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8

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© 2010 Exar Corporation

20/27

Rev. 1.0.3

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.

The user of the XRP7708 can choose to use an external source as the primary clock for the

XRP7708. This function can be configured using the SET_SYNC_MODE_CONFIG register. This

register sets the operation of the XRP7708 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 XRP7708s 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.

External Clock Synchronization

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.

XRP7708

Configured for

external clock use

CLK_IN

GPIO1

Fig. 22:XRP7708 Configured For External Clock Use

Synchronized Operation as a Master and Slave Unit

Two XRP7708s can be synchronized together. This Master-Slave configuration is described below.

•

Master

When the XRP7708 powers up as a master unit after the internal configuration memory is loaded

the unit will send CLK_OUT and SYNC_OUT signals to the slave on the preconfigured GPIO pins.

•

Slave