XRP7714 Exar, XRP7714 Datasheet - Page 20

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

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.
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.
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.
F
The user of the XRP7714 can choose to use an external source as the primary clock for the
XRP7714. This function can be configured using the SET_SYNC_MODE_CONFIG register. This
register sets the operation of the XRP7714 when an external clock is required. By selecting the
appropriate bit combination the user can configure the IC to function as a master or a slave when
two or more XRP7714s are used to convert power in a system. Automatic clock selection is also
provided to allow operation even if the external clock fails by switching the IC back to an internal
clock.
Even when configured to use an external clock, the chip initially powers up with its internal clock.
The user can set the percent target that the frequency detector will use when comparing the
internal clock with the clock frequency input on the GPIO pin. If the external clock frequency is
detected to be within the window specified by the user, then a switchover will occur to the external
clock. If the IC does not find a clock in the specified frequency target range then the external clock
will not be used and the IC will run on the internal clock that was specified by the user. If the
external clock fails the user can chose to have the internal clock take over, using the automatic
switch back mode in the SET_SYNC_MODE_CONFIG register.
© 2010 Exar Corporation
Efficiency
Component Selection and Frequency
REQUENCY
External Clock Synchronization
Maximum Duty Cycle
S
78%
86%
84%
89%
88%
88%
86%
YNCHRONIZATION
600KHZ
429KHZ
750KHz
500KHz
375KHz
1.5MHz
1.0MHz
48MHz
F
Q
Q
UNCTION AND
u
u
a
a
d
d
Fig. 25: PWM Frequency
44.8MHz 41.6MHz 38.4MHz 35.2MHz
467KHZ
400KHZ
933KHz
700KHz
560KHz
350KHz
1.4MHz
C
C
h
h
a
a
20/27
n
n
n
n
867KHz
650KHz
520KHz
433KHz
370KHz
325KHz
1.3MHz
E
e
e
XTERNAL
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
C
l
l
LOCK
P
P
W
W
1.1MHz
733KHz
550KHz
440KHz
367KHz
314KHz
M
M
NA
S
S
t
t
e
e
1.0MHz
667KHz
500KHz
400KHz
333KHz
p
p
32MHz
NA
NA
D
D
o
o
w
w
28.8Mhz
900KHz
600KHz
450KHz
360KHz
300KHz
n
n
www.DataSheet4U.com
X
X
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
o
o
7
7
NA
NA
NA
l
l
1
1
l
l
e
e
4
4
r
r