RT8167B datasheet(31/38 Pages) RICHTEK

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RT8167B

DS8167B-00

October 2011

www.richtek.com

Since the DCR of inductor is temperature dependent, it

affects the output accuracy in high temperature conditions.

Temperature compensation is recommended for the

lossless inductor DCR current sense method. Figure 10

shows a simple but effective way of compensating the

temperature variations of the sense resistor using an NTC

thermistor placed in the feedback path.

Figure 10. Loop Setting with Temperature Compensation

VCC_SENSE

VSS_SENSE

FBx

RGNDx

COMPx

R1b

R1a

NTC

VREFx

Figure 8. Simplified Schematic for Droop and Remote

Sense in CCM

Disable GFX VR : Before EN = High

GFX VR enable or disable is determined by the internal

circuitry that monitors the ISENAN voltage during start

up. Before EN = high, GFX VR detects whether the voltage

of ISENAN is higher than

“VCC − 1V” to disable GFX

VR. The unused driver pins can be connected to GND or

left floating.

GFX VR Forced-DEM Function Enable : After

VRA_Ready = High

The GFX VR's forced-DEM function can be enabled or

disabled with GFXPS2 pin. The RT8167B detects the

voltage of GFXPS2 for forced-DEM function. If the voltage

at GFXPS2 pin is higher than 4.3V, the GFX VR operates

in forced-DEM. If this voltage is lower than 0.7V, the GFX

VR follows SVID power state command.

Loop Control

Both CORE and GFX VR adopt Richtek's proprietary G-

NAVP

TM topology. G-NAVPTM is based on the finite-gain

valley current mode with CCRCOT (Constant Current

Ripple Constant On Time) topology. The output voltage,

VCORE or VGFX, will decrease with increasing output load

current. The control loop consists of PWM modulator with

power stage, current sense amplifier and error amplifier

as shown in Figure 8.

Similar to the valley current mode control with finite

compensator gain, the high side MOSFET on-time is

determined by the CCRCOT PWM generator. When load

current increases, VCS increases, the steady state COMP

voltage also increases which makes the output voltage

decrease, thus achieving AVP.

Droop Setting (with Temperature Compensation)

It's very easy to achieve the Active Voltage Positioning

(AVP) by properly setting the error amplifier gain due to

the native droop characteristics. The target is to have

VOUT = VREFx

− ILOAD x RDROOP

(1)

Then solving the switching condition VCOMPx = VCSx in

Figure 8 yields the desired error amplifier gain as

where AI is the internal current sense amplifier gain and

RSENSE is the current sense resistance. If no external sense

resistor is present, the DCR of the inductor will act as

RSENSE. RDROOP is the resistive slope value of the converter

output and is the desired static output impedance.

Figure 9. Error Amplifier Gain (AV) Influence on VOUT

Accuracy

AV1

AV2

AV2 > AV1

VOUT

Load Current

== I

SENSE

DROOP

(2)

VIN

ISENxP

ISENxN

FBx

RGNDx

High Side

MOSFET

CORE/GFX VR

VCC_SENSE

COMPx

VCSx

VREFx

GFX/CORE VR

CCRCOT

PWM Generator

Driver

Logic

Control

CByp

UGATEx

PHASEx

LGATEx

CORE/GFX VR

VSS_SENSE

VOUT

(VCORE/VGFX)

CMP

Low Side

MOSFET

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