Technical Note

6/20

www.rohm.com

2010.08 - Rev.A

© 2010 ROHM Co., Ltd. All rights reserved.

BD8119FM-M

●

5V voltage reference (VREG)

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 4.5 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〉

1

2

3

4

L

L

ON

ON

ON

ON

H

L

ON

ON

ON

OFF

L

H

ON

ON

OFF

OFF

H

H

ON

OFF

OFF

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 0.1 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

2950

3000

3050

3100

3150

3200

3250

3300

3350

0

20

40

60

80

100

120

140

160

ILED[mA]

ILED[mA]

GAIN

10

3215

20

3080

30

3030

40

2995

50

3000

60

3020

70

3040

80

3070

90

3105

100

3140

110

3175

120

3210

130

3245

140

3280

150

3330

PWM

ILED(50mA/div)

PWM

ILED

PWM=150Hz Duty=50%

PWM=150Hz Duty=0.38%