RT5712A
Application Information
The RT5712A 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 (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.
voltage at the FB pin, 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.
Over-Voltage Protection (OVP)
The RT5712AL provide Over-Voltage Protection
function when output voltage over 120%. The IC will be
into Latch-off mode.
UVLO Protection
Output Voltage Setting
The RT5712A 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
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 :
R1
R2
VOUT = VREF 1
where VREF is the feedback reference voltage 0.6V
(typ.).
threshold voltage has
noise-caused reset.
a
hysteresis to prevent
V
OUT
Inductor Selection
R1
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as
shown below :
FB
R2
GND
V
V V
IN OUT
OUT
L =
Figure 1. Setting VOUT with a Voltage Divider
f
LIR I
V
SW
LOAD(MAX) IN
Chip Enable and Disable
where LIR is the ratio of the peak-to-peak ripple current
to the average inductor current.
The EN pin allows for power sequencing between the
controller bias voltage and another voltage rail. The
RT5712A remains in shutdown if the EN pin is lower
than 400mV. When the EN pin rises above the VEN trip
point, the RT5712A begins a new initialization and
soft-start cycle.
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) :
LIR
2
IPEAK = ILOAD(MAX)
+
ILOAD(MAX)
Internal Soft-Start
The RT5712A 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
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.
Inductor saturation current should be chosen over IC’s
current limit.
Copyright © 2019 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS5712A-04 June 2019
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11
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