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.
where,
Maximum OFF time percentage
Switching period.
Table 5. Switching Frequency Adjustment
FREQUENCY
200
VOLTAGE APPLIED TO PIN FREQ
2.341 – 2.500
2.185 - 2.340
253
307
2.029 - 2.184
Drain – to – source resistance of FET
Winding resistance of Inductor
360
1.873 - 2.028
413
1.717 – 1.872
1.561 – 1.716
1.405 - 1.560
466
520
573
1.249 - 1.404
Output current ripple.
627
1.093 - 1.248
OUTPUT FILTER CAPACITOR
680
0.936 - 1.092
For the output capacitor, the following considerations are
more important than the actual capacitance value, the
physical size, the ESR and the voltage rating:
733
0.781 - 0.936
787
0.625 - 0.780
Transient Response percentage, TR_%
Maximum Transient Voltage, TR_v_dip = Vo*TR_%
Maximum current step,
840
0.469 - 0.624
893
0.313 - 0.468
947
0.157 - 0.312
1000
0.000 - 0.156
Inductor Current rise time,
where,
D_max = Maximum ON time percentage.
IO = Rated output current.
Vin_min = Minimum input voltage at PVIN
Figure 7. Resistor Divider for Frequency Adjustment
As a result, it is possible to calculate
SELECTION OF THE INDUCTOR
Inductor calculation is straight forward, being
34713
Analog Integrated Circuit Device Data
Freescale Semiconductor
15
FREESCALE [ Freescale ]
