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© 2012 Exar Corporation

12/28

Rev. 1.2.2

FUNCTIONAL DESCRIPTION AND OPERATION

The XRP7708 and XRP7740 are quad-output digital pulse width modulation (DPWM) controllers with

integrated gate drivers for the use in synchronous buck switching regulators. Each output voltage

can be programmed from 0.9V to 5.1V without the need of an external voltage divider. The wide

range of the programmable DPWM switching frequency (from 300 kHz to 1.5 MHz) enables the user

to optimize for efficiency or component sizes. The digital regulation loop requires no external

passive components for network compensation. The loop performance does not need to be

compromised due to component tolerance, aging, and operating condition. Each digital controller

provides a number of safety features, such as over-current protection (OCP) and over-voltage

protection (OVP). The chip also provides over-temperature protection (OTP) and under-voltage

lock-out (UVLO) for two input voltage rails. The XRP7708/40 also has up to 6 GPIOs and a Standby

well as to communicate with the host for fault reporting and handling, power rail monitoring,

channel enable and disable, Standby Low Drop-out Regulator voltage reconfiguration, and Standby

LDO enable and disable.

The XRP7708 and XRP7740 offer a complete solution for soft-start and soft-stop. The delay and

ramp of each PWM regulator can be independently controlled. When a pre-bias voltage is present,

the device holds both high-side and low-side MOSFETs off until the reference voltage ramps up

higher than the output voltage. As a result, large negative inductor current and output voltage

disturbance are avoided. During soft-stop, the output voltage ramps down with a programmable

slope until it reaches a pre-set stop voltage. This pre-set value can be programmed between within

zero volts and the target voltage with the same set target voltage resolution (see shutdown

waveforms in Applications).

For brevity, only the XRP7708 will be referred to unless there is a specific difference between the

devices.

There are two types of registers in the XRP7708: read/write registers and read-only registers. The

read/write registers are used for the control functions of the IC and can be programmed using

for feedback functions such as error/warning flags and for reading the output voltage or current.

The non-volatile memory (NVM) in XRP7708 stores the configuration data for the chip and all of the

power rails. This memory is normally configured during manufacturing time. Once a specific bit of

the NVM is programmed, that bit can never be reprogrammed again [i.e. one-time programmable].

During chip power up, the contents in the NVM are automatically transferred to the internal

registers of the chip. Programmed cells have been verified to be permanent for at least 10 years

and are highly reliable.

The XRP7708 can be programmed to sequence its outputs for nearly any imaginable loading

requirement. However, there are some important sequencing requirements for the XRP7708 itself.

When power is applied to the XRP7708, the 5V VCCA and 1.8V AVDD regulators must come up and

stabilize to provide power for the analog and digital blocks of the IC. The Enable Pin must remain

below its logic level high threshold until the AVDD is regulating to ensure proper loading of the

configuration registers. For systems that control the Enable signal through a microcontroller or

other processor, this is simply a matter of providing the proper delay to the Enable signal after