LM3421 datasheet(63/72 Pages) TI1

63 / 72 page

LM3421, LM3421-Q1, LM3423, LM3423-Q1

www.ti.com

SNVS574F – JULY 2008 – REVISED JULY 2015

9 Power Supply Recommendations

9.1 General Recommendations

The device is designed to operate from an input voltage supply range from 4.5 V to 75 V. This input supply

should be well regulated. If the input supply is located more than a few inches from the EVM or PCB, additional

bulk capacitance may be required in addition to the ceramic bypass capacitors.

9.2 Input Supply Current Limit

It is important to set the output current limit of your input supply to an appropriate value to avoid delays in your

converter analysis and optimization. If not set high enough, current limit can be tripped during start-up or when

your converter output power is increased, causing a foldback or shutdown condition. It is a common oversight

when powering up a converter for the first time.

10 Layout

10.1 Layout Guidelines

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 maximize 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 LM34xx-Q1 buck regulator contains the input capacitor

(CIN), the recirculating diode (D1), the N-channel MOSFET (Q1), and the sense resistor (RLIM). In the LM34xx-Q1

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 LM34xx-Q1, 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.

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

LM3421 Datasheet(PDF) 63 Page - Texas Instruments