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LT1763 Series

1763fe

The LT1763 series are 500mA low dropout regulators with

micropower quiescent current and shutdown. The devices

are capable of supplying 500mA at a dropout voltage of

300mV. Output voltage noise can be lowered to 20μVRMS

over a 10Hz to 100kHz bandwidth with the addition of

a 0.01μF reference bypass capacitor. Additionally, the

reference bypass capacitor will improve transient response

of the regulator, lowering the settling time for transient load

conditions. The low operating quiescent current (30μA)

drops to less than 1μA in shutdown. In addition to the

low quiescent current, the LT1763 regulators incorporate

several protection features which make them ideal for use

in battery-powered systems. The devices are protected

against both reverse input and reverse output voltages.

In battery backup applications where the output can be

held up by a backup battery when the input is pulled to

ground, the LT1763-X acts like it has a diode in series with

its output and prevents reverse current ﬂow. Additionally,

in dual supply applications where the regulator load is

returned to a negative supply, the output can be pulled

below ground by as much as 20V and still allow the device

to start and operate.

Adjustable Operation

The adjustable version of the LT1763 has an output voltage

range of 1.22V to 20V. The output voltage is set by the

ratio of two external resistors, as shown in Figure 2. The

device servos the output to maintain the ADJ pin voltage

at 1.22V referenced to ground. The current in R1 is then

equal to 1.22V/R1 and the current in R2 is the current

in R1 plus the ADJ pin bias current. The ADJ pin bias

current, 30nA at 25°C, ﬂows through R2 into the ADJ pin.

The output voltage can be calculated using the formula in

Figure 2. The value of R1 should be no greater than 250k

to minimize errors in the output voltage caused by the

ADJ pin bias current. Note that in shutdown the output

is turned off and the divider current will be zero. Curves

of ADJ Pin Voltage vs Temperature and ADJ Pin Bias

Current vs Temperature appear in the Typical Performance

Characteristics section.

The adjustable device is tested and speciﬁed with the ADJ

pin tied to the OUT pin for an output voltage of 1.22V.

Speciﬁcations for output voltages greater than 1.22V will

be proportional to the ratio of the desired output voltage

APPLICATIONS INFORMATION

Figure 2. Adjustable Operation

to 1.22V: VOUT/1.22V. For example, load regulation for an

output current change of 1mA to 500mA is –2mV typical

at VOUT = 1.22V. At VOUT = 12V, load regulation is:

(12V/1.22V)(–2mV) = –19.6mV

Bypass Capacitance and Low Noise Performance

The LT1763 regulators may be used with the addition of

a bypass capacitor from VOUT to the BYP pin to lower

output voltage noise. A good quality low leakage capacitor

is recommended. This capacitor will bypass the reference

of the regulator, providing a low frequency noise pole.

The noise pole provided by this bypass capacitor will

lower the output voltage noise to as low as 20μVRMS

with the addition of a 0.01μF bypass capacitor. Using

a bypass capacitor has the added beneﬁt of improving

transient response. With no bypass capacitor and a 10μF

output capacitor, a 10mA to 500mA load step will settle

to within 1% of its ﬁnal value in less than 100μs. With

the addition of a 0.01μF bypass capacitor, the output will

settle to within 1% for a 10mA to 500mA load step in less

than 10μs, with total output voltage deviation of less than

2.5% (see the LT1763-5 Transient Response curve in the

Typical Performance Characteristics section). However,

regulator start-up time is proportional to the size of the

bypass capacitor, slowing to 15ms with a 0.01μF bypass

capacitor and 10μF output capacitor.

1763 F02

OUT

VIN

VOUT

ADJ

GND

LT1763

InA

OUT

ADJ

⎛

⎝

⎜

⎞

⎠

⎟ +

()( )

=°

122

AT 25 C

OUTPUT RANGE = 1.22V TO 20V

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