TABLE 3. Recommended Capacitors (Continued)

Capacitance (µF)

Part Number

Chemistry

Vendor

220

TPSD227M006#0050

Tantalum

AVX

220

NOSD227M0040060

Niobium Oxide

AVX

SPLIT-RAIL OPERATION

The LM2853 can be powered using two separate voltages

for AVIN and PVIN. AVIN is the supply for the control logic;

PVIN is the supply for the power FETs. The output filter

components need to be chosen based on the value of PVIN.

For PVIN levels lower than 3.3V, use output filter component

values recommended for 3.3V. PVIN must always be equal

to or less than AVIN.

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SWITCH NODE PROTECTION

The LM2853 includes protection circuitry that monitors the

voltage on the switch pin. Under certain fault conditions,

switching is disabled in order to protect the switching de-

vices. One side effect of the protection circuitry may be

observed when power to the LM2853 is applied with no or

light load on the output. The output will regulate to the rated

voltage, but no switching may be observed. As soon as the

output is loaded, the LM2853 will begin normal switching

operation.

LAYOUT GUIDELINES

These are several guidelines to follow while designing the

PCB layout for an LM2853 application.

1.

The input bulk capacitor, C

close to the PVIN pin to keep the resistance as low as

possible between the capacitor and the pin. High current

levels will be present in this connection.

2.

All ground connections must be tied together. Use a

broad ground plane, for example a completely filled back

plane, to establish the lowest resistance possible be-

tween all ground connections.

3.

The sense pin connection should be made as close to

the load as possible so that the voltage at the load is the

expected regulated value. The sense line should not run

too close to nodes with high dV/dt or dl/dt (such as the

switch node) to minimize interference.

4.

The switch node connections should be low resistance

to reduce power losses. Low resistance means the trace

between the switch pin and the inductor should be wide.

However, the area of the switch node should not be too

large since EMI increases with greater area. So connect

the inductor to the switch pin with a short, but wide trace.

Other high current connections in the application such

as PVIN and V

low resistance and EMI.

5.

Allow area under the chip to solder the entire exposed

die attach pad to ground for improved thermal perfor-

mance. Lab measurements also show improved regula-

tion performance when the exposed pad is well

grounded.

LM2853 Example Circuit Schematic

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FIGURE 1.

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