CAT6243

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11

APPLICATIONS INFORMATION

A ceramic or tantalum 1

mF capacitor is recommended and

should be connected close to the CAT6243’s package.

Higher capacitance and lower ESR will improve the overall

line and load transient response.

The minimum output decoupling value is 2.2

mF and can

be

augmented

to

fulfill

stringent

load

transient

requirements. The regulator works with ceramic chip

capacitors. Larger values, up to 22

mF, improve noise

rejection and load regulation transient response. The

CAT6243 is a highly stable regulator and performs well over

a wide range capacitor Equivalent Series Resistances (ESR).

No−Load Regulation Considerations

The CAT6243 adjustable regulator will operate properly

under conditions where the only load current is through the

resistor divider that sets the output voltage. However, in the

case where the CAT6243 is configured to provide a 0.8 V

output, there is no resistor divider and the ADJ pin is

conditions, leakage current through the pass transistor at

junction temperatures above 85

°C can approach several

microamperes,

especially

as

junction

temperature

approaches 150

°C. If this leakage current is not directed into

a load, the output voltage will rise above nominal until a load

is applied. For this reason it is recommended that a minimum

load of 100

mA be present at all times. Normally the voltage

setting resistor divider will serve this function but if no

W

should be provided.

Output Voltage Adjust

The output voltage can be adjusted from 0.8 V to 5.0 V

using resistors between the output and the ADJ input. The

output voltage and resistors are chosen using Equation 1 and

Equation 2.

V

R

1

R

2

) I

ADJ

R

1

(eq. 1)

R

0.8 V

I

DIV

(eq. 2)

R

V

OUT

0.8 V

* 1

(eq. 3)

Figure 27. Adjustable Output Resistor Divider

VIN

ENABLE

CAT6243

ADJ

VOUT

GND

VIN

VOUT

BYP

ignored

mA, then, when there is no

mA to keep CAT6243 in

regulation. This then sets R2’s value using Equation 2 to

8K

W, which minimizes output noise. Use Equation 3 to find

can be considered, but not lower than 10

mA. The price will

Thermal Considerations

As power in the CAT6243 increases, it may become

necessary to provide thermal relief. The maximum power

dissipation supported by this device is dependent upon

board design and layout. Mounting pad configuration on the

PCB, the board material, and the ambient temperature affect

the rate of junction temperature rise for the part. When the

CAT6243 has good thermal conductivity through the PCB,

the junction temperature will be relatively low even with

high power applications. The maximum dissipation the

CAT6243 can handle is given by:

P

T

(eq. 4)

°C, then with

FR4 PCB material can dissipate in excess of 1 W when the

°C. Note that this assumes the

pad in the center of the package is soldered to the dissipating

dissipation can be calculated from the following equations:

P

(eq. 5)

or

V

P

I

I

(eq. 6)