XRP7714 Exar, XRP7714 Datasheet - Page 19

• Manufacturer Part Number: XRP7714
• Description: Quad Channel Digital PWM Step Down Controller
• Manufacturer: Exar

[2:0] set the clock divider. The tables below summarize the available Main Oscillator and PWM
switching frequency settings in the XRP7714.
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 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 XRP7714 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, 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:
© 2010 Exar Corporation
Main Oscillator Frequency
PWM Switching Frequency
Minimum On Time
Maximum Duty Cycle
SET_SW_FREQUENCY[6:4]
SET_SW_FREQUENCY[2:0]
Main Oscillator Frequency
WITCHING
000
001
010
011
100
101
110
111
Ts
F
REQUENCY
600KHZ
429KHZ
750KHz
500KHz
375KHz
1.5MHz
1.0MHz
C
48MHz
20.8ns
000
000
ONSIDERATIONS
NA
Q
Q
u
u
%
a
a
d
d
44.8MHz 41.6MHz 38.4MHz 35.2MHz
467KHZ
400KHZ
933KHz
700KHz
560KHz
350KHz
1.4MHz
22.3ns
001
001
C
C
NA
h
h
a
a
19/27
n
n
n
n
867KHz
650KHz
520KHz
433KHz
370KHz
325KHz
1.3MHz
24ns
e
e
010
010
NA
l
l
SET_SW_FREQUENCY[6:4]
%
D
D
i
i
g
g
800KHz
600KHz
480KHz
400KHz
343KHz
300KHz
1.2MHz
i
i
t
26ns
t
011
011
NA
a
a
l
l
P
P
W
W
1.1MHz
733KHz
550KHz
440KHz
367KHz
314KHz
28.4ns
M
M
100
100
NA
NA
S
S
t
t
e
e
31.25ns
1.0MHz
667KHz
500KHz
400KHz
333KHz
p
p
32MHz
101
101
100
NA
NA
NA
D
D
o
o
w
w
28.8Mhz
900KHz
600KHz
450KHz
360KHz
300KHz
n
n
34.7ns
www.DataSheet4U.com
110
110
X
X
NA
NA
NA
C
C
R
R
o
o
P
n
P
n
Rev. 1.1.3
t
t
7
7
25.6MHz
800KHz
533KHz
400KHz
320KHz
r
r
39ns
o
o
111
111
7
7
NA
NA
NA
NA
l
l
1
1
l
l
e
e
4
4
r
r