TLC5960
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SBVS147 –SEPTEMBER 2010
CONSTANT CURRENT CONTROL: PWM AND ANALOG DIMMING
The dedicated FET gate drivers help regulate the constant currents on the LED strings, as shown in Figure 15. A
sense resistor (RS) sets the constant current value on the corresponding LED channel. The gate driver regulates
this source node (Sn) voltage to be the same as the internal reference voltage (VREF). The default setting of VREF
is 0.6V. This internal reference voltage can be configured through the external input, VADJ. This function can be
used as a linear or analog dimming function for all LED strings simultaneously. As shown in Figure 16, the
TLC5960 uses halved VADJ input as the internal VREF. If VADJ is greater than 1.4V, the internal VREF falls back
to the typical 0.6V (100% analog dimming). Here, 1.4V is a positive-going threshold and is designed to have
100mV negative-going hysteresis as a noise margin. The VREF fallback mechanism is also applied in case VADJ
= open. The analog dimming linearity range is specified from 10% to 100%.
D1
VREG5
G1
On/Off
Control
S1
RS
VREF
VADJ
0.6V TYP
Channel 1
Figure 15. Constant Current Drive Amplifier
0.7
0.6
10% to 100%
Dimming
Area
0.06
0
0.12
1.2
1.4
VADJ (V)
Figure 16. VADJ vs Internal Reference Voltage
The constant current value in the LED strings is set using the following equations:
VADJ/2
If VADJ < 1.4V, then ILED
=
RS
0.6V
(1)
(2)
If VADJ > 1.4V, then ILED
=
RS
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