LM3450MT/NOPB National Semiconductor, LM3450MT/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM3450MT/NOPB
• Description: IC LED DRIVER DIMMING 16-TSSOP
• Manufacturer: National Semiconductor

Specifications of LM3450MT/NOPB

• Featured Product: LM3450 LED Driver
• Internal Driver: No
• Type - Primary: General Purpose
• Type - Secondary: High Brightness LED (HBLED)
• Mounting Type: Surface Mount
• Topology: PWM
• Number Of Outputs: 1
• Voltage - Supply: 8.5 V ~ 20 V
• Voltage - Output: 11.5V
• Package / Case: 16-TSSOP (0.173", 4.40mm Width)
• Operating Temperature: -40°C ~ 125°C
• Led Driver Application: General Lighting
• No. Of Outputs: 1
• Output Current: 200mA
• Output Voltage: 11.5V
• Input Voltage: 8.5V To 20V
• Dimming Control Type: PWM / Analog
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Frequency: -

CRM BACKGROUND
During critical conduction mode (CRM), a converter operates
at the boundary of continuous conduction mode (CCM) and
discontinuous conduction mode (DCM). This is usually im-
plemented as follows. The main switching MosFET (Q
turned on and the inductor current rises to a peak threshold.
Q
zero. At this point, Q
Near zero voltage switching, enabled by the inductor current
return to zero, gives CRM topologies an efficiency improve-
ment compared to CCM topologies.
sulting inductor current waveform, where the average induc-
tor current (I
In a CRM flyback PFC application, the rectified AC input is fed
forward to the control loop, creating a sinusoidal primary peak
current envelope (I
SW
is then turned off and the current falls until it reaches
L
) is half of the peak current (I
P-pk
SW
) as shown in
is turned on and the cycle repeats.
Figure
Figure 3
FIGURE 3. Basic CRM Inductor Current Waveform
FIGURE 4. CRM Flyback Current Waveforms
L-MAX
4. The secondary
shows the re-
).
SW
) is
13
peak current envelope (I
of the primary according to the turns ratio of the transformer.
Assuming good attenuation of the switching ripple via the EMI
filter, the average input current (I
line in
portional to the duty cycle (D(t)):
Since CRM operation is hysteretic and the input voltage is fed-
forward, the input current shaping loop is as fast as possible.
Only the output voltage needs to be regulated with a narrow
bandwidth error amplifier, which greatly simplifies the system
dynamics.
Figure
4, can also be approximated as a sinusoid pro-
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S-pk
) will simply be a scaled version
IN
), represented by the red
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