RT5701
Operation
The RT5701 is a low voltage synchronous step-down
converter that can support the input voltage range from
2.5V to 5.5V and can deliver up to 4 A at an I
2
C selectable
voltage ranging from 0.3 V to 5.5 V, distributed into 4
banks of output voltages. The converter can operate in
Auto mode or forced PWM mode. Operating modes and
output voltage can be selected via I
2
C.
By adapting current mode architecture, converter decides
its switching duty by inductor current sense information,
compensation ramp and error amplifier output. The
converter turns on the high-side P-MOSFET whenever the
raising edge of the switching clock. After the decided duty
time, the high-side P-MOSFET would be turned off and
the low-side N-MOSFET would be turned on until the next
frequency clock raising in forced PWM mode or turn off
by ZC (Zero Current Detection) in auto mode. The error
amplifier may adjust its output, COMP, with selected
voltage reference and output feed-back voltage information.
Different selection of voltage reference and different loading
at output node regulate the required COMP voltage, which
regulating the output voltage.
VSEL Function for Immediately Voltage Change
To address different performance operating points and
startup conditions, the device offers two output voltage /
mode presets, which can be chosen via a dedicated VSEL
pin; this allows simple and zero latency output voltage
transition.
Operating Mode Selection
The converter can operate in Auto mode or forced PWM
mode. It can be selected by programming register
0x14.PWM0 for VSEL = Low or 0x14.PWM1 for VSEL =
High. If Auto mode is selected, the converter automatically
switches the operation mode between PWM and PSM
according to the load conditions. If forced PWM mode is
selected, the converter works only in PWM mode.
PWM (Pulse Width Modulation) Operating Mode
The converter operates in PWM (Pulse Width Modulation)
mode from medium to heavy load. In PWM mode, the
converter operates with its nominal switching frequency
of 3MHz, and adapts its duty cycle to regulate the desired
output voltage. In this mode, the inductor current is in
CCM (Continuous Current Mode) and the voltage is
regulated by PWM.
Forced PWM (FPWM) Operating Mode
If forced PWM mode is selected, the converter works only
in PWM mode, and disables the transition from PWM to
PSM as the load decreases. The advantage of forced PWM
is that the switching frequency is fixed to be 3MHz, and
thus it is easy for EMI immunity design. The disadvantage
is that the efficiency at light load is poor due to the negative
inductor current.
Pulse Skipping Modulation (PSM) Operating Mode
When in Forced PWM operating mode without output
current loading, half of FPWM inductor current is negative
to balance total average zero output current. If that negative
inductor
current could be saved, the efficiency will be
improved strongly. PSM is one Buck operation mode with
zero inductor current detection. Whenever zero current
occur, the low-side N-MOSFET is turned off immediately
and save the resident power into output capacitor (store
energy in higher V
OUT
). Then, the converter skips internal
synchronous clock and keeps sleep-state until output
voltage is discharged to below its target value. Once skips
occurs, the power MOSFETs of converter is turned off
and lots of sub-block circuits is in sleep state to save
quiescent consumption. If the output loading is increasing,
the discharge time is shortened, i.e. the switching
frequency depends on the output loading.
The switch
frequency is decayed from 3MHz; the lighter output
loading, the lower switch frequency. As result, PSM
V
OUT
ripple would be slightly larger than FPWM but PSM gains
significant efficiency improvement.
Auto-Zero Current Detector
The Auto-Zero Current (AZC) detector circuit senses the
LX waveform to adjust the inductor zero current threshold
voltage automatically. In traditional trimmed zero current
detectors, the zero current threshold changes due to
V
IN
/
V
OUT
variation. This would degrade efficiency due to
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS5701-02 March 2018
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
