LM3478MASLASHNOPB datasheet(28/43 Pages) TI2

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LM3478

SNVS085X – JULY 2000 – REVISED DECEMBER 2017

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9 Power Supply Recommendations

The LM3478 is designed to operate from various DC power supply including a car battery. If so, VIN input should

be protected from reversal voltage and voltage dump over 40 volts. The impedance of the input supply rail

should be low enough that the input current transient does not cause drop below VIN UVLO level. If the input

supply is connected by using long wires, additional bulk capacitance may be required in addition to normal input

capacitor.

10 Layout

10.1 Layout Guidelines

Good board layout is critical for switching controllers. First the ground plane area must be sufficient for thermal

dissipation purposes and second, appropriate guidelines must be followed to reduce the effects of switching

noise. Switching converters are very fast switching devices. In such devices, the rapid increase of input current

combined with the parasitic trace inductance generates unwanted Ldi/dt noise spikes. The magnitude of this

noise tends to increase as the output current increases. This parasitic spike noise may turn into electromagnetic

interference (EMI), and can also cause problems in device performance. Therefore, care must be taken in layout

to minimize the effect of this switching noise. The current sensing circuit in current mode devices can be easily

affected by switching noise. This noise can cause duty cycle jittering which leads to increased spectral noise.

Although the LM3478 has 325 ns blanking time at the beginning of every cycle to ignore this noise, some noise

may remain after the blanking time.

The most important layout rule is to keep the AC current loops as small as possible. Figure 37 shows the current

flow of a boost converter. The top schematic shows a dotted line which represents the current flow during on-

state and the middle schematic shows the current flow during off-state. The bottom schematic shows the currents

we refer to as AC currents. They are the most critical ones since current is changing in very short time periods.

The dotted lined traces of the bottom schematic are the once to make as short as possible.

The PGND and AGND pins have to be connected to the same ground very close to the IC. To avoid ground loop

currents, attach all the grounds of the system only at one point.

A ceramic input capacitor should be connected as close as possible to the Vin pin and grounded close to the

GND pin.

For more information about layout in switch mode power supplies please refer to AN-1229 Simple Switcher PCB

Layout Guidelines, SNVA054.

Figure 37. Current Flow in a Boost Application

LM3478MA/NOPB Datasheet(PDF) 28 Page - Texas Instruments