LM2622 datasheet(11/16 Pages) NSC

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Operation (Continued)

gin can also be improved by adding C

C2 as discussed earlier

in the section. The equation for A

DC is given below with

additional equations required for the calculation:

mc ) 0.072fs (in V/s)

where R

L is the minimum load resistance, VIN is the mini-

mum input voltage, g

m is the error amplifier transconduc-

tance found in the Electrical Characteristics table, and R

D-

SON

is the value chosen from the graph "R

DSON vs. VIN "in

the Typical Performance Characteristics section.

LAYOUT CONSIDERATIONS

The input bypass capacitor C

IN, as shown in the typical

operating circuit, must be placed close to the IC. This will

reduce copper trace resistance which effects input voltage

ripple of the IC. For additional input voltage filtering, a 100nF

bypass capacitor can be placed in parallel with C

IN, close to

the V

IN pin, to shunt any high frequency noise to ground. The

output capacitor, C

OUT, should also be placed close to the

IC. Any copper trace connections for the C

OUT capacitor can

increase the series resistance, which directly effects output

voltage ripple. The feedback network, resistors R

FB1 and

FB2, should be kept close to the FB pin, and away from the

inductor, to minimize copper trace connections that can in-

ject noise into the system. Trace connections made to the

inductor and schottky diode should be minimized to reduce

power dissipation and increase overall efficiency. For more

detail on switching power supply layout considerations see

Application Note AN-1149: Layout Guidelines for Switching

Power Supplies.

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