RT8567A Datasheet, PDF (11/13 Pages) Richtek Technology Corporation – 6-CH 43V Constant Current LED Driver with PWM Dimming Control

RT8567A
output voltage can be clamped at a certain voltage level.
This voltage level can be calculated by the following
equation :
V_{OUT, OVP}
=
V_OVP
×
(1+
R_OVP2
R_OVP1
)
where R_OVP1 and R_OVP2 are the resistors in the voltage
divider connected to the OVP pin. If at least one string is
in normal operation, the controller will automatically ignore
the open strings and continue to regulate the current for
the string(s) in normal operation.
Current Limit Protection
The RT8567A can limit the peak current to achieve over
current protection. The RT8567A senses the inductor
current through LX pin in the ON period. The duty cycle
depends on the current sense signal summed with the
internal slope compensation and compared to the COMP
signal. The internal N-MOSFET will be turned off when
the current signal is larger than the COMP signal. In the
OFF period, the inductor current will descend. The internal
MOSFET is turned on by the oscillator in the next starting
cycle.
Over Temperature Protection
The RT8567A has an over temperature protection (OTP)
function to prevent excessive power dissipation from
overheating the device. The OTP will shut down switching
operation when the junction temperature exceeds 150°C.
Inductor Selection
The value of the output inductor (L), where the transition
from discontinuous to continuous mode occurs is
approximated by the following equation :
L=
(V_OUT − V_IN )× V_IN2
2×I_OUT × f × V_OUT2
where,
V_OUT = maximum output voltage.
V_IN = minimum input voltage.
f = operating frequency.
I_OUT = sum of current from all LED strings.
η is the efficiency of the power converter.
The Boost converter operates in discontinuous mode over
the entire input voltage range when the L1 inductor value
is less than this value L. With an inductance greater than
L, the converter operates in continuous mode at the
minimum input voltage and may be discontinuous at higher
voltages.
The selected inductor must be selected with saturation
current rating greater than the peak current provided by
the following equation :
I_PEAK
=
( ^{VOUT ×IOUT}
η x V_IN
)
+
(
V_IN × T _{) × (}
2×L
V_OUT − V_{IN )}
V_OUT
Diode Selection
Schottky diode is a good choice for any asynchronous
Boost converter due to its small forward voltage and fast
switching Speed. 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. Choose a suitable diode with
reverse voltage rating greater than the maximum output
voltage.
Capacitor Selection
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most applications, a 10μF ceramic capacitor is sufficient.
A value higher or lower may be used depending on the
noise level from the input supply and the input current to
the converter. It is recommended to choose a ceramic
capacitor based on the output voltage ripple requirements.
The minimum value of the output capacitor C_OUT, can be
calulated by the following equation :
C_OUT
=
(V_OUT − V_IN )×I_OUT
η × V_RIPPLE × V_OUT × f
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
Copyright ©2012 Richtek Technology Corporation. All rights reserved.
DS8567A-00 April 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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