LM3503ITL-25 National Semiconductor, LM3503ITL-25 Datasheet - Page 15

• Manufacturer Part Number: LM3503ITL-25
• Description: LM3503 Dual-Display Constant Current LED Driver with Analog Brightness Control; Package: MICRO SMD; No of Pins: 10; Qty per Container: 250; Container: Reel
• Manufacturer: National Semiconductor

Application Information
CONTINUOUS AND DISCONTINUOUS MODES OF
OPERATION
Since the LM3503 is a constant frequency pulse-width-
modulated step-up regulator, care must be taken to make
sure the maximum duty cycle specification is not violated.
The duty cycle equation depends on which mode of opera-
tion the LM3503 is in. The two operational modes of the
LM3503 are continuous conduction mode (CCM) and dis-
continuous conduction mode (DCM). Continuous conduction
mode refers to the mode of operation where during the
switching cycle, the inductor current never goes to and stays
at zero for any significant amount of time during the switch-
ing cycle. Discontinuous conduction mode refers to the
mode of operation where during the switching cycle, the
inductor current goes to and stays at zero for a significant
amount of time during the switching cycle. Figure 4 illus-
trates the threshold between CCM and DCM operation. In
Figure 4 the inductor current is right on the CCM/DCM
operational threshold. Using this as a reference, a factor can
be introduced to calculate when a particular application is in
CCM or DCM operation. R is a CCM/DCM factor we can use
to compute which mode of operation a particular application
is in. If R is ≥ 1, then the application is operating in CCM.
Conversely, if R is
The R factor inequalities are a result of the components that
PWM Dimming Duty Cycle vs. LED Current
The results are based on the 2LEDs on Main display and 2LEDs on Sub display
Duty
(%)
10
20
30
40
50
60
70
80
90
R = 787k ohms
0.78mA
1.85mA
2.88mA
3.96mA
5.05mA
6.08mA
7.13mA
8.17mA
9.24mA
200Hz
<
1, the application is operating in DCM.
R =316k ohms
12.94mA
14.83mA
16.73mA
11.03mA
1.59mA
3.46mA
5.35mA
7.24mA
9.12mA
500Hz
(Continued)
FIGURE 4. Inductor Current Waveform
R = 158kohms
12.45mA
15.03mA
17.61mA
20.20mA
22.79mA
2.23mA
4.78mA
7.33mA
9.88mA
1KHz
15
make up the R factor. From Figure 4, the R factor is equal to
the average inductor current, I
inductor ripple current, ∆i
equation can be used to compute R factor:
R=16.2k ohms
10.77mA
14.48mA
21.86mA
25.71mA
29.53mA
33.32mA
3.42mA
7.09mA
19.1mA
10KHz
R=3.16k ohms
L
15.12mA
19.06mA
22.98mA
30.83mA
34.78mA
11.25mA
. Using Figure 4, the following
3.58mA
7.41mA
26.9mA
50KHz
L
(avg), divided by half the
20128637
R=1.62k ohms
11.34mA
15.24mA
19.16mA
23.10mA
27.05mA
31.00mA
35.00mA
100kHz
3.61mA
7.48mA
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