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LM3421, LM3421-Q1
LM3423, LM3423-Q1
www.ti.com
Boost
V RD-MAX = V O
Buck-boost
V RD-MAX = V IN-MAX + V O
SNVS574E – JULY 2008 – REVISED MAY 2013
(84)
(85)
The current rating should be at least 10% higher than the maximum average diode current (I D-MAX ):
Buck
I D-MAX = (1 - D MIN ) x I LED
Boost and Buck-boost
I D-MAX = I LED
(86)
(87)
Replace D MAX with D in the I D-MAX equation to solve for the average diode current (I D ). Given a diode with forward
voltage (V FD ), solve for the nominal power dissipation (P D ):
P D = I D x V FD
(88)
11. OUTPUT OVLO
For boost and buck-boost regulators, output OVLO is programmed with the turn-off threshold voltage (V TURN-OFF )
and the desired hysteresis (V HYSO ). To set V HYSO , solve for R OV2 :
R OV2 =
V HYSO
23 P A
(89)
To set V TURN-OFF , solve for R OV1 :
Boost
R OV1 =
Buck-boost
R OV1 =
1.24V x R OV2
V TURN - OFF - 1.24V
1.24V x R OV2
V TURN - OFF - 620 m V
(90)
(91)
A small filter capacitor (C OVP = 47 pF) should be added from the OVP pin to ground to reduce coupled switching
noise.
12. INPUT UVLO
For all topologies, input UVLO is programmed with the turn-on threshold voltage (V TURN-ON ) and the desired
hysteresis (V HYS ).
Method #1: If no PWM dimming is required, a two resistor network can be used. To set V HYS , solve for R UV2 :
R UV2 =
V HYS
23 P A
(92)
To set V TURN-ON , solve for R UV1 :
R UV1 =
1.24V x R UV2
V TURN - ON - 1.24V
(93)
Copyright ? 2008–2013, Texas Instruments Incorporated
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