LM3409MY-NOPB Datasheet, PDF (15/43 Pages) Texas Instruments – PFET Buck Controller for High Power LED Drivers

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LM3409/09HV
22 #A
ON/OFF
UVLO
1.24V
LM3409, LM3409HV, LM3409-Q1
SNVS602J – MARCH 2009 – REVISED MAY 2013
V_IN
R_UV2
R_UV1
Figure 28. UVLO Circuit
The turn-on threshold (V_TURN-ON) is defined as follows:
( ) _VTURN-ON
=
1.24V x
R_UV1 + R_UV2
R_UV1
(16)
The hysteresis (V_HYS) is defined as follows:
V_HYS = R_UV2 x 22 PA
(17)
LOW POWER SHUTDOWN
The LM3409/09HV can be placed into a low power shutdown (typically 110µA) by grounding the EN terminal
(any voltage below 0.5V) until V_CC drops below the V_CC UVLO threshold (typically 3.73V). During normal
operation this terminal should be tied to a voltage above 1.74V and below absolute maximum input voltage
rating.
THERMAL SHUTDOWN
Internal thermal shutdown circuitry is provided to protect the IC in the event that the maximum junction
temperature is exceeded. The threshold for thermal shutdown is 160°C with 15°C of hysteresis (both values
typical). During thermal shutdown the PFET and driver are disabled.
Design Considerations
OPERATION NEAR DROPOUT
Because the power MosFET is a PFET, the LM3409/09HV can be operated into dropout which occurs when the
input voltage is approximately equal to output voltage. Once the input voltage drops below the nominal output
voltage, the switch remains constantly on (D=1) causing the output voltage to decrease with the input voltage. In
normal operation, the average LED current is regulated to the peak current threshold minus half of the ripple. As
the converter goes into dropout, the LED current is exactly at the peak current threshold because it is no longer
switching. This causes the LED current to increase by half of the set ripple current as it makes the transition into
dropout. Therefore, the inductor current ripple should be kept as small as possible (while remaining above the
previously established minimum) and output capacitance should be added to help maintain good line regulation
when approaching dropout.
LED RIPPLE CURRENT
Selection of the ripple current through the LED array is analogous to the selection of output ripple voltage in a
standard voltage regulator. Where the output voltage ripple in a voltage regulator is commonly ±1% to ±5% of the
DC output voltage, LED manufacturers generally recommend values for Δi_LED-PP ranging from ±5% to ±20% of
I_LED. For a nominal system operating point, a larger Δi_LED-PP specification can reduce the necessary inductor size
and/or allow for smaller output capacitors (or no output capacitors at all) which helps to minimize the total
solution size and cost. On the other hand, a smaller Δi_LED-PP specification would require more output inductance,
a higher switching frequency, or additional output capacitance.
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