LM5009MM National Semiconductor, LM5009MM Datasheet - Page 11

• Manufacturer Part Number: LM5009MM
• Description: IC,SMPS CONTROLLER,VOLTAGE-MODE,TSSOP,8PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM5009MM

• Rohs Compliant: NO

Applications Information
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-
ON
= 500K). In this case C4 should be increased so
(Continued)
FIGURE 5. LM5009 Example Circuit
11
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
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