Technical Note

7/22

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2011.08 - Rev.C

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BD8112EFV-M

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5V voltage reference (VREG)

5V (Typ.) is generated from the VCC input voltage when the enable pin is set high. This voltage is used to power internal

circuitry, as well as the voltage source for device pins that need to be fixed to a logical HIGH. UVLO protection is integrated

into the VREG pin. The voltage regulation circuitry operates uninterrupted for output voltages higher than 4.45 V (Typ.),

but if output voltage drops to 4.3 V (Typ.) or lower, UVLO engages and turns the IC off. Connect a capacitor (Creg = 2.2µF

Typ.) to the VREG terminal for phase compensation. Operation may become unstable if Creg is not connected.

●

Constant-current LED drivers

If less than four constant-current drivers are used, unused channels should be switched off via the LEDEN pin configuration.

The truth table for these pins is shown below. If a driver output is enabled but not used (i.e. left open), the IC’s open

circuit-detection circuitry will operate. Please keep the unused pins open. The LEDEN terminals are pulled down internally

in the IC, so if left open, the IC will recognize them as logic LO. However, they should be connected directly to VREG or

fixed to a logic HI when in use.

LED EN

LED

1

2

L

ON

ON

H

ON

OFF

・

Output current setting

LED current is computed via the following equation:

(min[VDAC , 2.0V] = the smaller value of either VDAC or VISET; GAIN = set by internal circuitry.)

In applications where an external signal is used for output current control, a control voltage in the range of 0.0 to 2.0 V

can be connected on the VDAC pin to control according to the above equation. If an external control signal is not used,

connect the VDAC pin to VREG (do not leave the pin open as this may cause the IC to malfunction). Also, do not switch

individual channels on or off via the LEDEN pin while operating in PWM mode.

The following diagram illustrates the relation between ILED and GAIN.

In PWM intensity control mode, the ON/OFF state of each current driver is controlled directly by the input signal on the

PWM pin; thus, the duty ratio of the input signal on the PWM pin equals the duty ratio of the LED current. When not

controlling intensity via PWM, fix the PWM terminal to a high voltage (100%). Output light intensity is greatest at 100%

input.

ILED vs GAIN

2850

2900

2950

3000

3050

3100

3150

0

20

40

60

80

100

120

140

160

ILED[mA]

PWM

ILED(50mA/div)

PWM

ILED

PWM=150Hz Duty=50%

PWM=150Hz Duty=0.