AME5259A datasheet(10/21 Pages) AME

10 / 21 page

AME

1.2A, 1.5MHz Synchronous

Step-Down Converter

AME5259A

Rev.A.02

Detailed Description

Using Ceramic Input and Output Capacitors

Higher values, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high ripple

current, high voltage rating and low ESR make them ideal

for switching regulator applications. However, care must

be taken when these capacitors are used at the input and

output. When a ceramic capacitor is used at the input

and the power is supplied by a wall adapter through long

wires, a load step at the output can induce ringing at the

input, V

IN. At best, this ringing can couple to the output

and be mistaken as loop instability. At worst, a sudden

inrush of current through the long wires can potentially

cause a voltage spike at V

IN large enough to damage the

part.

Output Voltage Programming

The output voltage is set by an external resistive divider

according to the following equation :

Where VREF equals to 0.6V typical. The resistive di-

vider allows the FB pin to sense a fraction of the output

voltage as shown in Figure 4.

)

(

VREF

VOUT

++++

====

⋅⋅⋅⋅

Thermal Considerations

In most applications the AME5259A does not dissipate

much heat due to its high efficiency. But, in applications

where the AME5259A is running at high ambient tem-

perature with low supply voltage and high duty cycles,

such as in dropout, the heat dissipated may exceed the

maximum junction temperature of the part. If the junc-

tion temperature reaches approximately 160OC, both

power switches will be turned off and the SW node will

become high impedance. To avoid the AME5259A from

exceeding the maximum junction temperature, the user

will need to do some thermal analysis. The goal of the

thermal analysis is to determine whether the power dissi-

pated exceeds the maximum junction temperature of the

part. The temperature rise is given by:

Where PD is the power dissipated by the regulator and

JA is the thermal resistance from the junction of the die

to the ambient temperature.

)

)(

(

Figure 4. Setting the AME5259A Output Voltage

AME5259 A

GND

0.6V VOUT 5.5V

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