RT5800
enable control, to enable or disable the device. If VEN is
held below a logic-low threshold voltage (VENL) of the
enable input (EN), the converter will enter into
shutdown mode and reset all digital function (I2C), that
is, the converter is disabled even if the VIN voltage is
above VIN under-voltage lockout threshold (VUVLO).
During shutdown mode, the supply current can be
reduced to ISHDN (20A or below). If the EN voltage
rises above the logic-high threshold voltage (VENH), the
device starts switching. When appropriate voltages are
present on the VIN, AVIN, VIO and EN pins, the
RT5800 will begin digital function, switching and initiate
a soft-start ramp of the output voltage. After the device
is turned on and VIO is ready, all digital functions
including I2C communication start to work in a boot
time with 230s (typ.). The voltage of VIO can be used
to supply power to digital function and it is recommend
that enable the device after VIO voltage is ready. The
RT5800 supports enable delay time setting (factory
setting) and soft-start slew rate setting for each Buck.
The soft-start function is used to prevent large inrush
current while the converter is powered up. Register
0x55[5:4], 0x6F[5:4] and 0x89[5:4] let soft-start time of
each be programmable. The start-up sequence is
shown in Figure 3. IC also implements enable control
by software, it can be set in the registers: 0x49[0],
0x4B[0], 0x63[0], 0x65[0], 0x7D[0] and 0x7F[0]. If the
output voltage of Buck is default disable which is only
set by factory, the output voltage starts to ramp up by
software and the Figure 4 shows the start-up sequence.
For disable function, the device supports disable delay
time setting (factory setting) by external EN pin and the
output voltage ramps down with default discharge
resistor. The discharge resistor can be controlled to on
or off by register 0x42[0], 0x5F[0] and 0x79[0] when the
converter is disabled by software. The power-off
sequence is shown in Figure 5 and Figure 6.
MODE Selection
Whether it is DVS0 or DVS1, there are two modes of
operation: forced continuous conduction mode (FCCM)
and automatic power saving mode (Auto mode). It is
set in the following registers: 0x49[5], 0x4B[5], 0x63[5],
0x65[5], 0x7D[5] and 0x7F[5]. For example, to set
DVS0 of Buck1 to FCCM, just write "1" at 0x49[5].
Auto Mode with Automatically Phase Adding/Shedding
Auto mode enables high efficiency at light load. At low
load current, the inductor current can drop to zero and
become negative. This is detected by internal zero
current-detect circuitry which utilizing the LSFET
RDS(ON) to sense the inductor current. The LSFET is
turned off when the inductor current drops to zero,
resulting in discontinuous operation (DCM). Both
HSFET and LSFET remain off with the output capacitor
supplying the load current until the feedback voltage
falls below the feedback reference voltage. DCM
operation maintains high efficiency at light load, while
setting MODE to Forced PWM (FCCM) operation helps
meet tight voltage regulation accuracy requirements.
For multiphase outputs, the RT5800 automatically
increases the number of operating phases as the load
continues to increase above 3A (typ). The two phases
are interleaved with 180 degrees apart. Interleaving
reduces ripple current at the input and output.
Therefore, the input and output capacitors are also
reduced. Conversely, when the load current per phase
drops below 2.6A (typ), the RT5800 automatically
sheds the number of phases.
FCCM Mode
Setting MODE to Forced PWM (FCCM) operation helps
meet
stringent
voltage
regulation
accuracy
requirements. Users must enable all the set outputs
before setting into the FCCM.
UVLO, Enable Control and Soft-Start
The RT5800 implements under-voltage lockout
protection (UVLO) to prevent operation without fully
turn on the internal HSFET and LSFET. The UVLO
monitors the voltage of AVIN. When the AVIN voltage
is lower than UVLO threshold, IC stops switching and
resets all digital functions.
The RT5800 provides an EN pin, as an external chip
Copyright © 2019 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
6
DS5800-00 November 2019
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