RT7259
Application Information
can also be externally pulled high by adding a REN resistor
Output Voltage Setting
and CEN capacitor from the VIN pin (see Figure 3).
The resistive divider allows the FB pin to sense the output
voltage as shown in Figure 1.
EN
R
EN
V
IN
EN
V
OUT
RT7259
GND
C
EN
R1
FB
RT7259
GND
R2
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 ENpin. MOSFET Q2 will be under logic control
to pull down the EN pin.
Figure 1. Output Voltage Setting
The output voltage is set by an external resistive voltage
divider according to the following equation :
R1
R2
⎛
⎝
⎞
⎟
⎠
VOUT = VREF 1+
⎜
R
EN
100k
Where VREF is the feedback voltage (0.808V typ.).
V
IN
EN
RT7259
GND
Q2
EN
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.
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 ENpin
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,
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
RT7259.Note that the external boot voltage must be lower
than 5.5V.
5V
(R
(V
x V
)
)
EN1
EN_T
R
EN2
=
− V
IN_S
EN_T
BOOT
RT7259
1µF
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 REN2 is 10.2kΩ.
SW
Figure 2. External Bootstrap Diode
Chip Enable Operation
R
EN1
V
IN
EN
The EN pin is the chip enable input. Pulling the EN pin
low (<0.4V) will shutdown the device. During shutdown
mode, the RT7259 quiescent current drops to lower than
3μA. Driving the ENpin high (2V < EN< 5.5V) will turn on
the device again. For external timing control, the EN pin
R
EN2
RT7259
GND
Figure 5. ResistorDivider for Lockout Threshold Setting
Copyright 2012 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
10
DS7259-00 January 2012
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
