, so the practical maximum output must be evaluated against the minimum expected input in the application.
3. Key Parameters at a Glance
| Parameter | Value / Condition |
|---|---|
| Default output voltage | 5V fixed (VOUT/FB tied to output) |
| Factory output option | 5V fixed or adjustable |
| Internal reference voltage (adjustable mode) | 1V |
| Adjustable output range | 1V to 0.95 × VIN |
| Recommended RFBT || RFBB | 5kΩ to 10kΩ |
| Example divider for 3.3V | RFBT = 33.2kΩ, RFBB = 14.3kΩ |
| Example divider for 2.5V | RFBT = 24.9kΩ, RFBB = 16.5kΩ |
| Output current | 2A |
| Input voltage range | 3.0V to 36V (42V transient) |
4. Questions and Answers
Q: If I need a 3.3V output, should I use the LMR43620MSC5RPERQ1 with a resistor divider or a different part?
A: For a 3.3V rail, the cleaner choice is the LMR43620MSC3RPERQ1, which is factory-set to 3.3V fixed output. Using the 5V variant with a divider works electrically but adds two resistors and a small amount of board area. The 5V variant with a divider is appropriate when the required voltage is not covered by a fixed-output option in the family.
Q: How does the device know whether to operate in fixed or adjustable mode?
A: The converter senses the resistance of the feedback path during startup. If the VOUT/FB pin is tied directly to the output, the device recognizes the fixed-output configuration. If an external divider is present, it recognizes the adjustable configuration and regulates the FB pin to 1V. This auto-detection simplifies the design and prevents incorrect configuration from being latched.
Q: Can the output be set below 1V?
A: No. The internal reference voltage is 1V, and the feedback loop regulates the FB pin to that value. The minimum adjustable output is therefore 1V. For lower voltages, a different device or a post-regulation stage is required.
Q: What happens if the resistor divider values are too large or too small?
A: If the parallel combination is much larger than 10kΩ, the FB pin becomes more susceptible to leakage and noise, potentially degrading output accuracy. If it is much smaller than 5kΩ, the divider draws more bias current from the output, reducing light-load efficiency. Staying within the recommended 5kΩ to 10kΩ window balances accuracy and quiescent current.
Q: Does using the adjustable configuration affect the 2A current capability or other specifications?
A: No. The 2A output current rating, the 36V input capability, the 2.2MHz switching frequency, and the protection features remain unchanged. Only the output voltage setting mechanism differs. The same silicon die supports both fixed and adjustable operation, and the choice is made entirely through the external feedback network.
5. Summary
The LMR43620MSC5RPERQ1 produces 5V by default when its VOUT/FB pin is connected directly to the output. That same part can be configured for any output voltage between 1V and 0.95 × VIN by adding an external resistor divider with a parallel resistance of 5kΩ to 10kΩ. The internal 1V reference and startup auto-detection make the transition between fixed and adjustable modes transparent to the designer. The default answer is 5V, and the adjustability answer is yes — through a simple external divider, without changing the device.
