LM3444MM/NOPB National Semiconductor, LM3444MM/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM3444MM/NOPB
• Description: IC LED DRIVER AC-DC 10MSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3444MM/NOPB

• Constant Current: Yes
• Constant Voltage: Yes
• Internal Driver: No
• Type - Primary: General Purpose
• Type - Secondary: High Brightness LED (HBLED)
• Mounting Type: Surface Mount
• Topology: AC DC Offline Switcher, PWM, Step-Down (Buck)
• Number Of Outputs: 1
• Frequency: Adjustable/Selectable
• Voltage - Supply: 8 V ~ 13 V
• Package / Case: 10-TFSOP, 10-MSOP (0.118", 3.00mm Width)
• No. Of Outputs: 1
• Output Current: 100mA
• Operating Temperature Range: -40°C To +125°C
• Driver Case Style: MSOP
• No. Of Pins: 10
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Output: -
• Operating Temperature: -
• Other names: LM3444MM/NOPB; LM3444MM/NOPBTR

BUCK CONVERTER
The LM3444 is a buck controller that uses a proprietary con-
stant off-time method to maintain constant current through a
string of LEDs. While transistor Q2 is on, current ramps up
through the inductor and LED string. A resistor R3 senses this
current and this voltage is compared to the reference voltage
at FILTER. When this sensed voltage is equal to the reference
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
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 6. LM3444 Buck Regulation Circuit
IN
or V
O
changes.
BUCK
LED
),
,
9
voltage, transistor Q2 is turned off and diode D10 conducts
the current through the inductor and LEDs. Capacitor C12
eliminates most of the ripple current seen in the inductor. Re-
sistor R4, capacitor C11, and transistor Q3 provide a linear
current ramp that sets the constant off-time for a given output
voltage.
the ISNS pin. This sensed voltage across R3 is compared
against the voltage of FILTER, at which point Q2 is turned off
by the controller.
FIGURE 7. Inductor Current Waveform in CCM
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