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© 2010 Exar Corporation

12/27

Rev. 1.0.3

FUNCTIONAL DESCRIPTION AND OPERATION

The XRP7708 is a quad-output digital pulse width modulation (DPWM) controller 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 also has up to 6 GPIOs and a Standby Linear

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 offers 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).

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 figure below shows the power-up sequence of XRP7708 during the normal operation. The

startup stage is divided into three phases. The first phase is the internal LDO power-up phase. The

second phase is the configuration transfer phase. The third phase is chip ready phase. The power

up sequence is less than 1ms.