XRP7704 Datasheet, PDF (22/31 Pages) Exar Corporation – Quad-Output Digital PWM Buck Controller

REV 1.1.0
XRP7704
Quad-Output Digital PWM Buck Controller
PWM Switching Frequency
The PWM switching frequency is set by choosing the corresponding oscillator frequency and clock divider ratio in
the SET_SW_FREQUENCY register. Bits [6:4] set the oscillator frequency and bits [2:0] set the clock divider. The
tables below summarize the available Main Oscillator and PWM switching frequency settings in the XRP7704.
Main Oscillator Frequency
SET_SW_FREQUENCY[6:4] 000
Main Oscillator Frequency 48MHz
Ts
20.8ns
001
010
011
100
44.8MHz 41.6MHz 38.4MHz 35.2MHz
22.3ns 24ns
26ns
28.4ns
101
32MHz
31.25ns
110
111
28.8Mhz 25.6MHz
34.7ns 39ns
PWM Switching Frequency
SET_SW_FREQUENCY[2:0]
000
001
010
011
100
101
110
111
000
NA
1.5MHz
1.0MHz
750KHz
600KHZ
500KHz
429KHZ
375KHz
001
NA
1.4MHz
933KHz
700KHz
560KHz
467KHZ
400KHZ
350KHz
010
NA
1.3MHz
867KHz
650KHz
520KHz
433KHz
370KHz
325KHz
SET_SW_FREQUENCY[6:4]
011
100
101
NA
NA
NA
1.2MHz 1.1MHz 1.0MHz
800KHz 733KHz 667KHz
600KHz 550KHz 500KHz
480KHz 440KHz 400KHz
400KHz 367KHz 333KHz
343KHz 314KHz NA
300KHz NA
NA
110
NA
900KHz
600KHz
450KHz
360KHz
300KHz
NA
NA
111
NA
800KHz
533KHz
400KHz
320KHz
NA
NA
NA
Setting the PWM switching frequency
There are a number of options that could result in similar PWM switching frequency as shown above. In general,
the chip consumes less power at lower oscillator frequency. When synchronization to external clock is needed,
the user can choose the oscillator frequency to be within +/- 5% of the external clock frequency. A higher Main
Oscillator frequency will not improve accuracy or any performance efficiency.
PWM Switching Frequency Considerations
There are several considerations when choosing the PWM switching frequency.
Minimum On Time
Minimum on time determines the minimum duty cycle at the specific switching frequency. The minimum on time
for the XRP7704 is 40ns.
As an example the minimum duty cycle is 4% for 1MHz PWM frequency. This is important since the minimum on
time dictates the maximum conversion ratio that the PWM controller can achieve.
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