DC-DC｜Application

Important Points in the Design of a Power Supply Using a Linear RegulatorHow to determine efficiency and Thermal design for linear regulator ICs

2025.01.28

table of contents

How to Determine the Efficiency of a Linear Regulator

The efficiency of a power supply that uses a linear regulator can be calculated from the following equation. In essence, the efficiency η for a linear regulator is equal to the output power (POUT) divided by the input power (PIN), which is no different from the case for a switching regulator.
Expressed as an equation in a web browser-compatible form, the efficiency is as follows.

\(\eta = \displaystyle \frac{P_{OUT}}{P_{IN}} = \displaystyle \frac{V_{{OUT}} \times I_{{OUT}}}{V_{{CC}} \times (I_{{OUT}} + I_{{CC}})} \times 100[％]\)

VCC: Input voltage [V]
VOUT: Output voltage [V]
IOUT: Output current [A]
ICC: IC circuit current [A]

When, as one condition, ICC is very small compared with IOUT (ICC << IOUT), the following equation can be used for calculation.

\(\eta = \displaystyle \frac{V_{OUT}}{V_{CC}}\times 100[％]\)

From the equation, we see that the smaller the difference between the input and output voltages, the better the efficiency. However, the input-output voltage difference must be determined considering the effect on operating characteristics.

Thermal Design for Linear Regulator ICs

To ensure highly reliable operation, the junction temperature TJ of an IC must not exceed the absolute maximum rating TJMAX of 150°C*1. The value of TJ can be calculated in the following two methods.
*1: For the BDxxIC0 series, refer to the data sheet of a given IC.

Thermal Calculation Using the Thermal Characterization Parameter PsiJT

To estimate TJ for an IC using surface temperature measurements, calculations are performed using the thermal characterization parameter PsiJT. If a thermocouple can be securely fastened to the center of the top surface of the package, the temperature TT at the center of package top surface can be accurately measured, and so TJ can be calculated precisely using the thermal characterization parameter.

\(T_J = T_T + \psi_{JT} \times P[℃]\)

TT : Temperature at the center of package top surface [℃]
PsiJT : Thermal characterization parameter from junction to center of package top surface [℃/W]
P : IC power consumption [W]

P is the IC power consumption, calculated by the following equation.

\(P = (V_{CC} – V_{OUT}) \times I_{OUT} + (V_{CC} \times I_{CC}) [W]\)

VCC : Input voltage [V]
VOUT : Output voltage [V]
IOUT : Output current [A]
ICC : IC circuit current [A]

Moreover, the maximum output current that can flow continuously can be calculated by the following equation.

\(I_{OUT(MAX)} = \displaystyle \frac{T_{{J(MAX)} – T_T}}{(V_{CC} – V_{OUT}) \times \psi_{JT}} [A]\)

TJMAX : Absolute maximum rating for junction temperature [℃]
TT : Temperature at the center of package top surface [℃]
PsiJT : Thermal characterization parameter from junction to center of package top surface [℃/W]
VCC : Input voltage [V]
VOUT : Output voltage [V]

Thermal Calculation Using the Thermal Resistance θJA

A simplified junction temperature TJ can also be calculated using thermal resistance θJA.

\(T_J = T_A + \theta_{JA} \times P[℃]\)

TA : Ambient temperature [°C]
θJA : Thermal resistance between junction and ambient environment [°C/W]
P : IC power consumption [W]

Further, the maximum output current that can flow continuously can be calculated by the following equation.

\(I_{OUT(MAX)} = \displaystyle \frac{T_{{J(MAX)} – T_A}}{(V_{{CC}} – V_{{OUT}}) \times \theta_{JA}} [A]\)

TJMAX : Absolute maximum rating for junction temperature［℃］
TA : Ambient temperature［℃］
θJA : Thermal resistance between junction and ambient environment［℃/W］
VCC : Input voltage [V]
VOUT : Output voltage [V]

Examples of Actual Measured Values of the Thermal Characterization Parameter PsiJT and Thermal Resistance θJA

The values shown in the tables below for thermal characterization parameters PsiJT and thermal resistances θJA were measured using specific PCBs (printed circuit boards). The heat dissipation performance varies with the PCB characteristics, copper foil layout, component layout, housing shape, ambient environment, and other factors, and consequently the thermal characterization parameter and thermal resistance also change. Hence it must be considered that the values may differ from those of actual boards.

Thermal characterization parameters and thermal resistances

The specifications of the PCBs used in measurements are shown below. All are for HTSOP-J8 packages, and are, in order, a 1-layer (1s) board (conforming to JESD51-3/-7); a 2-layer (2s) board (conforming to JESD51-3/-5/-7); and a 4-layer (2s2p) board (conforming to JESD51-3/-5/-7).

HTSOP-J8 package PCB specifications,1layer(1s)

HTSOP-J8 package PCB specifications,2layer(2s)

HTSOP-J8 package PCB specifications,4layers(2s2p)

    DC-DC

    Basic

    Design

    Evaluation

    Application

    Product Information

    FAQ

    THIS WEBSITE USES COOKIES.
    By continuing to browse this website without changing your web-browser cookie settings, you are agreeing to our use of cookies.
    Privacy Policy
    OK