SNVS585M September 2008 – October 2020 LM22678 , LM22678-Q1

PRODUCTION DATA

- 1 Features
- 2 Applications
- 3 Description
- 4 Revision History
- 5 Pin Configuration and Functions
- 6 Specifications
- 7 Detailed Description
- 8 Application and Implementation
- 9 Layout
- 10Device and Documentation Support

- NDR|7

The components with the highest power dissipation are the power diode and the power MOSFET internal to the LM22678 regulator. The easiest method to determine the power dissipation within the LM22678 is to measure the total conversion losses then subtract the power losses in the diode and inductor. The total conversion loss is the difference between the input power and the output power. An approximation for the power diode loss is shown in Equation 17.

Equation 17.

where

- V
_{D}is the diode voltage drop.

An approximation for the inductor power is determined by Equation 18.

Equation 18.

where

- R
_{L}is the dc resistance of the inductor.

The 1.1 factor is an approximation for the ac losses.

The regulator has an exposed thermal pad to aid
power dissipation. Adding multiple vias under the device to the ground plane will
greatly reduce the regulator junction temperature. Selecting a diode with an exposed
pad will also aid the power dissipation of the diode. The most significant variables
that affect the power dissipation of the regulator are output current, input voltage
and operating frequency. The power dissipated while operating near the maximum
output current and maximum input voltage can be appreciable. The junction-to-ambient
thermal resistance of the LM22678 will vary with the application. The most
significant variables are the area of copper in the PC board, the number of vias
under the IC exposed pad and the amount of forced air cooling provided. A large
continuous ground plane on the top or bottom PCB layer will provide the most
effective heat dissipation. The integrity of the solder connection from the IC
exposed pad to the PC board is critical. Excessive voids will greatly diminish the
thermal dissipation capacity. The junction-to-ambient thermal resistance of the
LM22678 PFM package is specified in *Section 6.6*. See *AN-2020 Thermal Design By Insight, Not Hindsight*
(SNVA419) for more information.