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RT8061A Datasheet PDF : 12 Pages
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RT8061A
Application Information
The RT8061A is a single-phase buck PWM converter. It
provides single feedback loop, current mode control with
fast transient response. An internal 0.6V reference allows
the output voltage to be precisely regulated for low output
voltage applications. A fixed switching frequency (1MHz)
oscillator and internal compensation are integrated to
minimize external component count. Protection features
include over current protection, under voltage protection,
over voltage protection and over temperature protection.
PWM Operation
The RT8061A utilizes DEM control to improve light load
efficiency. Depending on the load current, the controller
automatically operates in Diode-Emulation Mode (DEM)
or in Continuous Conduction Mode (CCM) with fixed-
frequency PWM.
At light load condition, the RT8061A automatically operates
in diode-emulation mode to reduce switching frequency
and improve efficiency. As the output current decreases
from heavy load condition, the inductor current is also
reduced and eventually comes to the point where 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 negative current to flow when
the inductor freewheeling current reaches negative. As the
load current further decreases, it takes longer and longer
to discharge the output capacitor to the level that that
requires the next UGATE “ON” cycle. In contrast, 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.
The controller will then operate in continuous conduction
mode with 1MHz fixed PWM switching frequency.
Output Voltage Setting
Connect a resistive voltage divider at the FB between VOUT
and GND to adjust the output voltage. The output voltage
is set according to the following equation :
VOUT
=
VFB
× ⎛⎜⎝1+
R1
R2
⎞⎟⎠
where VFB is 0.6V (typ.).
VOUT
R1
FB
R2
GND
Figure 1. Setting VOUT with a Voltage Divider
Chip Enable and Disable
The EN pin allows for power sequencing between the
controller bias voltage and another voltage rail. The
RT8061A remains in shutdown if the EN pin is lower than
400mV. When the EN pin rises above the VEN trip point,
the RT8061A begins a new initialization and soft-start cycle.
Internal Soft-Start
The RT8061A provides an internal soft-start function to
prevent large inrush current and output voltage overshoot
when the converter starts up. The Soft-Start (SS)
automatically begins once the chip is enabled. During soft-
start, the internal soft-start capacitor becomes charged
and generates a linear ramping up voltage across the
capacitor. This voltage clamps the voltage at the FB pin,
causing PWM pulse width to increase slowly and in turn
reduce the output surge current. The internal 0.6V
reference takes over the loop control once the internal
ramping-up voltage becomes higher than 0.6V.
UVLO Protection
The RT8061A has input Under Voltage Lockout protection
(UVLO). If the input voltage exceeds the UVLO rising
threshold voltage (2.4V typ.), the converter resets and
prepares the PWM for operation. If the input voltage falls
below the UVLO falling threshold voltage during normal
operation, the device will stop switching. The UVLO rising
and falling threshold voltage has a hysteresis to prevent
noise-caused reset.
Inductor Selection
The switching frequency (on-time) and operating point (%
ripple or LIR) determine the inductor value as shown below :
L=
VOUT × ( VIN − VOUT )
fSW × LIR ×ILOAD(MAX) × VIN
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
www.richtek.com
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is a registered trademark of Richtek Technology Corporation.
DS8061A-04 September 2012
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