RT5771A
Application Information
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.
The RT5771A is a single-phase step-down 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. Afixed switching frequency (1MHz)
oscillator and internal compensation are integrated to
minimize external component count. Protection features
include over-current protection, under-voltage protection
and over-temperature protection.
UVLO Protection
The RT5771Ahas 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. The power sequence of the VCC and
VINneed to be considered if they are powered separately.
The driver voltage of high-side MOSET comes from VIN
input and internal control circuit is powered by VCC. The
VCC has to be powered earlier than the VIN to ensure
that the high-side MOSFET has never turned on before
the internal control circuit is ready.At power off, the voltage
at the VINhas to be removed before the VCC goes below
the threshold of UVLO.
Output Voltage Setting
Connect a resistive voltage divider at the FB between VOUT
andGNDto adjust the output voltage. The output voltage
is set according to the following equation :
R1
R2
VOUT = VREF 1
where VREF is the feedback reference voltage 0.6V (typ.).
V
OUT
R1
FB
R2
GND
Inductor Selection
The switching frequency (on-time) and operating point (%
ripple or LIR) determine the inductor value as shown below:
Figure 1. Setting VOUT with a Voltage Divider
V
V V
IN OUT
OUT
L =
f
LIRI
V
Chip Enable and Disable
SW
LOAD(MAX)
IN
The EN pin allows for power sequencing between the
controller bias voltage and another voltage rail. The
RT5771Aremains in shutdown if the ENpin is lower than
400mV. When the EN pin rises above the VEN trip point,
the RT5771Abegins a new initialization and soft-start cycle.
where LIR is the ratio of the peak-to-peak ripple current to
the average inductor current.
Find a low loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice, although powdered iron is
inexpensive and can work well at 200kHz. The core must
be large enough not to saturate at the peak inductor current
(IPEAK) :
Internal Soft-Start
The RT5771A 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,
LIR
2
IPEAK = ILOAD(MAX)
+
ILOAD(MAX)
The calculation above serves as a general reference. To
further improve transient response, the output inductor
can be further reduced. This relation should be considered
along with the selection of the output capacitor.
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is a registered trademark of Richtek Technology Corporation.
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10
DS5771A-02 June 2019
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
