XRP7714_1009 Datasheet, PDF (20/29 Pages) Exar Corporation – Quad Channel Digital PWM Step Down Controller

XRP7714
Quad Channel Digital PWM Step Down Controller
PWM Switching Frequency
SET_SW_FREQUENCY[6:4]
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
011
NA
1.2MHz
800KHz
600KHz
480KHz
400KHz
343KHz
300KHz
100
NA
1.1MHz
733KHz
550KHz
440KHz
367KHz
314KHz
NA
101
NA
1.0MHz
667KHz
500KHz
400KHz
333KHz
NA
NA
110
NA
900KHz
600KHz
450KHz
360KHz
300KHz
NA
NA
111
NA
800KHz
533KHz
400KHz
320KHz
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 ±5% 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.
𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝐷𝑢𝑡𝑦 𝐶𝑦𝑐𝑙𝑒% = 𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝑜𝑛𝑡𝑖𝑚𝑒 × 𝑃𝑊𝑀 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 × 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.
𝑉𝑜𝑢𝑡
𝑀𝑖𝑛𝑖𝑚𝑢𝑚 𝐷𝑢𝑡𝑦 𝐶𝑦𝑐𝑙𝑒% > _{𝑉𝑖𝑛𝑚𝑎𝑥}
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:
Maximum Duty Cycle
78%
86%
84%
89%
88%
88%
86%
© 2010 Exar Corporation
Main Oscillator Frequency
48MHz 44.8MHz 41.6MHz 38.4MHz 35.2MHz
1.5MHz
1.0MHz
750KHz
600KHZ
500KHz
429KHZ
375KHz
1.4MHz
933KHz
700KHz
560KHz
467KHZ
400KHZ
350KHz
1.3MHz
867KHz
650KHz
520KHz
433KHz
370KHz
325KHz
1.2MHz
800KHz
600KHz
480KHz
400KHz
343KHz
300KHz
1.1MHz
733KHz
550KHz
440KHz
367KHz
314KHz
NA
Fig. 26: PWM Frequency
32MHz
1.0MHz
667KHz
500KHz
400KHz
333KHz
NA
NA
28.8Mhz
900KHz
600KHz
450KHz
360KHz
300KHz
NA
NA
25.6MHz
800KHz
533KHz
400KHz
320KHz
NA
NA
NA
20/29
Rev. 1.1.4