LTC3577EUFF#PBF Linear Technology, LTC3577EUFF#PBF Datasheet - Page 36

• Manufacturer Part Number: LTC3577EUFF#PBF
• Description: IC PWR MANAGEMENT HANDHELD 44QFN
• Manufacturer: Linear Technology

Specifications of LTC3577EUFF#PBF

• Applications: Handheld/Mobile Devices
• Voltage - Supply: 4.35 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 44-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Supply: -

OPERATION
LTC3577/LTC3577-1
where DAC is the decimal value programmed into the I
“LED DAC register”. For example with I
DAC[5:0] = 000000 (0 decimal) I
DAC[5:0] = 111111 (63 decimal) I
fi nal example DAC[5:0] = 101010 is 42 decimal and equates
to I
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
36
DAC Codes
LED
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
0
1
2
3
4
5
6
7
8
9
= 2mA for I
Output Current
LED(FS)
20.0μA
23.5μA
27.0μA
30.5μA
34.0μA
37.6μA
41.1μA
44.6μA
48.1μA
56.5μA
65.0μA
73.4μA
81.9μA
90.3μA
98.7μA
107μA
116μA
136μA
156μA
177μA
197μA
217μA
237μA
258μA
278μA
327μA
376μA
424μA
473μA
522μA
571μA
620μA
= 20mA. The DAC approximates
DAC Codes
LED
LED
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
equates to 20μA, while
equates to 20mA. As a
LED(FS)
Output Current
= 20mA and
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
668μA
786μA
903μA
2
C
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
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
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
register” which allows transitions times of approximately
15ms, one-half second, one second and two seconds.
See the “I
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
register.” When both bits are 0 the LED boost converter is
in constant current (CC) mode and the I
is constant whose value is set by the DAC[5:0] bits of the
I
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.
2
C “LED DAC register.”
LED
current in a case where LED_FS is shorted to
2
C Interface” section for more information. The
2
C “LED control register.”
2
2
LED
C “LED control
C “LED control
current sink
3577fa