LP38501-ADJ datasheet(12/16 Pages) NSC

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30028139

FIGURE 9. 10 µF Ceramic Paralleled By 220 µF Generic

Aluminum Electrolytic

A solid Tantalum should work better, so the aluminum elec-

trolytic is replaced by a 220 µF Tantalum (Figure 10). The

peak amplitude of the output transient is now reduced to about

130 mV, just slightly worse than the value of the 47 µF ceramic

alone.

30028141

FIGURE 10. 10 µF Ceramic Paralleled By 220 µF Tantalum

The OSCON (ultra low ESR) aluminum electrolytic is the best

of the electrolytics. Figure 11 shows the output voltage tran-

sient is reduced down to about 90 mV (about 5% of V

OUT)

when a 220 µF OSCON is added to the 10 µF ceramic. This

indicates that some kind of ultra-low ESR aluminum elec-

trolytic used in parallel with some ceramic capacitance is

probably the best approach for extremely fast transients, but

each application must be dialed in for it’s specific load re-

quirements.

30028140

FIGURE 11. 10 µF Ceramic Paralleled By 220 µF OSCON

PRINTED CIRCUIT BOARD LAYOUT

Good layout practices will minimize voltage error and prevent

instability which can result from ground loops. The input and

output capacitors should be directly connected to the IC pins

with short traces that have no other current flowing in them

(Kelvin connect).

The best way to do this is to place the capacitors very near

the IC and make connections directly to the IC pins via short

traces on the top layer of the PCB. The regulator’s ground pin

should be connected through vias to the internal or backside

ground plane so that the regulator has a single point ground.

The external resistors which set the output voltage must also

be located very near the IC with all connections directly tied

via short traces to the pins of the IC (Kelvin connect). Do not

connect the resistive divider to the load point or DC error will

be induced.

RFI/EMI SUSCEPTIBILITY

RFI (Radio Frequency Interference) and EMI (Electro-Mag-

netic Interference) can degrade any integrated circuit's per-

formance because of the small dimensions of the geometries

inside the device. In applications where circuit sources are

present which generate signals with significant high frequen-

cy energy content (> 1 MHz), care must be taken to ensure

that this does not affect the IC regulator.

If RFI/EMI noise is present on the input side of the regulator

(such as applications where the input source comes from the

output of a switching regulator), good ceramic bypass capac-

itors must be used at the input pin of the IC to reduce the

amount of EMI conducted into the IC.

If the LP38501/3-ADJ output is connected to a load which

switches at high speed (such as a clock), the high-frequency

current pulses required by the load must be supplied by the

capacitors on the IC output. Since the bandwidth of the reg-

ulator loop is less than 300 kHz, the control circuitry cannot

respond to load changes above that frequency. This means

the effective output impedance of the IC at frequencies above

300 kHz is determined only by the output capacitor(s). Ce-

ramic capacitors provide the best performance in this type of

application.

In applications where the load is switching at high speed, the

output of the IC may need RF isolation from the load. In such

cases, it is recommended that some inductance be placed

between the output capacitor and the load, and good RF by-

pass capacitors be placed directly across the load. PCB

layout is also critical in high noise environments, since RFI/

EMI is easily radiated directly into PC traces. Noisy circuitry

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