RT9287 View Datasheet(PDF) - Richtek Technology
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RT9287 Datasheet PDF : 13 Pages
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RT9287
Preliminary
Application Information
Boost Converter
Power Sequence
The RT9287 must take notice of the power sequence.
The power sequence of RT9287 has to VDD2 early than
ENB or else the RT9287 maybe fall into the unknown
state to result in RT9287 turn off.
VDD2 > 2.7V
VDD2
ENB < 0.4V
ENB
ENB > 1.5V
VOUT
Figure 1
LED Current Control
As shown in Figure 2, the RT9287 regulates the LED
current by setting the current sense resistor (RSET)
connected between FB pin and ground. The reference
voltage of FB pin is 1.235V in typical. The LED current
(ILED) can be calculated by the following Equation.
ILED = VREF / RSET
(1)
In order to have an accurate LED current, a precision
resistor is preferred (1% is recommended).
L1
D1
4.7uH to 22uH SS0520
+
VIN
2.7V to 5.5V
C1
1uF
RT9287
7 VDD2 LX 6
VENB>1.5V 4 ENB
VENB<0.4V
8 GND
FB 5
C2
1uF/16V
ILED
RSET
Figure 2. Application for Driving 3 Series WLEDs
Inductor Selection
The recommended value of the inductor is from 10μH to
22μH for 4 to 5 WLEDs applications. For 3WLEDs, the
recommended value of the inductor is from 4.7μH to 22μH.
Small size and better efficiency are the major concerns
for portable devices, just as RT9287's application for mobile
phone. The inductor should have low core loss at 1MHz
and low DCR for better efficiency.
The inductor saturation current rating should be considered
to cover the inductor peak current.
Capacitor Selection
Input and output ceramic capacitors of 1μF are
recommended for boost regulator. For better voltage
filtering, ceramic capacitors with low ESR are
recommended. X5R and X7R types are suitable because
they have better temperature characteristics.
Diode Selection
Schottky diode is suitable for RT9287 because of its low
forward voltage and fast reverse recovery. Using Schottky
diode can get better efficiency. The high speed rectification
is also a good characteristic of Schottky diode for high
switching frequency. Current rating of the diode must meet
the root mean square of the peak current and output average
current multiplication as following :
ID(RMS) ≈ IOUT ×IPEAK
The reverse breakdown voltage of the diode should be
higher than the output voltage.
Output Voltage Control
For fixed output voltage application, the output voltage
can be adjusted by the divider circuit on FB pin. Figure 3
shows a 2-level voltage control circuit for OLED application.
The output voltage can be calculated by the following
equations. Table 1 is the recommended resistance for
different conditions.
VOUT = RA x {(FB/RB) + (FB-GPIO)/RGPIO} + FB
(3)
As GPIO = 0V,
VOUT = RA x {(1.235/RB) + (1.235/RGPIO)} + 1.235 (4)
As GPIO = 2.8V,
VOUT = RA x {(1.235/RB) + (1.235-2.8)/RGPIO} + 1.235 (5)
As GPIO = 1.8V,
VOUT = RA x {(1.235/RB) + (1.235-1.8)/RGPIO} + 1.235 (6)
For Efficiency Consideration set RA = 990kΩ.
www.richtek.com
10
DS9287-01 August 2007
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