FB

VIN

SW

RTN

BST

LM5009

8

6

5

3

4

1

2

7

SHUTDOWN

VCC

RON/SD

12V - 90V

Input

237k

C1

1.0 PF

169k

R2

1.0k

R1

3.01k

R3

3.0

GND

C2

15 PF

10.0V

L1

150 PH

C3

0.1 PF

C4

0.022 PF

D1

C5

0.1 PF

LM5009

www.ti.com

SNVS402G – FEBRUARY 2006 – REVISED FEBRUARY 2013

FINAL CIRCUIT

The final circuit is shown in Figure 12. The circuit was tested, and the resulting performance is shown in

Figure 13 through Figure 16. For these graphs, the load current was varied from 50mA to 200mA.

MINIMUM LOAD CURRENT

A minimum load current of 1 mA is required to maintain proper operation. If the load current falls below that level,

the bootstrap capacitor may discharge during the long off-time, and the circuit will either shutdown, or cycle on

and off at a low frequency. If the load current is expected to drop below 1 mA in the application, the feedback

resistors should be chosen low enough in value so they provide the minimum required current at nominal Vout.

PC BOARD LAYOUT

The LM5009 regulation and over-voltage comparators are very fast, and as such will respond to short duration

noise pulses. Layout considerations are therefore critical for optimum performance. The components at pins 1, 2,

3, 5, and 6 should be as physically close as possible to the IC, thereby minimizing noise pickup in the PC tracks.

The current loop formed by D1, L1, and C2 should be as small as possible. The ground connection from C2 to

C1 should be as short and direct as possible.

If the internal dissipation of the LM5009 produces excessive junction temperatures during normal operation, good

use of the PC board’s ground plane can help considerably to dissipate heat. The exposed pad on the bottom of

the WSON-8 package can be soldered to a ground plane on the PC board, and that plane should extend out

from beneath the IC to help dissipate the heat. Additionally, the use of wide PC board traces, where possible,

can also help conduct heat away from the IC. Judicious positioning of the PC board within the end product, along

with use of any available air flow (forced or natural convection) can help reduce the junction temperatures.

Figure 12. LM5009 Example Circuit

Copyright © 2006–2013, Texas Instruments Incorporated

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