Conceptual
RT9293
By the above equation and the application circuit shown
Application for Driving 3 x 13 WLEDs
in Figure 3, and assume the RT9293B is selected which
VREF is equal to 0.3V. Figure 4 shows the relationship
between the LED current and PWM duty cycle. For
example, when the PWM duty is equal to 60%, the LED
current will be equal to 8.6mA. When the PWM duty is
equal to 40%, the LED current will be equal to 12.7mA.
The RT9293 can driver different WLEDs topology. For
example, the Figure 6 shows the 3x13 WLEDs and total
current is equal to 260mA. The total WLEDs current can
be set by the RREF which is equal to following equation.
VREF
ITotal
=
RSET
20
18
16
14
12
10
8
V
OUT
V
IN
L
22uH
D
C
1uF
OUT
C
IN
2.2uF
RT9293
LX
VIN
GND
FB
VOUT
EN
…
Chip Enable
3 x 13 WLEDs
SET
6
4
R
2
0
Figure 6.Application forDriving 3 X 13 WLEDs
Soft-Start
0
20
40
60
80
100
PWM Duty (%)
The function of soft-start is made for suppressing the inrush
current to an acceptable value at the beginning of power-
on. The soft-start function is built-in the RT9293 by
clamping the output voltage of error amplifier so that the
duty cycle of the PWM will be increased gradually in the
soft-start period.
Figure 4
Constant Output Voltage Control
The output voltage of R9293 can be adjusted by the divider
circuit on FB pin. Figure 5 shows the application circuit
for the constant output voltage. The output voltage can be
calculated by the following Equations.
Current Limiting
R1+ R2
The current flow through inductor as charging period is
detected by a current sensing circuit. As the value comes
across the current limiting threshold, theN-MOSFET will
be turned off so that the inductor will be forced to leave
charging stage and enter discharging stage. Therefore,
the inductor current will not increase over the current
limiting threshold.
V
= V
´
; R2 >10k
OUT
REF
R2
V
IN
V
OUT
2.5V to 5.5V
L
D
10uH to 22uH
C
1uF
OUT
C
IN
2.2uF
RT9293
LX
VOUT
EN
VIN
GND
OVP/UVLO/OTP
R1
Chip Enable
FB
The Over Voltage Protection is detected by a junction
breakdown detecting circuit. Once VOUT goes over the
detecting voltage, LX pin stops switching and the power
N-MOSFET will be turned off. Then, the VOUT will be
clamped to be near VOVP. As the output voltage is higher
than a specified value or input voltage is lower than a
specified value, the chip will enter protection mode to
R2
Figure 5. Application for Constant Output Voltage
DS9293-03C April 2008
www.richtek.com
9
ETC [ ETC ]
