RT7285C
12
DS7285C-03 July 2014
www.richtek.com
©
Copyright 2014 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
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 :
V
OUT
= 0.6 x (1 + R1 / R2)
Figure 5. Output Voltage Setting
RT7285C
GND
FB
R1
R2
V
OUT
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.
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
OUT
R2 (V
0.6)
R1
0.6
Place the FB resistors within 5mm of the FB pin. Choose
R2 between 10k
Ω
and 100k
Ω
to minimize power
consumption without excessive noise pick-up and
calculate R1 as follows :
SW
BOOT
5V
0.1µF
RT7285C
Over-Temperature Protection
The RT7285C 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.
Under-Voltage Protection
Hiccup Mode
The RT7285C provides Hiccup Mode Under-Voltage
Protection (UVP). When the VFB voltage drops below
0.4V, the UVP function will be triggered to shut down
switching operation. If the UVP condition remains for a
period, the RT7285C will retry automatically. When the
UVP condition is removed, the converter will resume
operation. The UVP is disabled during soft-start period.
Figure 6. External Bootstrap Diode
turn-on can be slowed by placing a small (<47
Ω
)
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.
