Documentation sections

LDO Linear Regulator Dropout Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug linear-regulator-dropout. Calculate LDO regulator power dissipation, junction temperature rise, minimum input voltage, efficiency, and headroom for linear regulator design

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of linear-regulator-dropout
Name	Unit	Range	Default	Description
inputVoltagenumber	V	at least 0.1	5	Input Voltage
outputVoltagenumber	V	at least 0.1	3.3	Output Voltage
loadCurrentnumber	mA	at least 0.001	500	Load Current
dropoutVoltagenumber	mV	at least 10	300	Minimum input-to-output differential for regulation (from datasheet)
thetaJAnumber	°C/W	at least 1	50	Junction-to-ambient thermal resistance (from datasheet)
targetHeadroomnumber	mV	0 … 10000	500	The headroom above dropout the design should keep for ripple and transients. The status fails below 0 mV, where the regulator is in dropout.

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of linear-regulator-dropout
Field	Unit	Description
powerDissipation	mW	Power Dissipation
junctionTempRise	°C	Junction Temp Rise
minInputVoltage	V	Minimum Input Voltage
efficiency	%	Efficiency
headroomheadline	mV	Input Headroom

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "linear-regulator-dropout",
    "inputs": {
      "inputVoltage": 5,
      "outputVoltage": 3.3,
      "loadCurrent": 500,
      "dropoutVoltage": 300,
      "thetaJA": 50,
      "targetHeadroom": 500
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("linear-regulator-dropout", {
    "inputVoltage": 5,
    "outputVoltage": 3.3,
    "loadCurrent": 500,
    "dropoutVoltage": 300,
    "thetaJA": 50,
    "targetHeadroom": 500,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "linear-regulator-dropout",
    "inputs": {
      "inputVoltage": 5,
      "outputVoltage": 3.3,
      "loadCurrent": 500,
      "dropoutVoltage": 300,
      "thetaJA": 50,
      "targetHeadroom": 500
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "linear-regulator-dropout",
	  "inputs": {
	    "inputVoltage": 5,
	    "outputVoltage": 3.3,
	    "loadCurrent": 500,
	    "dropoutVoltage": 300,
	    "thetaJA": 50,
	    "targetHeadroom": 500
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "linear-regulator-dropout",
  "values": {
    "powerDissipation": 850.0000000000001,
    "junctionTempRise": 42.50000000000001,
    "minInputVoltage": 3.5999999999999996,
    "efficiency": 65.99999999999999,
    "headroom": 1400.0000000000002
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:linear-regulator-dropout",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "inputVoltage": {
          "min": 0.1,
          "max": null,
          "unit": "V"
        },
        "outputVoltage": {
          "min": 0.1,
          "max": null,
          "unit": "V"
        },
        "loadCurrent": {
          "min": 0.001,
          "max": null,
          "unit": "mA"
        },
        "dropoutVoltage": {
          "min": 10,
          "max": null,
          "unit": "mV"
        },
        "thetaJA": {
          "min": 1,
          "max": null,
          "unit": "°C/W"
        },
        "targetHeadroom": {
          "min": 0,
          "max": 10000,
          "unit": "mV"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "inputVoltage": 5,
      "outputVoltage": 3.3,
      "loadCurrent": 500,
      "dropoutVoltage": 300,
      "thetaJA": 50,
      "targetHeadroom": 500
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "inputVoltage"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageLDO Linear Regulator Dropout Calculator, at these values