LTC3577-4_15 Datasheet, PDF (34/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
C1
22μF
V_OUT ³⁹
LTC3577-3/
LTC3577-4
18
SW
SW
19
20
SW
L1
10μH
LPS4018-103ML
D12
ZLLS400
BOOST
R1
LED_OV ^{9 10M}
R2
3 ^20k
I_{LED_FS}
C2
1μF
C3
22nF
D1
D2
D3
D4
D5
50V
50V
22
I_LED
D10 D9 D8 D7 D6
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Figure 11. LED Boost Application Circuit
The boost converter will be operated in either continuous
conduction mode, discontinuous conduction mode or
pulse-skipping mode depending on the inductor current
required for regulation.
LED Constant Current Sink
The LED driver uses a precision current sink to regulate the
LED current up to 20mA. The current sink is programmed
via I²C “LED DAC Register” and utilizes a 6-bit 60dB expo-
nential DAC. This DAC provides accurate current control
from 20μA to 20mA with approximately 1dB per step for
I_LED(FS) = 20mA. The LED current can be approximated
by the following equations:
I_LED
=
I_LED(FS)
•
⎛
10^⎝⎜
3
•
DAC –
63
63⎞
⎠⎟
(1)
I_LED(FS)
=
0.8V
R2
•
500
where DAC is the decimal value programmed into the I²C
“LED DAC register”. For example with I_LED(FS) = 20mA
and DAC[5:0] = 000000 (0 decimal) I_LED equates to 20μA,
while DAC[5:0] = 111111 (63 decimal) I_LED equates to
20mA. As a final example DAC[5:0] = 101010 is 42 decimal
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.
34
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.
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