XRP7708 Exar Corporation, XRP7708 Datasheet - Page 19

• Manufacturer Part Number: XRP7708
• Description: Quad Channel Digital Pwm Step Down Controller
• Manufacturer: Exar Corporation

within +/- 5% of the external clock frequency. A higher Main Oscillator frequency will not improve
accuracy or any performance efficiency.
PWM S
There are several considerations when choosing the PWM switching frequency.
Minimum on time determines the minimum duty cycle at the specific switching frequency. The
minimum on time for the XRP7708 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.
The maximum duty cycle is dictated by the minimum required time to sample the current when the
low side MOSFET is on. For the XRP7708, 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:
On the other hand, if the 48MHz main oscillator frequency was chosen for the 1MHz PWM
frequency, the maximum duty cycle would be:
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:
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.
© 2010 Exar Corporation
Minimum On Time
Maximum Duty Cycle
Efficiency
Maximum Duty Cycle
WITCHING
47%
64%
72%
77%
80%
83%
85%
F
REQUENCY
%
%
600KHZ
429KHZ
750KHz
500KHz
375KHz
1.5MHz
1.0MHz
C
48MHz
ONSIDERATIONS
1
1
Q
Q
u
u
%
a
a
d
d
Fig. 21: PWM Frequency
44.8MHz 41.6MHz 38.4MHz 35.2MHz
467KHZ
400KHZ
933KHz
700KHz
560KHz
350KHz
1.4MHz
C
C
h
h
16
16
a
a
19/27
n
n
n
n
867KHz
650KHz
520KHz
433KHz
370KHz
325KHz
1.3MHz
e
e
l
l
%
Main Oscillator Frequency
D
D
i
i
g
g
800KHz
600KHz
480KHz
400KHz
343KHz
300KHz
1.2MHz
i
i
t
t
a
a
l
l
P
P
W
W
733KHz
550KHz
440KHz
367KHz
314KHz
1.1MHz
M
M
NA
S
S
0.03
0.03
t
t
e
e
667KHz
500KHz
400KHz
333KHz
1.0MHz
p
p
32MHz
100
NA
NA
D
D
o
o
100
100
w
w
28.8Mhz
900KHz
600KHz
450KHz
360KHz
300KHz
n
n
X
X
NA
NA
47%
64%
C
C
R
R
o
o
n
P
n
P
Rev. 1.0.3
t
t
7
7
25.6MHz
800KHz
533KHz
400KHz
320KHz
r
r
o
7
o
7
NA
NA
NA
l
l
0
0
l
l
e
e
8
8
r
r