RT6218A/B
R
EN1
5V
V
IN
EN
D
R
BOOT
EN2
RT6218A/B
GND
BOOT
RT6218A/B
LX
C
0.1μF
BOOT
Figure 4. Resistor Divider for Lockout Threshold
Setting
Figure 6. External Bootstrap Diode
Resistor at BOOT Pin
Output Voltage Setting
Set the desired output voltage using a resistive divider
from the output to ground with the midpoint connected
to FB. The output voltage is set according to the
following equation :
The gate driver of an internal power MOSFET, utilized
as a high-side switch, is optimized for turning on the
switch not only fast enough for reducing switching
power loss, but also slow enough for minimizing EMI.
The EMI issue is worse when the switch is turned on
rapidly due to high di/dt noises induced. When the
high-side switch is being turned off, the LX node will be
discharged relatively slowly by the inductor current due
to the presence of the dead time when both the
high-side and low-side switches are turned off.
VOUT = 0.765V x (1 + R1 / R2)
V
OUT
R1
FB
RT6218A/B
R2
GND
In some cases, it is desirable to reduce EMI further,
even at the expense of some additional power
dissipation. The turn-on rate of the high-side switch can
be slowed by placing a small (< 47) resistor between
the BOOT pin and the external bootstrap capacitor.
This will slow down the rates of the high-side switch
turn-on and the rise of VLX. The recommended
application circuit is shown in Figure 7, which includes
an external bootstrap diode for charging the bootstrap
capacitor and a bootstrap resistor RBOOT being placed
between the BOOT pin and the capacitor/diode
connection.
Figure 5. Output Voltage Setting
Place the FB resistors within 5mm of the FB pin.
Choose R2 between 10k and 100k to minimize
power consumption without excessive noise pick-up
and calculate R1 as follows :
R2(VOUT VREF
)
R1
VREF
For output voltage accuracy, use divider resistors with
1% or better tolerance.
External BOOT Bootstrap Diode
5V
A bootstrap capacitor of 0.1F low-ESR ceramic
capacitor is connected between the BOOT and LX pins
to supply the high-side gate driver. It is recommended
to add an external bootstrap diode between an external
5V voltage supply and the BOOT pin to improve
enhancement of the internal MOSFET switch and
improve efficiency when the input voltage is below 5.5V.
The bootstrap diode can be a low-cost one, such as
1N4148 or BAT54. The external 5V can be a fixed 5V
voltage supply from the system, or a 5V output voltage
generated by the RT6218A/B. Note that the BOOT
voltage VBOOT must be lower than 5.5V.
D
C
BOOT
BOOT
R
BOOT
BOOT
RT6218A/B
SW
0.1μF
Figure 7. External Bootstrap Diode and Resistor at the
BOOT Pin
Power-Good Output
The PGOOD pin is an open-drain power-good
indication output and is to be connected to an external
Copyright © 2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS6218A/B-01 October 2018
www.richtek.com
15
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
