Datasheet
LTC3577/LTC3577-1
36
3577fa
OPERATION
where DAC is the decimal value programmed into the I
2
C
“LED DAC register”. For example with I
LED(FS)
= 20mA and
DAC[5:0] = 000000 (0 decimal) I
LED
equates to 20A, while
DAC[5:0] = 111111 (63 decimal) I
LED
equates to 20mA. As a
fi nal example DAC[5:0] = 101010 is 42 decimal and equates
to I
LED
= 2mA for I
LED(FS)
= 20mA. The DAC approximates
the Equation 1 using the nominal values in Table 5. The
differences between the approximation equation and the
table are due to design of the DAC using eight linear seg-
ments that approximate the exponential function.
Table 5. LED DAC Codes to Output Current
DAC Codes Output Current DAC Codes Output Current
0 20.0µA 32 668µA
1 23.5µA 33 786µA
2 27.0µA 34 903µA
3 30.5µA 35 1.02mA
4 34.0µA 36 1.14mA
5 37.6µA 37 1.26mA
6 41.1µA 38 1.37mA
7 44.6µA 39 1.49mA
8 48.1µA 40 1.61mA
9 56.5µA 41 1.89mA
10 65.0µA 42 2.17mA
11 73.4µA 43 2.45mA
12 81.9µA 44 2.74mA
13 90.3µA 45 3.02mA
14 98.7µA 46 3.30mA
15 107µA 47 3.58mA
16 116µA 48 3.86mA
17 136µA 49 4.54mA
18 156µA 50 5.22mA
19 177µA 51 5.90mA
20 197µA 52 6.58mA
21 217µA 53 7.26mA
22 237µA 54 7.93mA
23 258µA 55 8.61mA
24 278µA 56 9.29mA
25 327µA 57 10.8mA
26 376µA 58 12.4mA
27 424µA 59 13.9mA
28 473µA 60 15.4mA
29 522µA 61 17.0mA
30 571µA 62 18.5mA
31 620µA 63 20.0mA
The full-scale LED current is set using a resistor (R2 in
Figure 16) connected between the LED_FS pin and ground.
Typically R2 should be set to 20k to give 20mA of LED
current at full-scale. The resistance may be increased to
decrease the current or the resistance may be decreased
to increase the LED current. The DAC has been optimized
for best performance at 20mA full-scale. The full-scale
current may be adjusted but the accuracy of the output
current will be degraded the further it is programmed
from 20mA. The LED_FS pin is current limited and will
only source about 80µA. This protects the pin and limits
the I
LED
current in a case where LED_FS is shorted to
ground, it is not recommended to program the LED cur-
rent above 25mA.
LED Gradation
The LED driver features an automatic gradation circuit.
The gradation circuit ramps the LED current up when
the LED driver is enabled and ramps the current down
when the LED driver is disabled. The DAC is enabled and
disabled with the EN bit of the I
2
C “LED control register.”
The gradation function is automatic when enabling and
disabling the LED driver; only the gradation speed needs to
be programmed to use this function. The gradation speed
is set by the GR1 and GR2 bits of the I
2
C “LED control
register” which allows transitions times of approximately
15ms, one-half second, one second and two seconds.
See the “I
2
C Interface” section for more information. The
gradation function allows the LEDs to turn on and off
gradually as opposed to an abrupt step.
LED PWM vs Constant Current Operation
The LED driver provides both linear LED current mode as
well as PWM LED current mode. These modes are selected
through the MD1 and MD2 bits of the I
2
C “LED control
register.” When both bits are 0 the LED boost converter is
in constant current (CC) mode and the I
LED
current sink
is constant whose value is set by the DAC[5:0] bits of the
I
2
C “LED DAC register.”
Setting MD1 to 0 and MD2 to 1 selects the LED PWM mode.
In this mode the LED driver is pulsed using an internally
generated PWM signal. The PWM mode may be used to re-
duce the LED intensity for a given programmed current.