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LM3445
www.ti.com
SNVS570L – JANUARY 2009 – REVISED MAY 2013
t OFF =
Finally:
1
1 V LED
K u V BUCK
f SW
(21)
K u V BUCK
L2 =
V LED 1
f SW x ' i
1 V LED
(22)
Refer to DESIGN EXAMPLE to better understand the design process.
SETTING THE LED CURRENT
The LM3445 constant off-time control loop regulates the peak inductor current (I L2 ). The average inductor current
equals the average LED current (I AVE ). Therefore the average LED current is regulated by regulating the peak
inductor current.
I L2-PK
' i L
I AVE
I L2-MIN
I L2 (t)
t ON
t OFF
t
Figure 28. Inductor Current Waveform in CCM
Knowing the desired average LED current, I AVE and the nominal inductor current ripple, Δ i L , the peak current for
an application running in continuous conduction mode (CCM) is defined as follows:
I L2-PK = I AVE + ' 2 i L
Or, the maximum, or "undimmed", LED current would then be,
I AVE(UNDIM) = I L2-PK(UNDIM) - ' 2 i L
(23)
(24)
This is important to calculate because this peak current multiplied by the sense resistor R3 will determine when
the internal comparator is tripped. The internal comparator turns the control MOSFET off once the peak sensed
voltage reaches 750 mV.
I L-PK(UNDIM) =
750 mV
R3
(25)
Current Limit : Under normal circumstances, the trip voltage on the PWM comparator would be less than or
equal to 750 mV, depending on the amount of dimming. However, if there is a short circuit or an excessive load
on the output, higher than normal switch currents will cause a voltage above 1.27V on the ISNS pin which will
trip the I-LIM comparator. The I-LIM comparator will reset the RS latch, turning off Q2. It will also inhibit the Start
Pulse Generator and the COFF comparator by holding the COFF pin low. A delay circuit will prevent the start of
another cycle for 180 μs.
VALLEY FILL CAPACITORS
Determining voltage rating and capacitance value of the valley-fill capacitors:
The maximum voltage seen by the valley-fill capacitors is:
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