Datasheet
SS
TGAIN
OVP
LM3424
nDIM
V
IN
GND
TSENSE
TREF
DDRV
VS
C
OUT
DAP
GATE
EN
COMP
VIN
CSH
RT/SYNC
IS
HSN
SLOPE
VCC
HSP
Q2
D1
L1
C
IN
R
SLP
C
BYP
R
LIM
Q1
C
CMP
R
CSH
R
T
C
SS
R
OV2
R
OV1
R
UV2
R
UV1
R
GAIN
NTC
R
REF1
R
REF2
R
BIAS
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
R
HSP
R
HSN
C
NTC
I
LED
R
SNS
R
FS
C
FS
C
OV
C
REF
Q3
PWM
R
UVH
LM3424
SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
www.ti.com
Discontinuous currents are the most likely to generate EMI, therefore care should be taken when routing these
paths. The main path for discontinuous current in the LM3424 buck regulator contains the input capacitor (C
IN
),
the recirculating diode (D1), the N-channel MosFET (Q1), and the sense resistor (R
LIM
). In the LM3424 boost
regulator, the discontinuous current flows through the output capacitor (C
O
), D1, Q1, and R
LIM
. In the buck-boost
regulator both loops are discontinuous and should be carefully layed out. These loops should be kept as small as
possible and the connections between all the components should be short and thick to minimize parasitic
inductance. In particular, the switch node (where L1, D1 and Q1 connect) should be just large enough to connect
the components. To minimize excessive heating, large copper pours can be placed adjacent to the short current
path of the switch node.
The RT, COMP, CSH, IS, TSENSE, TREF, HSP and HSN pins are all high-impedance inputs which couple
external noise easily, therefore the loops containing these nodes should be minimized whenever possible.
In some applications the LED or LED array can be far away (several inches or more) from the LM3424, or on a
separate PCB connected by a wiring harness. When an output capacitor is used and the LED array is large or
separated from the rest of the regulator, the output capacitor should be placed close to the LEDs to reduce the
effects of parasitic inductance on the AC impedance of the capacitor.
Basic Topology Schematics
BOOST REGULATOR (V
IN
< V
O
)
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