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LM3421 Datasheet(PDF) 36 Page - Texas Instruments

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Part # LM3421
Description  N-Channel Controllers for Constant Current LED Drivers
PDF  65 Pages
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

LM3421 Datasheet(HTML) 36 Page - Texas Instruments

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background image
(
)
R
R
+
x
2
UV
1
UV
A
23 P
(
)
x
HYS
A
23
V
x
P
-
R
2
UV
R
1
UV
UVH
R
=
R
UV1 =
1.24V
V
ON
TURN
-
-
R
1.24V
UV2
x
R
UV2 =
A
23 P
V
HYS
R
OV1 =
R
1.24V
OV2
x
V
m
620
V
OFF
TURN
-
-
R
OV1 =
1.24V
V
OFF
TURN
-
-
R
1.24V
OV2
x
R
OV2 =
V
HYSO
A
23 P
FD
D
D
V
I
P
x
=
ID-MAX = ILED
LM3421, LM3421-Q1
LM3423, LM3423-Q1
SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011
www.ti.com
Boost and Buck-boost
(87)
Replace DMAX with D in the ID-MAX equation to solve for the average diode current (ID). Given a diode with forward
voltage (VFD), solve for the nominal power dissipation (PD):
(88)
11. OUTPUT OVLO
For boost and buck-boost regulators, output OVLO is programmed with the turn-off threshold voltage (VTURN-OFF)
and the desired hysteresis (VHYSO). To set VHYSO, solve for ROV2:
(89)
To set VTURN-OFF, solve for ROV1:
Boost
(90)
Buck-boost
(91)
A small filter capacitor (COVP = 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 (VTURN-ON) and the desired
hysteresis (VHYS).
Method #1: If no PWM dimming is required, a two resistor network can be used. To set VHYS, solve for RUV2:
(92)
To set VTURN-ON, solve for RUV1:
(93)
Method #2: If PWM dimming is required, a three resistor network is suggested. To set VTURN-ON, assume RUV2 =
10 k
Ω and solve for RUV1 as in Method #1. To set VHYS, solve for RUVH:
(94)
13. PWM DIMMING METHOD
PWM dimming can be performed several ways:
Method #1: Connect the dimming MosFET (Q3) with the drain to the nDIM pin and the source to AGND. Apply
an external PWM signal to the gate of QDIM. A pull down resistor may be necessary to properly turn off Q3.
Method #2: Connect the anode of a Schottky diode to the nDIM pin. Apply an external inverted PWM signal to
the cathode of the same diode.
36
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Copyright © 2008–2011, Texas Instruments Incorporated
Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1



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