RT7271A
Application Information
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 RT7271Ais a single-phase Buck 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 (500kHz)
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.
UVLO Protection
The RT7271A provides input Under Voltage Lockout
Protection (UVLO). If the input voltage exceeds the UVLO
rising threshold voltage (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.
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 :
Inductor Selection
RFB1
RFB2
⎛
⎝
⎞
⎟
⎠
VOUT = VREF × 1 +
⎜
The switching frequency (on-time) and operating point (%
ripple or LIR) determine the inductor value as shown below:
where VREF is 0.6V (typ.).
V
× V − V
IN OUT
(
OUT
V
OUT
L =
f
× LIR × I
× V
SW
LOAD(MAX) IN
R
FB1
FB2
where LIR is the ratio of the peak-to-peak ripple current to
the average inductor current.
FB
R
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 and will not saturate at the peak inductor
current (IPEAK) :
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
RT7271Aremains in shutdown if the ENpin is lower than
1.25V. When the EN pin rises above the VEN threshold,
the RT7271Abegins a new initialization and soft-start cycle.
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.
Internal Soft-Start
The RT7271A 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
Input Capacitor Selection
High quality ceramic input decoupling capacitor, such as
X5R or X7R, with values greater than 20μF are
recommended for the input capacitor. The X5R and X7R
ceramic capacitors are usually selected for power regulator
capacitors because the dielectric material has less
capacitance variation and more temperature stability.
Copyright 2013 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS7271A-00 February 2013
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
