MIC4802 Datasheet, PDF (7/12 Pages) Micrel Semiconductor – High Efficiency 800mA Single Channel Linear WLED Driver with Ultra Fast PWM™ Control

Micrel Inc.
VIN
The input supply (V_IN) provides power to the linear LED
drivers and the control circuitry. The V_IN operating range
is 3V to 5.5V. A minimum bypass capacitor of 1µF
should be placed close to the input (VIN) pin and the
ground (GND) pin. Refer to the layout recommendations
section for details on placing the input capacitor (C1).
EN
The EN pin is equivalent to the enable pin for the linear
drivers on the MIC4802. It can also be used for dimming
applying a PWM signal. See the PWM Dimming
Interface in the Application Information section for
details. Pulling the EN low for more than 40ms puts the
MIC4802 into a low I_Q sleep mode. The EN pin cannot be
left floating; a floating enable pin may cause an
indeterminate state on the outputs. The first pulse on the EN
pin must be equal or greater than 60µs to wake the part up
in a known state. This equates to an 8.3kHz PWM signal at
equal or greater than 50% duty cycle. Higher PWM
frequencies may be used but the first pulse must be equal or
greater than 60µs.
RSET
The R_SET pin is used to set the peak current of the linear
driver by connecting a R_SET resistor to ground. The
theoretical average LED current can be estimated by
equation (1):
I_LED (mA) = 4920 * D / R_SET (kΩ)
(1)
R_SET (kΩ) = 4920 * D / I_LED (mA)
(2)
D is the duty cycle of the LED current during PWM
dimming. When the device is fully ON the duty cycle
equals 100% (D = 1). A plot of I_LED versus R_SET is shown
in Figure 2.
Due to DC losses across current paths internal and
external to the package, the calculated R_SET resistance
equation is modified by a factor K, where K is calculated
to be 0.280kΩ.
R_SET (kΩ) = 4920 * D / I_LED (mA) + 0.280 (kΩ)
(3)
I_LED (mA) = 4920 * D / ((R_SET (kΩ) – 0.280 (kΩ))
(4)
The modified LED current equation is more accurate in
determining the actual LED current based on the RSET
resistor value.
Peak LED Current
vs. R_SET
1000
MIC4802
100
10
1
1
10
100
1000 10000
RSET (kΩ)
Figure 2. Peak LED Current vs. R_SET
D1
The D1 pins are the linear driver inputs for the LED.
Connect the anode of the LED to VIN and the cathode to
the D1 pins. All the D1 pins must be connected together.
The D1 voltage at dropout is the minimum voltage
required by the linear driver in order for the LED to be
fully biased.
GND
The ground pin is the ground path for the linear driver.
The ground of the input capacitor should be routed with
low impedance traces to the GND pin and made as short
as possible. Refer to the layout recommendations for
more details.
January 2011
7
M9999-013111-B