RT6030 Datasheet, PDF (8/10 Pages) Richtek Technology Corporation – 4-CH LED Current Source Controller

RT6030
Applications Information
The RT6030 is a 4-CH LED current source controller. This
device can also drive an external BJT or N-MOSFET for
various applications. Refer to topology in Typical
Application Circuit for more details.
Capacitors Selection
Careful selection of the external capacitors for the RT6030
is highly recommended in order to maintain high stability
and performance. An input capacitor with minimum 1μF
must be connected between VCC and ground. The
capacitor improves the supply voltage stability for proper
operation.
Chip Enable Operation
Pull the EN pin low to drive the device into shutdown mode.
During shutdown mode, the standby current drops to
10mA(MAX). Drive the EN pin high to turn on the device
again. To control LED brightness, the RT6030 can perform
dimming function by applying a PWM signal to the EN
pin. The average LED current is proportional to the PWM
signal duty cycle. To obtain correct dimming, the
magnitude of the PWM signal should be higher than the
threshold voltage of the EN pin.
MOSFET Selection
The RT6030 is designed to drive external N-MOSFET pass
element. MOSFET selection criteria include threshold
voltage, V_GS (V_TH), maximum continuous drain current,
I_D, on resistance, R_DS(ON) ,maximum drain-to-source
voltage, V_DS, and package thermal resistance, θ_JA. The
most critical specification is the MOSFET R_DS(ON). R_DS(ON)
can be calculated from the following formula :
R_DS(ON)
=
(V_IN − V_OUT )
I_O
For example, the MOSFET operates up to 2A when the
input voltage is 1.5V and set the output voltage as 1.2V.
Then, R_DS(ON) = (1.5V − 1.2V) / 2A = 150mΩ. The
MOSFET's R_DS(ON) must be lower than 150mΩ. Philip
PHD3055E MOSFET with an R_DS(ON) of 120mΩ (typ.) is
a suitable solution.
The power dissipation is calculated as :
P_D = (V_IN − V_OUT ) x I_LOAD
The thermal resistance from junction to ambient is :
θ _JA
=
(T_J − T_A )
P_D
In this example, P_D = (1.5V−1.2V) x 2A = 0.6W. The
PHD3055E's θ_JA is 75°C/W for its D-PAK package, which
translates to a 45°C temperature rise above ambient. The
package provides exposed backsides that directly transfer
heat to the PCB board.
LED Current Setting
The RT6030 maintains an internal reference voltage of 0.8V.
As shown in Typical Application Circuit, the LED current
can be set accordingly via the Rx (x = 1, 2, 3, 4) resistor.
I_LEDx
=
0.8
R_x
(A)
NPN Transistor Selection
The RT6030 drives the external NPN transistor via the DRIx
pin (source Base current I_B). NPN transistor selection
criteria include DC current gain, h_FE, threshold voltage,
V_BE, collector emitter voltage, V_CE, maximum continuous
collector current, I_C, and package thermal resistance, θ_JA.
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 of
the RT6030, the maximum junction temperature is 125°C
and T_A is the ambient temperature. The junction to ambient
thermal resistance, θ_JA, is layout dependent. For
SOP-16 packages, the thermal resistance, θ_JA, is
95°C/W on a standard JEDEC 51-3 single-layer thermal
test board. The maximum power dissipation at T_A = 25°C
can be calculated by the following formula :
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DS6030-02 March 2011