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LT1964

200mA, Low Noise, Low Dropout Negative Micropower Regulator in ThinSOT

Linear Technology 

LT1964

PI FU CTIO S

collector logic with a pull-up resistor. The pull-up resistor

is required to supply the pull-up current of the open

collector gate, normally several microamperes, and the

SHDN pin current, typically 3µA out of the pin (for negative

logic) or 6µA into the pin (for positive logic). If unused,

the SHDN pin must be connected to VIN. The device will

be shut down if the SHDN pin is open circuit. For the

LT1964-BYP, the SHDN pin is internally connected to VIN.

A parasitic diode exists between the SHDN pin and the

input of the LT1964. The SHDN pin cannot be pulled more

negative than the input during normal operation, or more

than 0.5V below the input during a fault condition.

ADJ (Pin 4, Adjustable Devices only): For the Adjustable

LT1964, this is the Input to the Error Amplifier. The ADJ pin

has a bias current of 30nA that flows out of the pin. The

ADJ pin voltage is –1.22V referenced to ground, and the

output voltage range is –1.22V to –20V. A parasitic diode

exists between the ADJ pin and the input of the LT1964.

The ADJ pin cannot be pulled more negative than the input

during normal operation, or more than 0.5V more negative

than the input during a fault condition.

OUT (Pin 5): The Output Supplies Power to the Load. A

minimum output capacitor of 1µF is required to prevent

oscillations. Larger output capacitors will be required for

applications with large transient loads to limit peak voltage

transients. A parasitic diode exists between the output and

the input. The output cannot be pulled more negative than

the input during normal operation, or more than 0.5V

below the input during a fault condition. See the Applica-

tions Information section for more information on output

capacitance and reverse output characteristics.

APPLICATIO S I FOR ATIO

The LT1964 is a 200mA negative low dropout regulator

with micropower quiescent current and shutdown. The

device is capable of supplying 200mA at a dropout voltage

of 340mV. Output voltage noise can be lowered to 30µVRMS

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

0.01µF reference bypass capacitor. Additionally, the refer-

ence 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 3µA in shutdown. In addition to the low quiescent

current, the LT1964 incorporates several protection

features which make it ideal for use in battery-powered

systems. In dual supply applications where the regulator

load is returned to a positive supply, the output can be

pulled above ground by as much as 20V and still allow the

device to start and operate.

Adjustable Operation

The adjustable version of the LT1964 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 1. The

device servos the output to maintain the voltage at the ADJ

pin 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 out of the

ADJ pin. The output voltage can be calculated using the

formula in Figure 1. The value of R1 should be less than

250kΩ 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 Char-

acteristics section.

The adjustable device is tested and specified with the ADJ

pin tied to the OUT pin and a 5µA DC load (unless otherwise

specified) for an output voltage of –1.22V. Specifications

for output voltages greater than –1.22V will be propor-

tional to the ratio of the desired output voltage to –1.22V;

(VOUT/– 1.22V). For example, load regulation for an output

current change of 1mA to 200mA is 2mV typical at VOUT =

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

(–12V/–1.22V) • (2mV) = 19.6mV

1964f

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