Documentation sections

LDO Thermal Calculator

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

Slug ldo-thermal. Calculate LDO regulator power dissipation, junction temperature, thermal margin, and minimum dropout voltage for thermal design validation.

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 ldo-thermal
Name	Unit	Range	Default	Description
vinnumber	V	0.5 … 60	5	Input Voltage
voutnumber	V	0.5 … 59	3.3	Output Voltage
iloadnumber	mA	1 … 5000	500	Load Current
thetaJAnumber	°C/W	1 … 500	50	θJA from datasheet. SOT-23: ~150°C/W, TO-252: ~50°C/W, TO-220: ~5°C/W
tambnumber	°C	−40 … 85	25	Ambient Temperature
tjMaxnumber	°C	85 … 175	125	Max Junction Temperature
targetTjnumber	°C	25 … 175	100	The junction temperature the design should stay at or below, usually derated from the maximum. The status fails above 125 °C.

Outputs

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

Outputs of ldo-thermal
Field	Unit	Description
pdiss	W	Power Dissipation
tjheadline	°C	Junction Temperature
margin	°C	Thermal Headroom
dropoutV	V	Dropout Voltage
efficiencyPct	%	Efficiency

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": "ldo-thermal",
    "inputs": {
      "vin": 5,
      "vout": 3.3,
      "iload": 500,
      "thetaJA": 50,
      "tamb": 25,
      "tjMax": 125,
      "targetTj": 100
    }
  }'

import rftools

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

result = client.calculate("ldo-thermal", {
    "vin": 5,
    "vout": 3.3,
    "iload": 500,
    "thetaJA": 50,
    "tamb": 25,
    "tjMax": 125,
    "targetTj": 100,
})
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": "ldo-thermal",
    "inputs": {
      "vin": 5,
      "vout": 3.3,
      "iload": 500,
      "thetaJA": 50,
      "tamb": 25,
      "tjMax": 125,
      "targetTj": 100
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "ldo-thermal",
	  "inputs": {
	    "vin": 5,
	    "vout": 3.3,
	    "iload": 500,
	    "thetaJA": 50,
	    "tamb": 25,
	    "tjMax": 125,
	    "targetTj": 100
	  }
	}`)
	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": "ldo-thermal",
  "values": {
    "pdiss": 0.8500000000000001,
    "tj": 67.5,
    "margin": 57.5,
    "dropoutV": 1.7000000000000002,
    "efficiencyPct": 65.99999999999999
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:ldo-thermal",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "Texas Instruments Application Note SLVA061; IEC 60747-6",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "vin": {
          "min": 0.5,
          "max": 60,
          "unit": "V"
        },
        "vout": {
          "min": 0.5,
          "max": 59,
          "unit": "V"
        },
        "iload": {
          "min": 1,
          "max": 5000,
          "unit": "mA"
        },
        "thetaJA": {
          "min": 1,
          "max": 500,
          "unit": "°C/W"
        },
        "tamb": {
          "min": -40,
          "max": 85,
          "unit": "°C"
        },
        "tjMax": {
          "min": 85,
          "max": 175,
          "unit": "°C"
        },
        "targetTj": {
          "min": 25,
          "max": 175,
          "unit": "°C"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "vin": 5,
      "vout": 3.3,
      "iload": 500,
      "thetaJA": 50,
      "tamb": 25,
      "tjMax": 125,
      "targetTj": 100
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "vin"
      ],
      "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 Thermal Calculator, at these values