FDS6670AS 查看數據表（PDF） - Richtek Technology
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FDS6670AS Datasheet PDF : 18 Pages
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RT8298
Application Information
Output Voltage Setting
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
VOUT
R1
FB
RT8298
R2
GND
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
VOUT
=
VREF
⎛⎜⎝1+
R1
R2
⎞⎟⎠
Where VREF is the feedback reference voltage (0.808V
typ.).
External Bootstrap Diode
Connect a 1μF low ESR ceramic capacitor between the
BOOT pin and SW pin. This capacitor provides the gate
driver voltage for the high side MOSFET.
It is recommended to add an external bootstrap diode
between an external 5V and BOOT pin for efficiency
improvement when input voltage is lower than 5.5V or duty
ratio is higher than 65% .The bootstrap diode can be a
low cost one such as IN4148 or BAT54. The external 5V
can be a 5V fixed input from system or a 5V output of the
RT8298. Note that the external boot voltage must be lower
than 5.5V.
5V
BOOT
RT8298
1µF
SW
Figure 2. External Bootstrap Diode
Chip Enable Operation
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shutdown the device. During shutdown
mode, the RT8298 quiescent current drops to lower than
3μA. Driving the EN pin high (2V < EN < 5.5V) will turn on
Copyright ©2011 Richtek Technology Corporation. All rights reserved.
DS8298-01 November 2011
the device again. For external timing control, the EN pin
can also be externally pulled high by adding a REN resistor
and CEN capacitor from the VIN pin (see Figure 3).
EN
REN
VIN
CEN
EN
RT8298
GND
Figure 3. Enable Timing Control
An external MOSFET can be added to implement digital
control on the EN pin, as shown in Figure 4. In this case,
a 100kΩ pull-up resistor, REN, is connected between VIN
pin and the EN pin. MOSFET Q2 will be under logic control
to pull down the EN pin.
REN
100k
VIN
EN
EN
Q2
RT8298
GND
Figure 4. Digital Enable Control Circuit
The chip starts to operate when VIN rises to 4.2V (UVLO
threshold). During the VIN rising period, if an 8V output
voltage is set, VIN is lower than the VOUT target value and
it may cause the chip to shut down. To prevent this
situation, a resistive voltage divider can be placed between
the input voltage and ground and connected to the EN pin
to adjust enable threshold, as shown in Figure 5. For
example, the setting VOUT is 8V and VIN is from 0V to
12V, when VIN is higher than 10V, the chip is triggered to
enable the converter. Assume REN1 = 50kΩ. Then,
REN2
=
(REN1
(VIN_S
x
−
VEN_T )
VEN_T )
where VEN_T is the enable comparator's logic-high reference
threshold voltage (1.7V) and VIN_S is the target turn on
input voltage (10V in this example). According to the
equation, the suggested resistor R EN2 is 10.2kΩ.
REN1
VIN
REN2
EN
RT8298
GND
Figure 5. Resistor Divider for Lockout Threshold Setting
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