RT5710C
Application Information
The RT5710C is a single-phase step-down converter. It
provides single feedback loop, constant on-time 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 (1.5MHz) oscillator and internal
compensation are integrated to minimize external
component count. Protection features include over-current
protection, under-voltage protection and over-temperature
protection.
causing PWM pulse width to increase slowly and in turn
reduce the input 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 RT5710C has input Under-Voltage Lockout protection
(UVLO). If the input voltage exceeds the UVLO rising
threshold voltage (2.25V 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 :
R1
R2
Inductor Selection
VOUT = VREF 1
The switching frequency (on-time) and operating point (%
where VREF is the feedback reference voltage 0.6V (typ.).
ripple or LIR) determine the inductor value as shown below:
VOUT V VOUT
fSW LIR ILOAD(MAX) V
IN
V
OUT
L =
IN
where LIR is the ratio of the peak-to-peak ripple current to
the average inductor current.
R1
FB
R2
Find a low loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. The core
must be large enough not to saturate at the peak inductor
current (IPEAK) :
GND
Figure 1. Setting VOUT with a Voltage Divider
LIR
2
IPEAK = ILOAD(MAX)
+
ILOAD(MAX)
Chip Enable and Disable
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.
The EN pin allows for power sequencing between the
controller bias voltage and another voltage rail. The
RT5710C remains in shutdown if the ENpin is lower than
400mV. When the EN pin rises above the VEN trip point,
the RT5710C begins a new initialization and soft-start
cycle.
Inductor saturation current should be chosen over IC’s
current limit.
Internal Soft-Start
Input Capacitor Selection
The RT5710C 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,
High quality ceramic input decoupling capacitor, such as
X5R or X7R, with values greater than 10μF 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.
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DS5710C-04 November 2019
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
