XRP7714ILB-0X14-F【PDF 21/29 页】IC REG 5OUT BCK/LINEAR 40TQFN

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Sheet目录47

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?2011 Exar Corporation

21/29

Rev. 1.1.6

The maximum duty cycle obtained from the table above is programmed by the PowerArchitect

software 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.

It is possible for the user to program SET_DUTY_LIMITER_CHx register to a higher value, but the

OCP Fault and OCP WARN flags should then be ignored.

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.

FREQUENCY SYNCHRONIZATION FUNCTION AND EXTERNAL CLOCK

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

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.

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.

XRP7714

Configured for

external clock use

CLK_IN

GPIO1

Fig. 27:XRP7714 Configured For External Clock Use

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