LP38690-ADJ datasheet(10/13 Pages) NSC

Part #	LP38690-ADJ

Description	1A Low Dropout CMOS Linear Regulators with Adjustable Output Stable with Ceramic Output Capacitors

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Application Hints

EXTERNAL CAPACITORS

Like any low-dropout regulator, external capacitors are re-

quired to assure stability. These capacitors must be correctly

selected for proper performance.

INPUT CAPACITOR: An input capacitor of at least 1µF is

required (ceramic recommended). The capacitor must be

located not more than one centimeter from the input pin and

returned to a clean analog ground.

OUTPUT CAPACITOR: An output capacitor is required for

loop stability. It must be located less than 1 centimeter from

the device and connected directly to the output and ground

pins using traces which have no other currents flowing

through them.

The minimum amount of output capacitance that can be

used for stable operation is 1µF. Ceramic capacitors are

recommended (the LP38690/2-ADJ was designed for use

with ultra low ESR capacitors). The LP38690/2-ADJ is stable

with any output capacitor ESR between zero and 100 Ohms.

SETTING THE OUTPUT VOLTAGE: The output voltage is

set using the external resistors R1 and R2 (see Typical

Application Circuit). The output voltage will be given by the

equation:

V

OUT =VADJ X (1 + R1/R2)

Because the part has a minimum load current requirement of

100 µA, it is recommended that R2 always be 12k Ohms or

less to provide adequate loading. Even if a minimum load is

always provided by other means, it is not recommended that

very high value resistors be used for R1 and R2 because it

can make the ADJ node susceptible to noise pickup. A

maximum value of 100k is recommended for R2 to prevent

this from occurring.

ENABLE PIN (LP38692-ADJ only): The LP38692-ADJ has

an enable pin which turns the regulator output on and off.

Pulling the enable pin down to a logic low will turn the part

off. The voltage the pin has to be pulled up to in order to

assure the part is on depends on input voltage (refer to

Electrical Characteristics section). This pin should be tied to

V

IN if the enable function is not used.

FOLDBACK CURRENT LIMITING: Foldback current limit-

ing is built into the LP38690/2-ADJ which reduces the

amount of output current the part can deliver as the output

voltage is reduced. The amount of load current is dependent

on the differential voltage between V

IN and VOUT. Typically,

when this differential voltage exceeds 5V, the load current

will limit at about 450 mA. When the V

IN -VOUT differential is

reduced below 4V, load current is limited to about 1500 mA.

SELECTING A CAPACITOR

It is important to note that capacitance tolerance and varia-

tion with temperature must be taken into consideration when

selecting a capacitor so that the minimum required amount

of capacitance is provided over the full operating tempera-

ture range.

Capacitor Characteristics

CERAMIC: For values of capacitance in the 10 to 100 µF

range, ceramics are usually larger and more costly than

tantalums but give superior AC performance for bypassing

high frequency noise because of very low ESR (typically less

than 10 m

Ω). However, some dielectric types do not have

good capacitance characteristics as a function of voltage

and temperature.

Z5U and Y5V dielectric ceramics have capacitance that

drops severely with applied voltage. A typical Z5U or Y5V

capacitor can lose 60% of its rated capacitance with half of

the rated voltage applied to it. The Z5U and Y5V also exhibit

a severe temperature effect, losing more than 50% of nomi-

nal capacitance at high and low limits of the temperature

range.

X7R and X5R dielectric ceramic capacitors are strongly rec-

ommended if ceramics are used, as they typically maintain a

capacitance range within ±20% of nominal over full operat-

ing ratings of temperature and voltage. Of course, they are

typically larger and more costly than Z5U/Y5U types for a

given voltage and capacitance.

TANTALUM: Solid Tantalum capacitors have good tempera-

ture stability: a high quality Tantalum will typically show a

capacitance value that varies less than 10-15% across the

full temperature range of -40˚C to 125˚C. ESR will vary only

about 2X going from the high to low temperature limits.

The increasing ESR at lower temperatures can cause oscil-

lations when marginal quality capacitors are used (if the ESR

of the capacitor is near the upper limit of the stability range at

room temperature).

PCB LAYOUT

Good PC layout practices must be used or instability can be

induced because of ground loops and voltage drops. The

input and output capacitors must be directly connected to the

input, output, and ground pins of the regulator using traces

which do not have other currents flowing in them (Kelvin

connect).

The best way to do this is to lay out C

IN and COUT near the

device with short traces to the V

IN,VOUT, and ground pins.

The regulator ground pin should be connected to the exter-

nal circuit ground so that the regulator and its capacitors

have a "single point ground".

It should be noted that stability problems have been seen in

applications where "vias" to an internal ground plane were

used at the ground points of the IC and the input and output

capacitors. This was caused by varying ground potentials at

these nodes resulting from current flowing through the

ground plane. Using a single point ground technique for the

regulator and it’s capacitors fixed the problem. Since high

current flows through the traces going into V

IN and coming

from V

OUT, Kelvin connect the capacitor leads to these pins

so there is no voltage drop in series with the input and output

capacitors.

RFI/EMI SUSCEPTIBILITY

RFI (radio frequency interference) and EMI (electromagnetic

interference) can degrade any integrated circuit’s perfor-

mance because of the small dimensions of the geometries

inside the device. In applications where circuit sources are

present which generate signals with significant high fre-

quency 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 ca-

pacitors must be used at the input pin of the IC.

If a load is connected to the IC output 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 regulator loop is

less than 100 kHz, the control circuitry cannot respond to

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