supplies the surge current to charge the buck switch gate at

turn-on. A low ESR also ensures a quick recharge during

each off-time. At minimum VIN, when the on-time is at maxi-

mum, it is possible during start-up that C4 will not fully

recharge during each 300 ns off-time. The circuit will not be

able to complete the start-up, and achieve output regulation.

This can occur when the frequency is intended to be low

(e.g., R

it can maintain sufficient voltage across the buck switch

driver during each on-time.

C5: This capacitor helps avoid supply voltage transients and

ringing due to long lead inductance at VIN. A low ESR, 0.1µF

ceramic chip capacitor is recommended, located close to the

LM5009.

FINAL CIRCUIT

The final circuit is shown in Figure 5. The circuit was tested,

and the resulting performance is shown in Figure 6 through

Figure 9. For these graphs, the load current was varied from

50mA to 200mA.

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 opti-

mum 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 dissi-

pate heat. The exposed pad on the bottom of the LLP-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.

20165822

FIGURE 5. LM5009 Example Circuit

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