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LT1963A Series

1963aff

For more information www.linear.com/LT1963A

applicaTions inForMaTion

typically 40nV/√Hz over this frequency bandwidth for

the LT1963A (adjustable version). For higher output

voltages (generated by using a resistor divider), the

output voltage noise will be gained up accordingly. This

results in RMS noise over the 10Hz to 100kHz bandwidth

of 14µVRMS for the LT1963A increasing to 38µVRMS for

the LT1963A-3.3.

Higher values of output voltage noise may be measured

when care is not exercised with regard to circuit layout

and testing. Crosstalk from nearby traces can induce

unwanted noise onto the output of the LT1963A-X.

Powersupplyripplerejectionmustalsobeconsidered;the

LT1963A regulators do not have unlimited power supply

rejection and will pass a small portion of the input noise

through to the output.

Thermal Considerations

Thepowerhandlingcapabilityofthedeviceislimitedbythe

maximum rated junction temperature (125°C). The power

dissipated by the device is made up of two components:

1. Output current multiplied by the input/output voltage

differential: (IOUT)(VIN – VOUT), and

2. GND pin current multiplied by the input voltage: (IGND)

(VIN).

The GND pin current can be found using the GND Pin

CurrentcurvesintheTypicalPerformanceCharacteristics.

Power dissipation will be equal to the sum of the two

components listed above.

The LT1963A series regulators have internal thermal

limiting designed to protect the device during overload

conditions. For continuous normal conditions, the maxi-

mum junction temperature rating of 125°C must not be

exceeded. It is important to give careful consideration to

all sources of thermal resistance from junction to ambi-

ent. Additional heat sources mounted nearby must also

be considered.

For surface mount devices, heat sinking is accomplished

by using the heat spreading capabilities of the PC board

and its copper traces. Copper board stiffeners and plated

through-holes can also be used to spread the heat gener-

ated by power devices.

This resistor should be made using one of the inner

layers of the PC board which are well defined. The resistiv-

ity is determined primarily by the sheet resistance of the

copper laminate with no additional plating steps. Table

2 gives some sizes for 0.75A RMS current for various

copper thicknesses. More detailed information regarding

resistors made from PC traces can be found in Application

Note 69, Appendix A.

Overload Recovery

LikemanyICpowerregulators,theLT1963A-Xhassafeop-

eratingareaprotection.Thesafeareaprotectiondecreases

the current limit as input-to-output voltage increases and

keeps the power transistor inside a safe operating region

for all values of input-to-output voltage. The protection

is designed to provide some output current at all values

of input-to-output voltage up to the device breakdown.

When power is first turned on, as the input voltage rises,

the output follows the input, allowing the regulator to start

up into very heavy loads. During the start-up, as the input

voltage is rising, the input-to-output voltage differential

is small, allowing the regulator to supply large output

currents. With a high input voltage, a problem can occur

wherein removal of an output short will not allow the

output voltage to recover. Other regulators, such as the

LT1085, also exhibit this phenomenon, so it is not unique

to the LT1963A-X.

The problem occurs with a heavy output load when the

inputvoltageishighandtheoutputvoltageislow.Common

situations are immediately after the removal of a short-

circuit or when the shutdown pin is pulled high after the

input voltage has already been turned on. The load line for

such a load may intersect the output current curve at two

points. If this happens, there are two stable output operat-

ing points for the regulator. With this double intersection,

the input power supply may need to be cycled down to

zero and brought up again to make the output recover.

Output Voltage Noise

The LT1963A regulators have been designed to provide

low output voltage noise over the 10Hz to 100kHz band-

width while operating at full load. Output voltage noise is

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