RT7278 Datasheet, PDF (10/14 Pages) Richtek Technology Corporation – 3A, 18V, 700kHz ACOTTM Synchronous Step-Down Converter

RT7278
Application Information
The RT7278 is a synchronous high voltage Buck converter
that can support the input voltage range from 4.5V to 18V
and the output current up to 3A. It adopts ACOT^TM mode
control to provide a very fast transient response with few
external compensation components.
PWM Operation
It is suitable for low external component count
configuration with appropriate amount of Equivalent Series
Resistance (ESR) capacitors at the output. The output
ripple valley voltage is monitored at a feedback point
voltage. The synchronous high side MOSFET is turned
on at the beginning of each cycle. After the internal on-
time timer expires, the MOSFET is turned off. The pulse
width of this on-time is determined by the converter's input
and output voltages to keep the frequency fairly constant
over the entire input voltage range.
Advanced Constant On-Time Control
The RT7278 has a unique circuit which sets the on-time
by monitoring the input voltage and SW signal. The circuit
ensures the switching frequency operating at 700kHz over
input voltage range and loading range.
Soft-Start
The RT7278 contains an external soft-start clamp that
gradually raises the output voltage. The soft-start timing
can be programmed by the external capacitor between
SS pin and GND. The chip provides a 2μA charge current
for the external capacitor. If a 3.9nF capacitor is used,
the soft-start will be 2.6ms (typ.). The available capacitance
range is from 2.7nF to 220nF.
t _SS
(ms) =
C5 (nF)×1.365
I_SS (μA)
Chip Enable Operation
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shut down the device. During shutdown
mode, the RT7278's quiescent current drops to lower than
10μA. Driving the EN pin high (>2V, <18V) will turn on the
device again. For external timing control, the EN pin can
also be externally pulled high by adding a R_EN resistor
and C_EN capacitor from the VIN pin (see Figure 1).
EN
R_EN
V_IN
C_EN
EN
RT7278
GND
Figure 1. External Timing Control
An external MOSFET can be added to implement digital
control on the EN pin when the EN pin input voltage is
lower than 2V, as shown in Figure 2. In this case, a 100kΩ
pull-up resistor, R_EN, is connected between V_IN and the
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
R_EN
100k
V_IN
EN
EN
Q1
RT7278
GND
Figure 2. Digital Enable Control Circuit
To prevent enabling circuit when V_IN is smaller than the
V_OUT target value, a resistive voltage divider can be placed
between the input voltage and ground and connected to
the EN pin to adjust IC lockout threshold, as shown in
Figure 3. For example, if an 8V output voltage is regulated
from a 12V input voltage, the resistor R_EN2 can be selected
to set input lockout threshold larger than 8V.
R_EN1
V_IN
R_EN2
EN
RT7278
GND
Figure 3. Resistor Divider for Lockout Threshold Setting
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is a registered trademark of Richtek Technology Corporation.
DS7278-00 January 2013