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R
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P
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7
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7
1
1
4
4
 
 
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M
 
 
S
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D
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?2011 Exar Corporation
20/29
Rev. 1.1.6
PWM Switching Frequency
 
SET_SW_FREQUENCY[6:4]
SET_SW_FREQUENCY[2:0]
000
001
010
011
100
101
110
111
NA
NA
NA
NA
NA
NA
NA
NA
1.  MHz    1.4MHz    1.  MHz    1.2MHz    1.1MHz    1.  MHz
KHz
800KHz
1.  MHz
KHz
7KHz
KHz    7   KHz
7KHz
KHz
KHz
011
750KHz    700KHz    650KHz    600KHz    550KHz    500KHz    450KHz    400KHz
1
600KHZ    560KHz    520KHz    480KHz    440KHz    400KHz    360KHz    320KHz
1  1
500KHz    467KHZ    433KHz    400KHz    367KHz    333KHz    300KHz
NA
42  KHZ    4   KHZ
7  KHz
4  KHz
14KHz
NA
NA
NA
7  KHz
KHz
2  KHz
KHz
NA
NA
NA
NA
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 of the Main Oscillator Frequency to an external system clock is desired, the user
must choose the oscillator frequency to be within ?% of the external clock frequency. A higher
Main Oscillator frequency will not improve accuracy or any performance efficiency.  
Note: It is the intention of the synchronization feature to sync to a system clock or to another
compatible Exar device, not the switching frequency.  
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 XRP7714 is 40ns.
55V5[5V5Z5b5Z 575b5a5f 565f5P5Y5R% = 5@5V5[5V5Z5b5Z 5\5[5a5V5Z5R ?5C5J5@ 595_5R5^5b5R5[5P5f ?100
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.
55V5[5V5Z5b5Z 575b5a5f 565f5P5Y5R% >
55\5b5a
55V5[5Z5N5e
 
Maximum Duty Cycle
The maximum duty cycle is dictated by the minimum required time to sample the current when the
low side MOSFET is on, this depends on the frequency of the main oscillator and the selected PWM
frequency. It is best to choose the highest main oscillator frequency available for any specific PWM
frequency. The maximum duty cycle for each PWM frequency is shown in the table below:
 
 
Main Oscillator Frequency
Maximum Duty Cycle
48MHz
44.8MHz
41.6MHz
38.4MHz
35.2MHz
32MHz
28.8Mhz
25.6MHz
7
1.  MHz    1.4MHz    1.  MHz    1.2MHz    1.1MHz    1.  MHz
KHz
800KHz
1.  MHz
KHz
7KHz
KHz    7   KHz
7KHz
KHz
KHz
84%
750KHz    700KHz    650KHz    600KHz    550KHz    500KHz    450KHz    400KHz
600KHZ    560KHz    520KHz    480KHz    440KHz    400KHz    360KHz    320KHz
500KHz    467KHZ    433KHz    400KHz    367KHz    333KHz    300KHz
NA
42  KHZ    4   KHZ
7  KHz
4  KHz
14KHz
NA
NA
NA
7  KHz
KHz
2  KHz
KHz
NA
NA
NA
NA
Fig. 26: PWM Frequency
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