LT1764EQ

The LT ®1764 is a low dropout regulator optimized for fasttransient response. The device is capable of supplying 3Aof output current with a dropout voltage of 340mV

FEATURES

Optimized for Fast Transient Response

■ Output Current: 3A

■ Dropout Voltage: 340mV at 3A

■ Low Noise: 40µVRMS (10Hz to 100kHz)

■ 1mA Quiescent Current

■ Wide Input Voltage Range: 2.7V to 20V

■ No Protection Diodes Needed

■ Controlled Quiescent Current in Dropout

■ Fixed Output Voltages: 1.5V, 1.8V, 2.5V, 3.3V

■ Adjustable Output from 1.21V to 20V

■ <1µA Quiescent Current in Shutdown

■ Stable with 10µF Output Capacitor

■ Reverse Battery Protection

■ No Reverse Current

■ Thermal Limiting

■ Available in 5-Lead TO-220, DD and 16-LeadTSSOP Packages

APPLICATIO S

■ 3.3V to 2.5V Logic Power Supply■ Post Regulator for Switching Supplies

DESCRIPTIO U

The LT ®1764 is a low dropout regulator optimized for fasttransient response. 

The device is capable of supplying 3Aof output current with a dropout voltage of 340mV. 

Operating quiescent current is 1mA, dropping to <1µA inshutdown.

 Quiescent current is well controlled; it does notrise in dropout as it does with many other regulators. Inaddition to fast transient response, 

the LT1764 has verylow output voltage noise which makes the device ideal forsensitive RF supply applications.

Output voltage range is from 1.21V to 20V. The LT1764regulators are stable with output capacitors as low as 10µF.

Internal protection circuitry includes reverse battery protection, current limiting, thermal limiting and reverse current protection. 

The device is available in fixed outputvoltages of 1.5V, 1.8V, 2.5V, 3.3V and as an adjustabledevice with a 1.21V reference voltage. 

The LT1764 regulators are available in 5-lead TO-220, DD and Exposed Pad16-lead TSSOP packages.

ORDER PART NUMBER

LT1764EQ  LT1764EQ-1.5  LT1764EQ-1.8 LT1764EQ-2.5  LT1764EQ-3.3

LT1764ET LT1764ET-1.5 LT1764ET-1.8 LT1764ET-2.5  LT1764ET-3.3

LT1764EFE LT1764EFE-1.5 LT1764EFE-1.8 LT1764EFE-2.5 LT1764EFE-3.3

Applicatios Infomation 

The LT1764 series are 3A low dropout regulators optimized for fast transient response. 

The devices are capableof supplying 3A at a dropout voltage of 340mV. 

The lowoperating quiescent current (1mA) drops to less than 1µAin shutdown. 

In addition to the low quiescent current, theLT1764 regulators incorporate several protection featureswhich make them ideal for use in battery-powered systems.

 The devices are protected against both reverse inputand reverse output voltages. 

In battery backup applications where the output can be held up by a backup batterywhen the input is pulled to ground, 

the LT1764-X acts likeit has a diode in series with its output and prevents reversecurrent flow. 

Additionally, in dual supply applicationswhere the regulator load is returned to a negative supply,

the output can be pulled below ground by as much as 20Vand still allow the device to start and operate.

Adjustable Operation

The adjustable version of the LT1764 has an outputvoltage range of 1.21V to 20V. 

The output voltage is set bythe ratio of two external resistors as shown in Figure 2. 

Thedevice servos the output to maintain the voltage at the ADJpin at 1.21V referenced to ground. 

The current in R1 isthen equal to 1.21V/R1 and the current in R2 is the currentin R1 plus the ADJ pin bias current. 

The ADJ pin biascurrent, 3µA at 25°C, flows through R2 into the ADJ pin.The output voltage can be calculated using the formula inFigure 2.

 The value of R1 should be less than 4.17k tominimize errors in the output voltage caused by the ADJpin bias current. Note that in shutdown the output is turnedoff and the divider current will be zero.

The adjustable device is tested and specified with the ADJpin tied to the OUT pin for an output voltage of 1.21V.

Specifications for output voltages greater than 1.21V willbe proportional to the ratio of the desired output voltage to1.21V: VOUT/1.21V. For example, 

load regulation for anoutput current change of 1mA to 3A is –3mV typical atVOUT = 1.21V. At VOUT = 5V, load regulation is:(5V/1.21V)(–3mV) = –12.4mV

Output Capacitance and Transient Response

The LT1764 regulators are designed to be stable with awide range of output capacitors. The ESR of the outputcapacitor affects stability, most notably with small capacitors. A minimum output capacitor of 10µF with an ESR inthe range of 50mΩ to 3Ω is recommended to preventoscillations. Larger values of output capacitance can decrease the peak deviations and provide improved transient response for larger load current changes. 

Bypasscapacitors, used to decouple individual components powered by the LT1764-X, will increase the effective outputcapacitor value.

Extra consideration must be given to the use of ceramiccapacitors. Ceramic capacitors are manufactured with avariety of dielectrics, each with different behavior acrosstemperature and applied voltage. 

The most common dielectrics used are specified with EIA temperature characteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Y5Vdielectrics are good for providing high capacitances in asmall package,

 but they tend to have strong voltage andtemperature coefficients as shown in Figures 3 and 4.When used with a 5V regulator, a 16V 10µF Y5V capacitorcan exhibit an effective value as low as 1µF to 2µF for the

DC bias voltage applied and over the operating temperature range. The X5R and X7R dielectrics result in morestable characteristics and are more suitable for use as theoutput capacitor. 

The X7R type has better stability acrosstemperature, while the X5R is less expensive and isavailable in higher values. 

Care still must be exercisedwhen using X5R and X7R capacitors; the X5R and X7Rcodes only specify operating temperature range and maximum capacitance change over temperature. 

Capacitancechange due to DC bias with X5R and X7R capacitors isbetter than Y5V and Z5U capacitors, but can still besignificant enough to drop capacitor values below appropriate levels. 

Capacitor DC bias characteristics tend toimprove as component case size increases, but expectedcapacitance at operating voltage should be verified.

The protection is designed to provide some output currentat all values of input-to-output voltage up to the devicebreakdown.When power is first turned on, 

as the input voltage rises,the output follows the input, allowing the regulator to startup into very heavy loads. During the start-up, as the inputvoltage is rising, 

the input-to-output voltage differential issmall, allowing the regulator to supply large output currents. 

With a high input voltage, a problem can occurwherein removal of an output short will not allow theoutput voltage to recover. 

Other regulators, such as theLT1085, also exhibit this phenomenon, so it is not uniqueto the LT1764 series.

The problem occurs with a heavy output load when theinput voltage is high and the output voltage is low. 

Common situations are immediately after the removal of ashort circuit or when the SHDN pin is pulled high after theinput voltage has already been turned on. 

The load line forsuch a load may intersect the output current curve at twopoints. If this happens, there are two stable output operating points for the regulator. 

With this double intersection, the input power supply may need to be cycled downto zero and brought up again to make the output recover.

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