RT2856
Application Information
This IC is a single phase Buck PWM converter with two
integrated N-MOSFETs. It provides good performance
during load and line transients by implementing a single
feedback loop, current-mode control, and external
compensation. The integrated synchronous power
switches can increase efficiency and it is suitable for lower
duty cycle applications. The switching frequency can be
externally set from 200kHz to 1.6MHz which allows for
high efficiency and optimal size selection of output filter
components. In additional, there is a synchronization
mode control in this device which can be synchronized to
the external clock frequency, and easily switched from
internal switching mode to synchronization mode.
VIN and PVIN Pins
The VINand PVINpins can be used together or separately
for a variety of applications. In this device, the VIN pin is
an input for supplying internal reference and control
circuitry and the PVIN pin is an input for providing main
power to device system and internal high-side power
MOSFET. When the VINand PVINpins are tied together,
both pins can operate from 4.5V to 18V. When the VIN
and PVINpins are used separately, VINpin must be ranged
from 4.5V to 18V, and the PVIN pin can be applied down
to as low as 1.6V to 18V.
The device incorporates an internal Under-Voltage Lockout
(UVLO) circuitry on the VIN pin. If the VIN pin voltage
exceeds the UVLO rising threshold voltage 4V, the
converter resets and prepares the PWM for operation. If
the VIN pin voltage falls below the falling threshold voltage
3.85V during normal operation, the device is disabled.
Such wide internal UVLO hysteresis of 150mV can
efficiently prevent noise caused reset. There is also an
external UVLO circuitry which can be achieved by
configuring a resistive voltage divider on EN pin for both
input VIN and PVIN pins and it is able to provide either
input pins an adjustable UVLO function to ensure a proper
power up behavior. More discussions are located in the
section of Enable Operation.
The device contains a power good protection and an
external soft-start function that is able to monitor the
system output voltage for normal regulation and provides
a programmable power up sequence for avoiding inrush
currents efficiently. Furthermore, the device incorporates
a lot of protections such as OVP, OCP, OTP and etc.
Main Control Loop
The device implements an adjustable fixed frequency with
peak current-mode control which offers an excellent
performance over various line and loading.During normal
operation, the internal high-side power switch is turned
on by the internal oscillator initiating. Current in the
inductor increases until the high-side switch current reaches
the current reference converted by the output voltage VCOMP
of the error amplifier. The error amplifier adjusts its output
voltage by comparing the feedback signal from a resistive
voltage divider on the FB pin with an internal 0.8V
reference. When the load current increases, it causes a
reduction in the feedback voltage relative to the reference.
The error amplifier increases its current reference until
the average inductor current matches the new load current.
When the high-side power MOSFET turns off, the low-
side synchronous power switch (N-MOSFET) turns on
until the beginning of the next clock cycle.
Output Voltage Setting
The resistive voltage divider allows the FB pin to sense
the output voltage as shown in Figure 1.
V
OUT
R1
FB
RT2856
GND
R2
Figure 1. Setting the Output Voltage
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12
DS2856-12 June 2019
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
