34713 datasheet(15/21 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34713

TYPICAL APPLICATIONS

PROTECTION AND DIAGNOSTIC FEATURES

COMPONENT SELECTION

SWITCHING FREQUENCY SELECTION

The switching frequency defaults to a value of 1.0 MHz

when the FREQ pin is grounded, and 200 KHz when the

FREQ pin is connected to VDDI. Intermediate switching

frequencies can be obtained by connecting an external

resistor divider to the FREQ pin. The table below shows the

resulting switching frequency versus FREQ pin voltage.

Table 5. Switching Frequency Adjustment

Figure 7. Resistor Divider for Frequency Adjustment

SELECTION OF THE INDUCTOR

Inductor calculation is straight forward, being

where,

Maximum OFF time percentage

Switching period.

Drain – to – source resistance of FET

Winding resistance of Inductor

Output current ripple.

OUTPUT FILTER CAPACITOR

For the output capacitor, the following considerations are

more important than the actual capacitance value, the

physical size, the ESR and the voltage rating:

Transient Response percentage, TR_%

Maximum Transient Voltage, TR_v_dip = Vo*TR_%

Maximum current step,

Inductor Current rise time,

where,

D_max = Maximum ON time percentage.

IO = Rated output current.

Vin_min = Minimum input voltage at PVIN

As a result, it is possible to calculate

FREQUENCY

VOLTAGE APPLIED TO PIN FREQ

200

2.341 – 2.500

253

2.185 - 2.340

307

2.029 - 2.184

360

1.873 - 2.028

413

1.717 – 1.872

466

1.561 – 1.716

520

1.405 - 1.560

573

1.249 - 1.404

627

1.093 - 1.248

680

0.936 - 1.092

733

0.781 - 0.936

787

0.625 - 0.780

840

0.469 - 0.624

893

0.313 - 0.468

947

0.157 - 0.312

1000

0.000 - 0.156

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