XRP7704 Exar Corporation, XRP7704 Datasheet - Page 22

• Manufacturer Part Number: XRP7704
• Description: Quad-output Digital Pwm Buck Controller
• Manufacturer: Exar Corporation

REV 1.1.0
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
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.
SET_SW_FREQUENCY[6:4]
Main Oscillator Frequency
Ts
SET_SW_FREQUENCY[2:0]
000
001
010
011
100
101
110
111
EXAR CONFIDENTIAL. PROPRIETARY. DO NOT DISTRIBUTE OR COPY.
48MHz
000
20.8ns
000
NA
1.5MHz
1.0MHz
750KHz
600KHZ
500KHz
429KHZ
375KHz
Setting the PWM switching frequency
001
44.8MHz 41.6MHz 38.4MHz 35.2MHz
22.3ns
001
NA
1.4MHz
933KHz
700KHz
560KHz
467KHZ
400KHZ
350KHz
Quad-Output Digital PWM Buck Controller
010
24ns
010
NA
1.3MHz
867KHz
650KHz
520KHz
433KHz
370KHz
325KHz
22
SET_SW_FREQUENCY[6:4]
011
26ns
011
NA
1.2MHz
800KHz
600KHz
480KHz
400KHz
343KHz
300KHz
100
28.4ns
100
NA
1.1MHz
733KHz
550KHz
440KHz
367KHz
314KHz
NA
101
32MHz
31.25ns
101
NA
1.0MHz
667KHz
500KHz
400KHz
333KHz
NA
NA
110
28.8Mhz
34.7ns
110
NA
900KHz
600KHz
450KHz
360KHz
300KHz
NA
NA
XRP7704
111
25.6MHz
39ns
111
800KHz
533KHz
400KHz
320KHz
NA
NA
NA
NA