LM3502SQ-35/NOPB National Semiconductor, LM3502SQ-35/NOPB Datasheet - Page 14

• Manufacturer Part Number: LM3502SQ-35/NOPB
• Description: IC LED DRIVR WHITE BCKLGT 16-LLP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Backlight, White LEDr

Specifications of LM3502SQ-35/NOPB

• Topology: PWM, Step-Up (Boost)
• Number Of Outputs: 2
• Internal Driver: Yes
• Type - Primary: Backlight
• Type - Secondary: White LED
• Frequency: 800kHz ~ 1.2MHz
• Voltage - Supply: 2.5 V ~ 5.5 V
• Voltage - Output: 33 V ~ 34 V
• Mounting Type: Surface Mount
• Package / Case: 16-LLP
• Operating Temperature: -40°C ~ 85°C
• Current - Output / Channel: 750mA
• Internal Switch(s): Yes
• Efficiency: 80%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM3502SQ-35; LM3502SQ-35TR

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Application Information
CONTINUOUS AND DISCONTINUOUS MODES OF
OPERATION
Since the LM3502 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 LM3502 is in. The two operational modes of the
LM3502 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 6 illus-
trates the threshold between CCM and DCM operation. In
Figure 6, 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
make up the R factor. From Figure 6, the R factor is equal to
the average inductor current, I
inductor ripple current, ∆i
equation can be used to compute R factor:
<
1, the application is operating in DCM.
L
. Using Figure 6 the following
L
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(avg), divided by half the
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(Continued)
FIGURE 6. Inductor Current Waveform
14
V
V
Eff:
Fs:
I
L:
D:
∆i
I
For CCM operation, the duty cycle can be computed with:
D:
V
V
For DCM operation, the duty cycle can be computed with:
OUT
L
IN
OUT
OUT
IN
(avg): Average Inductor Current
L
:
:
:
:
:
: Output Voltage.
Duty Cycle for CCM Operation.
Input Voltage.
Input Voltage.
Output Voltage.
Efficiency of the LM3502.
Switching Frequency.
White LED Current/Load Current.
Inductance Magnitude/Inductor Value.
Duty Cycle for CCM Operation.
Inductor Ripple Current
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