XRP7704 Datasheet, PDF (23/31 Pages) Exar Corporation – Quad-Output Digital PWM Buck Controller

REV 1.1.0
XRP7704
Quad-Output Digital PWM Buck Controller
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. For the XRP7704, the minimum required sampling time is about 16 clock cycles at the main
oscillator frequency. When calculating maximum duty cycle, the sampling time needs to be subtracted using the
below equation. For example, if operating at 1MHz using the 32MHz main oscillator frequency, the maximum duty
cycle would be:
Maximum Duty Cycle% = (1 – ((16/clock frequency)*PWM frequency)) - 0.03) * 100 ≈ 47%
On the other hand, if the 48MHz main oscillator frequency was chosen for the 1MHz PWM frequency, the
maximum duty cycle would be:
Maximum Duty Cycle% = (1 – ((16/clock frequency)*PWM frequency) - 0.03) * 100 ≈ 64%
Therefore, it is best to choose the highest main oscillator frequency for a particular PWM frequency if duty cycle
limit might be encountered. The maximum duty cycle for any PWM frequency can easily be determined using the
following table:
Main Osc. Frequency →
Maximum Duty Cycle ↓
47%
64%
72%
77%
80%
83%
85%
48MHz
1.5MHz
1.0MHz
750KHz
600KHZ
500KHz
429KHZ
375KHz
44.8MHz 41.6MHz
1.4MHz
933KHz
700KHz
560KHz
467KHZ
400KHZ
350KHz
1.3MHz
867KHz
650KHz
520KHz
433KHz
370KHz
325KHz
38.4MHz 35.2MHz 32MHz
PWM Frequency ↓
1.2MHz 1.1MHz 1.0MHz
800KHz 733KHz 667KHz
600KHz 550KHz 500KHz
480KHz 440KHz 400KHz
400KHz 367KHz 333KHz
343KHz 314KHz NA
300KHz NA
NA
28.8Mhz 25.6MHz
900KHz
600KHz
450KHz
360KHz
300KHz
NA
NA
800KHz
533KHz
400KHz
320KHz
NA
NA
NA
It is highly recommended that the maximum duty cycle obtained from the table above be programmed into each of
the channels using the SET_DUTY_LIMITER_CHx register. This ensures that under all conditions (including
faults), there will always be sufficient sampling time to measure the output current. When the duty cycle limit is
reached, the output voltage will no longer regulate and will be clamped based on the maximum duty cycle limit
setting.
Efficiency
The PWM Switching frequency plays an important role on overall power conversion efficiency. As the switching
frequency increase, the switching losses also increase. Please see the APPLICATION INFORMATION, Typical
Performance Data for further examples.
Component Selection and Frequency
Typically the components become smaller as the frequency increases, as long as the ripple requirements remain
constant. At higher frequency the inductor can be smaller in value and have a smaller footprint while still
maintaining the same current rating.
EXAR CONFIDENTIAL. PROPRIETARY. DO NOT DISTRIBUTE OR COPY.
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