RT8511B Datasheet, PDF (9/11 Pages) Richtek Technology Corporation – 43V Asynchronous Boost WLED Driver

RT8511B
Over Voltage Protection
The RT8511B equips Over Voltage Protection (OVP)
function. When the voltage at the OVP pin reaches a
threshold of approximately 1.2V, the MOSFET drive output
will turn off. The MOSFET drive output will turn on again
once the voltage at the OVP pin drops below the threshold.
Thus, the output voltage can be clamped at a certain
voltage level, as shown in the following equation :
V_{OUT, OVP}
=
V_OVP

₁₊
R2
R1

where R1 and R2 make up the voltage divider connected
to the OVP pin.
Over Temperature Protection
The RT8511B has an Over Temperature Protection (OTP)
function to prevent overheating caused by excessive power
dissipation from overheating the device. The OTP will shut
down switching operation if the junction temperature
exceeds 160°C. The boost converter will start switching
again when the junction temperature is cooled down by
approximately 30°C.
Inductor Selection
The inductance depends on the maximum input current.
As a general rule, the inductor ripple current range is 20%
to 40% of the maximum input current. If 40% is selected
as an example, the inductor ripple current can be
calculated according to the following equation :
I_IN(MAX)
=
V_OUT I_OUT
_(MIN)  V_IN(MIN)
I_RIPPLE = 0.4 I_IN(MAX)
where η is the efficiency of the boost converter, I_IN(MAX) is
the maximum input current, I_OUT is the total current from
all LED strings, and I_RIPPLE is the inductor ripple current.
The input peak current can be calculated by maximum
input current plus half of inductor ripple current shown as
following equation :
I_PEAK = 1.2 x I_IN(MAX)
Note that the saturated current of the inductor must be
greater than I_PEAK. The inductance can eventually be
determined according to the following equation :
L=
   V_IN ²  (V_OUT  V_IN )
0.4   V_OUT ² I_OUT  f_OSC
Copyright ©2015 Richtek Technology Corporation. All rights reserved.
DS8511B-05 February 2015
where f_OSC is the switching frequency. For better efficiency,
it is suggested to choose an inductor with small series
resistance.
Diode Selection
The Schottky diode is a good choice for an asynchronous
boost converter due to its small forward voltage. However,
when selecting a Schottky diode, important parameters
such as power dissipation, reverse voltage rating, and
pulsating peak current must all be taken into
consideration. A suitable Schottky diode's reverse voltage
rating must be greater than the maximum output voltage,
and its average current rating must exceed the average
output current.
Capacitor Selection
Two 1μF ceramic input capacitors and two 1μF ceramic
output capacitors are recommended for driving 10 WLEDs
in series. For better voltage filtering, ceramic capacitors
with low ESR are recommended. Note that the X5R and
X7R types are suitable because of their wide voltage and
temperature ranges.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
P_D(MAX) = (T_J(MAX) − T_A) / θ_JA
where T_J(MAX) is the maximum junction temperature, T_A is
the ambient temperature, and θ_JA is the junction to ambient
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θ_JA, is layout dependent. For
WDFN-8L 2x2 package, the thermal resistance, θ_JA, is
120°C/W on a standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at T_A = 25°C
can be calculated by the following formulas :
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