RT6224A/B
Output Voltage Setting
can be slowed by placing a small (<47Ω) resistance
between BOOT and the external bootstrap capacitor. This
will slow the high-side switch turn-on and VSW's rise. To
remove the resistor from the capacitor charging path
(avoiding poor enhancement due to undercharging the
BOOT capacitor), use the external diode shown in Figure
6 to charge the BOOT capacitor and place the resistance
between BOOT and the capacitor/diode connection.
5V
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 :
VOUT = 0.6 x (1 + R1 / R2)
V
OUT
R1
FB
RT6224A/B
BOOT
R2
RT6224A/B
SW
0.1µF
GND
Figure 5. Output Voltage Setting
Figure 6. External Bootstrap Diode
Over-Temperature Protection
Place the FB resistors within 5mm of the FB pin. Choose
R2 between 10kΩ and 100kΩ to minimize power
consumption without excessive noise pick-up and
calculate R1 as follows :
The RT6224A/B features an over-temperature protection
(OTP) circuitry to prevent from overheating due to
excessive power dissipation. The OTP will shut down
switching operation when junction temperature exceeds
160°C. Once the junction temperature cools down by
approximately 20°C, the converter will resume operation.
To maintain continuous operation, the maximum junction
temperature should be lower than 125°C.
R2(V
0.6)
OUT
R1
0.6
For output voltage accuracy, use divider resistors with 1%
or better tolerance.
External BOOT Bootstrap Diode
When the input voltage is lower than 5.5V it is
recommended to add an external bootstrap diode between
VIN(or VINR) and the BOOT pin to improve enhancement
of the internal MOSFET switch and improve efficiency.
The bootstrap diode can be a low cost one such as 1N4148
or BAT54.
Under-Voltage Protection
Hiccup Mode
The RT6224A/B provides Hiccup Mode under-voltage
protection (UVP). When the VFB voltage drops below
0.45V, the UVP function will be triggered to shut down
switching operation. If the UVP condition remains for a
period, the RT6224A/B will retry automatically. When the
UVP condition is removed, the converter will resume
operation. The UVP is disabled during soft-start period.
External BOOT Capacitor Series Resistance
The internal power MOSFET switch gate driver is
optimized to turn the switch on fast enough for low power
loss and good efficiency, but also slow enough to reduce
EMI. Switch turn-on is when most EMI occurs since VSW
rises rapidly. During switch turn-off, SW is discharged
relatively slowly by the inductor current during the deadtime
between high-side and low-side switch on-times. In some
cases it is desirable to reduce EMI further, at the expense
of some additional power dissipation. The switch turn-on
Thermal Considerations
The junction temperature should never exceed the
absolute maximum junction temperature TJ(MAX), listed
under Absolute Maximum Ratings, to avoid permanent
damage to the device. The maximum allowable power
dissipation depends on the thermal resistance of the IC
Copyright 2018 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS6224A/B-02 August 2018
www.richtek.com
13
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