Technical Note
6/20
www.rohm.com
2011.08 - Rev.B
© 2011 ROHM Co., Ltd. All rights reserved.
BD8119FM-M
●5V voltage reference (VREG)
5V (Typ.) is generated from the V
CC
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 4.5 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:
I
LED
= min[VDAC , VISET(=2.0V)] / RSET x GAIN [A]
(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]
GAIN
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%
