1

2

3

4

5

8

7

6

Exposed Pad on Bottom

Connect to Ground

FB

PGATE

ISEN

GND

VCC

ADJ

RT

VIN

RT and ADJ

Connections

L1

D1

GND

Keep

Loop Small

Q1

VOUT

LM5085, LM5085-Q1

www.ti.com

SNVS565I – NOVEMBER 2008 – REVISED AUGUST 2015

9 Power Supply Recommendations

The devices are designed to operate from an input voltage supply range between 4.5 V and 75 V. This input

supply must be well regulated. If the input supply is located more than a few inches from the device, additional

bulk capacitance may be required at the input terminals of the converter in addition to the calculated values to

limit the inductive spikes due to the input cables or wires.

10 Layout

10.1 Layout Guidelines

In most applications, the heat sink pad or tab of Q1 is connected to the switch node, i.e. the junction of Q1, L1

and D1. While it is common to extend the PC board pad from under these devices to aid in heat dissipation, the

pad size should be limited to minimize EMI radiation from this switching node. If the PC board layout allows, a

similarly sized copper pad can be placed on the underside of the PC board, and connected with as many vias as

possible to aid in heat dissipation.

The voltage regulation, over-voltage, and current limit comparators are very fast and can respond to short

duration noise pulses. Layout considerations are therefore critical for optimum performance. The layout must be

as neat and compact as possible with all the components as close as possible to their associated pins. Two

major current loops conduct currents which switch very fast, requiring the loops to be as small as possible to

be as close as possible to the VIN and VCC pins.

If the anticipated internal power dissipation of the LM5085 will produce excessive junction temperatures during

normal operation, a package option with an exposed pad must be used (HVSSOP-8 or WSON-8). Effective use

of the PC board ground plane can help dissipate heat. Additionally, the use of wide PC board traces, where

possible, helps conduct heat away from the IC. Judicious positioning of the PC board within the end product,

along with the use of any available air flow (forced or natural convection) also helps reduce the junction

temperature.

10.2 Layout Example

Figure 33. LM5085 Buck Converter Layout Example

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Product Folder Links: LM5085 LM5085-Q1