RT9285A Datasheet, PDF (3/6 Pages) Richtek Technology Corporation – Tiny Package, High Performance, Diode Embedded White LED Driver

Function Block Diagram
VDD
EN
FB
1.0MHz OSC
Slope
Compensation
OVP
UVLO/P
-
GOOD
+
V_REF
Timer
+
Dimming
Controller
-
Current
Sense
OCP
PWM
Logic
Soft
Start/Cla
mping
Product specification
RT9285A/B
LX
VOUT
GND
Operation
Soft-Start
The Soft-Start function is made by clamping the output
voltage of error amplifier with another voltage source that
is increased slowly from zero to near V_IN in the Soft-Start
period. Therefore, the duty cycle of the PWM will be
increased from zero to maximum in this period. The soft-
start time is decided by a timer of 1.5ms. The charging
time of the inductor will be limited as the smaller duty so
that the inrush current can be reduced to an acceptable
value.
Over Voltage Protection
The Over Voltage Protection is detected by a junction
breakdown detecting circuit. Once V_OUT goes over the
detecting voltage, LX pin stops switching and the power
NMOS is turned off. Then, the V_OUT is clamped to be near
V_OVP.
LED Current Setting
The RT9285 regulates the LED current by setting the current
sense resistor (R_SET) connecting to feedback and ground.
The internal feedback reference voltage is 0.25V. The LED
current can be set from following equation easily.
I_LED (mA) = 0.25/R_SET
In order to have an accurate LED current, precision resistors
are preferred (1% is recommended). The table for R_SET
selection is shown below.
Table 1. R_SET Value Selection
I_LED (mA)
5
R_SET (Ω)
49.9
10
24.9
12
21
15
16.5
20
12.4
Digital Pulse Dimming (RT9285A)
The digital pulse dimming is implemented by checking
the low-level duration time of EN pin. As the duration time
is in the range of T_UP, the LED current will increase 1/16. If
the duration time is in the range of T_DOWN, the LED current
will decrease 1/16. The high-level duration time of EN pin
needs larger than T_DELAY to make sure the logic can be
detected correctly. As the LED current is set to MAX (16/
16), it will keep MAX until the decreasing signal is
detected. On the other hand, as the LED current is set to
MIN (1/16), it will keep MIN until the increasing signal is
detected. When the chip turns on, the initial state of LED
current is MAX.
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