LT1964IS5-5PBF datasheet(11/16 Pages) LINER

Part #	LT1964IS5-5PBF

Description	200mA, Low Noise,Low Dropout Negative Micropower Regulator

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Manufacturer	LINER [Linear Technology]

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LT1964

11

1964fb

APPLICATIONS INFORMATION

Figure 1. Adjustable Operation

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 quies-

cent 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 volt-

age 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 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 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

Bypass Capacitance and Low Noise Performance

The LT1964 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 recom-

mended. This capacitor will bypass the reference of the

LT1964, providing a low frequency noise pole. The noise

pole provided by this bypass capacitor will lower the out-

put voltage noise to as low as 30μVRMS 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 –200mA 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 stay within 1%

for the same –10mA to –200mA load step (see LT1964-5

Transient Response in the Typical Characteristics section).

However, regulator start-up time is proportional to the size

of the bypass capacitor.

Higher values of output voltage noise may be measured

if care is not exercised with regard to circuit layout and

testing. Crosstalk from nearby traces can induce unwanted

noise onto the output of the LT1964-X.

1964 F01

GND

ADJ

IN

OUT

LT1964

VIN

VOUT

+

R1

R2

VOUT = –1.22V(1 +

) – (IADJ)(R2)

VADJ = –1.22V

IADJ = 30nA AT 25°C

OUTPUT RANGE = –1.22V TO –20V

R2

R1

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