RT8800GS datasheet(19/26 Pages) RICHTEK

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RT8800/B

DS8800/B-06

March 2007

www.richtek.com

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DAC Offset Voltage Tuning

The Intel specification requires that at no load the nominal

output voltage of the regulator be offset to a value lower

than the nominal voltage corresponding to the VID code.

The offset is tuning from RG in the DAC generator as

Figure 18.

If VID0~6 is set at VSS (Ground), and to suppose that

shunt resistance is Rs.

From below equation, we can tune the value of RG to

increase or decrease the base voltage of VDACQ.

REF

DACQ

x V

)

REF

(0.8V)

VDACFB

VDACQ

VID0

VID1

VID2

VID3

VID4

VID5

Figure 18. The Structure of Discrete DAC Generator

Over Current Protection

OCP comparator co\mpares each inductor current sensed

& sample/hold by current sense circuit with this reference

current(150uA). RT8800/B uses hiccup mode to eliminate

fault detection of OCP or reduce output current when

output is shorted to ground.

Figure 19. The Over Current Protection in the interval

CH1:(5V/Div)

CH2:(5V/Div)

Over Current Protection

Time (25ms/Div)

PWM

Figure 20. Over Current Protection at steady state

CH1:(5V/Div)

CH2:(5V/Div)

Over Current Protection

Time (25ms/Div)

PWM

VSS

Fault Detection

The

“hiccup mode” operation of over current protection

is adopted to reduce the short circuit current. The in-rush

current at the start up is suppressed by the soft start

circuit through clamping the pulse width and output voltage

by an internal slow rising ramp.

Figure 17. Load Droop Circuit

DACQ

DRP

2xI

SINK

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