RT2875A/B
Application Information
Output Voltage Setting
Chip Enable Operation
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shutdown the device. During shutdown
mode, the RT2875A/B quiescent current drops to lower
than 10μA.Driving the ENpin high (>1.6V) will turn on 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).
V
OUT
R1
FB
RT2875A/B
GND
R2
EN
R
EN
V
IN
EN
RT2875A/B
Figure 1. Output Voltage Setting
C
EN
GND
The output voltage is set by an external resistive voltage
divider according to the following equation :
Figure 3. Enable Timing Control
R1
R2
VOUT = VREF 1
An external MOSFET can be added to implement digital
control on the EN pin when no system voltage above 2.5V
is available, as shown in Figure 4. In this case, a 100kΩ
pull-up resistor, REN, is connected between VIN and the
EN pin. MOSFET Q1 will be under logic control to pull
down the EN pin.
Where VREF is the reference voltage (0.6V typ.).
External Bootstrap Diode
Connect a 0.1μF low ESR ceramic capacitor between the
BOOT and SW pins. This capacitor provides the gate driver
voltage for the high side MOSFET.
R
EN
100k
V
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
RT2875A/B. Note that the external boot voltage must be
lower than 5.5V
EN
RT2875A/B
GND
IN
Q1
EN
Figure 4. Digital Enable Control Circuit
Under Voltage Protection
5V
Hiccup Mode
The RT2875B provides Hiccup Mode Under Voltage
Protection (UVP). When the VFB voltage drops below
0.3V, the UVP function will be triggered to shut down
switching operation. If the UVP condition remains for a
period, the RT2875B will retry automatically. When the
UVP condition is removed, the converter will resume
operation. The UVP is disabled during soft-start period.
BOOT
100nF
RT2875A/B
SW
Figure 2. External Bootstrap Diode
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12
DS2875A/B-05 October 2016
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
