LM3445MM/NOPB National Semiconductor, LM3445MM/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM3445MM/NOPB
• Description: IC LED DRIVER TRIAC DIMM 10-MSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Triac Dimmabler

Specifications of LM3445MM/NOPB

• Mfg Application Notes: LM3445 Reference Design
• Constant Current: Yes
• Topology: PWM, Step-Down (Buck)
• Number Of Outputs: 1
• Internal Driver: No
• Type - Secondary: High Brightness LED (HBLED)
• Frequency: 30kHz ~ 1MHz
• Voltage - Supply: 8 V ~ 12 V
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Operating Temperature: -40°C ~ 125°C
• Current - Output / Channel: 1A
• Internal Switch(s): Yes
• Efficiency: 85%
• For Use With: LM3445-220VEVAL - BOARD EVAL LM3445 220VLM3445-120VSMEV - BOARD EVAL LM3445 110VLM3445TRIACEVAL - BOARD EVAL LM3445MM
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Other names: LM3445MMTR

OVERVIEW OF CONSTANT OFF-TIME CONTROL
A buck converter’s conversion ratio is defined as:
Constant off-time control architecture operates by simply
defining the off-time and allowing the on-time, and therefore
the switching frequency, to vary as either V
The output voltage is equal to the LED string voltage (V
and should not change significantly for a given application.
The input voltage or V
input line varies. The length of the on-time is determined by
the sensed inductor current through a resistor to a voltage
reference at a comparator. During the on-time, denoted by
t
increase. During the on-time, current flows from V
through the LEDs, through L2, Q2, and finally through R3 to
ground. At some point in time, the inductor current reaches a
maximum (I
the ISNS pin. This sensed voltage across R3 is compared
ON
, MOSFET switch Q2 is on causing the inductor current to
L2-PK
) determined by the voltage sensed at R3 and
BUCK
in this analysis will vary as the
FIGURE 11. LM3445 Buck Regulation Circuit
IN
or V
O
changes.
BUCK
LED
),
,
13
against the voltage of dim decoder output, FLTR2, at which
point Q2 is turned off by the controller.
During the off-period denoted by t
continues to flow through the LEDs via D10.
FIGURE 12. Inductor Current Waveform in CCM
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OFF
, the current through L2
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