RT8886A Datasheet, PDF (21/45 Pages) Richtek Technology Corporation – 2-Phase Controller with Dual Integrated Drivers

Applications information
The RT8886A is a 2/1 multiphase synchronous Buck
controller designed to meet Intel VR12.5 and VR12.6
compatible CPU specification with a serial SVID control
interface. The controller uses an ADC to implement all
kinds of settings to save a total number of pins for easily
using and increasing PCB space utilization.
G-NAVP^TM Control Mode
The RT8886A adopts the G-NAVP^TM controller, which is a
current mode constant on-time control with DC offset
cancellation. The approach can not only improve DC offset
problem for increasing system accuracy but also provide
fast transient response. For the RT8886A, when current
feedback signal reaches comp signal to generate an on-
time width to achieve PWM modulation. Figure 1 shows
the basic G-NAVP^TM behavior waveforms in continuous
conduct mode (CCM).
Current feedback signal
PWM1
PWM2
Comp signal
PWM3
Figure 1 (a). G-NAVP^TM Behavior Waveforms in CCM in
Steady State
RT8886A
Diode Emulation Mode (DEM)
As well-known, the dominate power loss is switching
related loss during light load, hence VR needs to be
operated in asynchronous mode (or called discontinuous
conduct mode, DCM) to reduce switching related loss
since switching frequency is dependent on loading in the
asynchronous mode. The RT8886A can operate in Diode
Emulation Mode (DEM) in order to improve light load
efficiency. In DEM operation, the behavior of the low-side
MOSFET(s) needs to work like a diode, that is, the low-
side MOSFET(s) will be turned on when the phase voltage
is a negative value, i.e. the inductor current follows from
Source to Drain of low-side MOSFET(s). The low-side
MOSFET(s) will be turned off when phase voltage is a
positive value, i.e. reversed current is not allowed. The
positive voltage threshold (ZCD threshold) of low-side
MOSFET(s) turn off is set by the SET3 pin in Table 9.
Figure 2 shows the control behavior in DEM. Figure 3
shows the G-NAVP^TM operation in DEM to illustrate the
control behaviors. When the load decreases, the discharge
time of output capacitors increases during UGATE and
LGATE are turned off. Hence, the switching frequency
and switching losses will be reduced to improve efficiency
in light load condition.
Inductor current
Phase node
Current feedback signal
Comp signal
PWM1
PWM2
PWM3
Figure 1 (b). G-NAVP^TM Behavior Waveforms in CCM in
Load Transient
UGATE
LGATE
Figure 2. Diode Emulation Mode (DEM) in Steady State
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
DS8886A-00 November 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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