RT9284A-20GJ6E Richtek USA Inc, RT9284A-20GJ6E Datasheet - Page 10

• Manufacturer Part Number: RT9284A-20GJ6E
• Description: IC LED DVR WHITE BCKLGT TSOT23-6
• Manufacturer: Richtek USA Inc

Specifications of RT9284A-20GJ6E

• Constant Current: Yes
• Internal Driver: Yes
• Type - Primary: Backlight, Flash/Torch
• Type - Secondary: White LED
• Mounting Type: Surface Mount
• Topology: PWM
• Number Of Outputs: 1
• Frequency: 900kHz ~ 1.5MHz
• Voltage - Supply: 2.7 V ~ 5.5 V
• Package / Case: SOT-23-6 Thin, TSOT-23-6
• Operating Temperature: -40°C ~ 85°C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -

RT9284A/B
Application Information
Power Sequence
The RT9284A/B must take notice of the power sequence.
The power sequence of RT9284A/B has to VDD early than
EN or else the RT9284A/B maybe fall into the unknown
state to result in RT9284A/B turn off.
LED Current Control
The RT9284A/B regulates the LED current by setting the
current sense resistor (R1) connecting to feedback and
ground. As shown in Table 1, the RT9284A feedback
voltage (V
(V
resistor R1. In order to have an accurate LED current, a
precision resistor is preferred (1% is recommended).
Table 1
Dimming Control
a. Using a PWM Signal to EN Pin
For controlling the LED brightness, the RT9284A/B can
perform the dimming control by applying a PWM signal
to EN pin. The internal soft start and the wide range
dimming frequency from100 to100kHz can eliminate inrush
current and audio noise when dimming. The average LED
current is proportional to the PWM signal duty cycle. The
magnitude of the PWM signal should be higher than the
maximum enable voltage of EN pin, in order to let the
dimming control perform correctly.
10
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RT9284A
RT9284B
Version
FB
V
V
OUT
) is 0.25V. The LED current (I
EN
DD
FB
EN < 0.4V
Feedback Reference
) is 1.235V and the RT9284B feedback voltage
Voltage V
1.235
0.25
Figure 5
FB
(V)
V
EN > 1.5V
DD
> 2.7V
LED
Setting I
I
LED
I
) can be set by a
LED Current
LED
= 1.235/R1
= 0.25/R1
LED
(A)
b. Using a DC Voltage
Using a variable DC voltage to adjust the brightness is a
popular method in some applications. The dimming control
using a DC voltage circuit is shown in Figure 6. According
to the Superposition Theorem, as the DC voltage
increases, the voltage contributed to V
the voltage drop on R2 decreases, i.e. the LED current
decreases. For example, if the V
2.8V, the selection of resistors in Figure 6 sets dimming
control of LED current from 20mA to 0mA.
The LED current can be calculated by the following
equation.
c. Using a Filtered PWM signal:
Another common application is using a filtered PWM
signal as an adjustable DC voltage for LED dimming control.
A filtered PWM signal acts as the DC voltage to regulate
the output current. The recommended application circuit
is shown in the Figure 7. In this circuit, the output ripple
depends on the frequency of PWM signal. For smaller
output voltage ripple (<100mV), the recommended
frequency of 2.8V PWM signal should be above 2kHz. To
fix the frequency of PWM signal and change the duty
cycle of PWM signal can get different output current.
Figure 8. shows the relationship between LED current and
PWM duty cycle.
2.7V to 5.5V
V
I
LED
EN
Figure 6. Dimming Control Using a DC Voltage for
< 0.4V
=
V
V
EN
V
IN
FB
> 1.5V
C1
−
R2 (V
6
4
×
2
4.7uH to 22uH
R1
VDD
EN
GND
RT9284A
DC
R3
L1
RT9284A
−
OVP
V
FB
LX
FB
DS9284A/B-06 June 2009
1
5
3
)
V
SS0520
DC
0V to 2.8V
DC
D1
Dimming
range is from 0V to
13k
R3
FB
increases and
10k
C2
R2
110
R1
I
LED