DS8817A Datasheet, PDF (13/21 Pages) Richtek Technology Corporation – Dual-Phase PWM Controller with PWM-VID Reference

RT8817A
Active Phase Circuit setting
The RT8817A operates as active phase being 2 phase,
and 1 phase. When programming active phase being 1
phase, The UGATE2, BOOT2, PHASE2, and LGATE2
pins are floating. The active phase number can be
programmed by PSI pin voltage. Refer to Table 1 for
detail
Mode Selection
The RT8817A can operate into 2 phases with force CCM,
1 phase with force CCM, 1 phase with DEM and 2 phases
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 RT8817A
automatically reduces the operation frequency at light
load conditions for saving power loss. If PSI voltage is
pulled between 0.7V to 0.88V, the controller will switch
operation into 1 phase with force CCM. If PSI voltage is
pulled between 1.08V to 1.35V, the controller will switch
operation into 2 phase with DEM. If PSI voltage is pulled
between 1.6V to 5.5V, the controller will switch
operation into 2 phase with force CCM. 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
1phase with DEM
0V to 0.4V
1phase with CCM
0.7V to 0.88V
2phase with DEM
1.08V to 1.35V
2phase with CCM
1.6V to 5.5V
Diode-Emulation Mode
In diode-emulation mode, the RT8817A 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 5 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 5. Boundary condition of CCM/DEM
The switching waveforms may be noisy and asynchronous
in light loading diode-emulation operation condition, but
this is a normal operating condition that results in high
light-load efficiency. Trade-off in DEM noise vs. light-load
efficiency is made by varying the inductor value. Generally,
low inductor values produce a broad high efficiency range
vs. load curve, while higher values result in higher full load
efficiency (assuming that the coil resistance remains fixed)
and less output voltage ripple. The disadvantages for using
higher inductor values include larger physical size and
degraded load-transient response (especially at low input
voltage levels).
Forced-CCM Mode
The low noise, forced-CCM mode disables the zero-
crossing comparator, which controls the low-side switch
on-time. This causes the low-side gate drive waveform to
be the complement of the high-side gate drive waveform.
This in turn causes the inductor current to reverse at light
loads as the PWM loop to maintain a duty ratio V_OUT/V_IN.
The benefit of forced-CCM mode is to keep the switching
frequency fairly constant.
Copyright ©2016 Richtek Technology Corporation. All rights reserved.
DS8817A-00 September 2016
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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