DS8813D Datasheet, PDF (15/23 Pages) Richtek Technology Corporation – Multi-Phase PWM Controller with PWM-VID Reference

800
700
600
500
400
300
200
150 250 350 450 550 650 750
R_TON (kΩ)
Figure 6. Frequency vs. R_TON
Active Phase Circuit setting : Before POR
The RT8813D can operate in 3/2/1 phase. When PVCC is
higher than POR threshold and EN is higher than logic-
high level, the RT8813D will detect the VCC/ISEN1 pin to
determine how many phases should be active. For three
phases operation, the VCC/ISEN1 pin is connected to
PHASE1, the TALERT/ISEN2 pin is connected to
PHASE2, the TSNS/ISEN3 pin is connected to PHASE3,
and external MOSEFT driver's PWM pin is connected to
PWM3. For two phases operation, the VCC/ISEN1 pin is
connected to PVCC, the TALERT/ISEN2 pin is connected
to TALERT signal, the TSNS/ISEN3 pin is connected to
TSNS signal, and the PWM3 pin is connected to GND.
For one phase operation, the VCC/ISEN1 pin is connected
to PVCC, TALERT/ISEN2 pin is connected to TALERT
signal, the TSNS/ISEN3 pin is connected to TSNS signal,
the PWM3 pin is connected to GND, and UGATE2,
BOOT2, PHASE2, and LGATE 2 pins are floating. The
voltage setting at PSI pin can't higher than 1.8V.
Mode Selection
The RT8813D can operate in 3 phases or 2 phases with
force CCM, 1 phase with force CCM, and 1 phase with
DEM according to PSI voltage setting. If PSI voltage is
pulled below 0.4V, the controller will operate into 1 phase
with DEM. In DEM operation, the RT8813D automatically
reduces the operation frequency at light load conditions
for saving power loss. If PSI voltage is pulled between
Copyright ©2016 Richtek Technology Corporation. All rights reserved.
DS8813D-00 September 2016
RT8813D
0.8V to 1V, the controller will switch operation into 1 phase
with force CCM. If PSI voltage is pulled between 1.4V to
5.5V, the controller will switch operation into active phase
(only for 2 or 3 phase). The operation mode is summarized
in Table 1. Moreover, the PSI pin is valid after POR of VR.
Table 1
Operation Phase Number PSI Voltage Setting
1 phase with DEM
0V to 0.4V
1 phase with CCM
0.8V to 1V
Active phase with CCM
1.4V to 5.5V
Diode-Emulation Mode
In diode-emulation mode, the RT8813D automatically
reduces switching frequency at light-load conditions to
maintain high efficiency. As the output current decreases
from heavy-load condition, the inductor current is also
reduced, and eventually comes to the point that its valley
touches zero current, which is the boundary between
continuous conduction and discontinuous conduction
modes. By emulating the behavior of diodes, the low side
MOSFET allows only partial of negative current when the
inductor freewheeling current reaches negative value. As
the load current is further decreased, it takes a longer
time to discharge the output capacitor to the level that
requires the next “ON” cycle. In reverse, when the output
current increases from light load to heavy load, the
switching frequency increases to the preset value as the
inductor current reaches the continuous conduction
condition. The transition load point to the light load
operation is shown in Figure 7 and can be calculated as
follows :
I_LOAD(SKIP)

(V_IN
 V_OUT
2L
)
 t_ON
where t_ON is on-time.
I_{L Slope = (VIN - VOUT) / L}
I_PEAK
I_LOAD = I_PEAK/2
t
0
t_ON
Figure 7. Boundary condition of CCM/DEM
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