LM3421 datasheet(27/65 Pages) TI1

27 / 65 page

OVP

nDIM

LM3421

AGND

VIN

PGND

DDRV

DAP

GATE

COMP

VIN

CSH

RCT

HSP

RPD

VCC

HSN

CIN

CBYP

RLIM

CCMP

RCSH

ROV2

ROV1

RUV2

RUV1

RHSN

RHSP

ILED

RSNS

RFS

CFS

COV

PWM

RUVH

LM3421, LM3421-Q1

LM3423, LM3423-Q1

www.ti.com

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

CIRCUIT LAYOUT

The performance of any switching regulator depends as much upon the layout of the PCB as the component

selection. Following a few simple guidelines will maximimize noise rejection and minimize the generation of EMI

within the circuit.

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 LM3421/23 buck regulator contains the input capacitor

(CIN), the recirculating diode (D1), the N-channel MosFET (Q1), and the sense resistor (RLIM). In the LM3421/23

boost regulator, the discontinuous current flows through the output capacitor (CO), D1, Q1, and RLIM. 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, 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 LM3421/23, 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 (VIN < VO)

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Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1

LM3421 Datasheet(PDF) 27 Page - Texas Instruments

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