LTC3577-3 Datasheet, PDF (35/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
and equates to I_LED = 2mA for I_LED(FS) = 20mA. The DAC
approximates 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 segments that approximate the exponential
function.
Table 5. LED DAC Codes to Output Current
DAC Codes Output Current DAC Codes
0
20.0μA
32
1
23.5μA
33
2
27.0μA
34
3
30.5μA
35
4
34.0μA
36
5
37.6μA
37
6
41.1μA
38
7
44.6μA
39
8
48.1μA
40
9
56.5μA
41
10
65.0μA
42
11
73.4μA
43
12
81.9μA
44
13
90.3μA
45
14
98.7μA
46
15
107μA
47
16
116μA
48
17
136μA
49
18
156μA
50
19
177μA
51
20
197μA
52
21
217μA
53
22
237μA
54
23
258μA
55
24
278μA
56
25
327μA
57
26
376μA
58
27
424μA
59
28
473μA
60
29
522μA
61
30
571μA
62
31
620μA
63
Output Current
668μA
786μA
903μA
1.02mA
1.14mA
1.26mA
1.37mA
1.49mA
1.61mA
1.89mA
2.17mA
2.45mA
2.74mA
3.02mA
3.30mA
3.58mA
3.86mA
4.54mA
5.22mA
5.90mA
6.58mA
7.26mA
7.93mA
8.61mA
9.29mA
10.8mA
12.4mA
13.9mA
15.4mA
17.0mA
18.5mA
20.0mA
The full-scale LED current is set using a resistor (R2 in
Figure 11) 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 I²C “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 I²C “LED control
register” which allows transitions times of approximately
15ms, one-half second, one second and two seconds.
See the “I²C 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 I²C “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
I²C“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 reduce the LED intensity for a given programmed
current.
When dimming via PWM the LED driver and boost converter
are both turned on and off together. This allows some
degree of additional control over the LED current, and in
some cases may offer a more efficient method of dimming
since the boost could be operated at an optimal efficiency
point and pulsed for the desired LED intensity.
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