The RT8856 is a 1/2-phase DC/DC controller and includes
embedded gate drivers for reduced system cost and board
area. The number of phases is not only user selectable,
but also dynamically changeable based on Intel's
IMVP6.5 control signals to optimize efficiency. Phase
currents are continuously sensed for loop control, droop
tuning, and over current protection. The internal 7-bit VID
DAC and a low offset differential amplifier allow the
controller to maintain high voltage regulating accuracy
to meet Intel's IMVP6.5 specification.
To reduce the efforts and errors caused by manual
calculations, a user friendly design tool is now available
This design tool calculates all necessary design
parameters by entering user's requirements. Please
contact Richtek's representatives for details.
Phase Selection and Operation Modes
The maximum number of operating phase is programmable
by setting ISEN2_N. After the initial turn-on of the RT8856,
an internal comparator checks the voltage at the ISEN2_N
pin. To set the RT8856 as a pure single phase PWM
controller, connect ISEN2_N to a voltage higher than (VCC
- 1V) at power on. The controller will then disable phase 2
(hold UGATE2 and LGATE2 low) and operate as a single
phase PWM controller.
The RT8856 also works in conjunction with Intel's IIMVP6.5
control signals, such as PSI and DPRSLPVR. Table 2
shows the control signal truth table for operation modes
of the RT8856.
For high current demand, the controller will operate with
both phases active. These two phase gate signals are
interleaved. This achieves minimal output voltage ripple
and best transient performance.
For reduced current demand, only one phase is active.
For 1-phase operation, the power stage can minimize
switching losses and maintain transient response
At lowest current levels, the controller enters single phase
Ringing-Free Mode (RFM) to achieve highest efficiency.
Table 2. Control signal truth table for operation
1 Multi-phase CCM
0 Single-phase CCM
S Single-phase RFM,
Differential Remote Sense Setting
The RT8856 includes differential, remote sense inputs to
eliminate the effects of voltage drops along the PC board
traces, CPU internal power routes and socket contacts.
The CPU contains on-die sense pin voltages, VCC_SENSE
and VSS_SENSE. VSS_SENSE is connected to RGND pin. The
VCC_SENSE is connected to FB pin with a resistor to build
the negative input path of the error amplifier. Connect VSEN
to VCC_SENSE for CLKEN, PGOOD, OVP, and UVP sense.
The 7-bit VID DAC and the precision voltage reference are
referred to RGND for accurate remote sensing.
Current Sense Setting
The RT8856 continuously sense the output current of each
phase. Therefore, the controller can be less noise sensitive
and get more accurate current sharing between phases.
Low offset amplifiers are used for loop control and current
limit. The internal current sense amplifier gain (AI) is fixed
to be 10. The ISENx and ISENx_N denote the positive
and negative input of the current sense amplifier of each
phase, respectively. Users can either use a current-sense
resistor or the inductor's DCR for current sensing.
Using inductor's DCR allows higher efficiency as shown
in Figure 1. If
then the current sense performance will be optimum. For
example, choosing L = 0.36μH with 1mΩ DCR and
CX = 100nF, yields RX :
DS8856-03 June 2011