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LT3021/LT3021-1.2/

LT3021-1.5/LT3021-1.8

3021fa

APPLICATIO S I FOR ATIO

The LT3021 is a very low dropout linear regulator capable

of 1V input supply operation. Devices supply 500mA of

output current and dropout voltage is typically 155mV.

Quiescent current is typically 120

µA and drops to 3µA in

shutdown. The LT3021 incorporates several protection

features, making it ideal for use in battery-powered sys-

tems. The device protects itself against reverse-input and

reverse-output voltages. In battery backup applications

where the output is held up by a backup battery when the

input is pulled to ground, the LT3021 acts as if a diode is

in series with its output which prevents reverse current

flow. 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 10V without affecting start-

up or normal operation.

Adjustable Operation

The LT3021’s output voltage range is 0.2V to 9.5V. Figure

1 shows that the output voltage is set by the ratio of two

external resistors. The device regulates the output to

maintain the ADJ pin voltage at 200mV referenced to

ground. The current in R1 equals 200mV/R1 and the

current in R2 is the current in R1 minus the ADJ pin bias

current. The ADJ pin bias current of 20nA flows out of the

pin. Use the formula in Figure 1 to calculate output voltage.

An R1 value of 20k sets the resistor divider current to

µA. Note that in shutdown the output is turned off and

the divider current is zero. Curves of ADJ Pin Voltage vs

Temperature and ADJ Pin Bias Current vs Temperature

appear in the Typical Performance Characteristics section.

0.4mV at VADJ= 200mV. At VOUT = 1.5V, load regulation is:

(1.5V/200mV) • (0.4mV) = 3mV

Output Capacitance and Transient Response

The LT3021’s design is stable with a wide range of output

capacitors, but is optimized for low ESR ceramic capaci-

tors. The output capacitor’s ESR affects stability, most

notably with small value capacitors. Use a minimum

output capacitor of 3.3

µF with an ESR of 0.2Ω or less to

prevent oscillations. The LT3021 is a low voltage device,

and output load transient response is a function of output

capacitance. Larger values of output capacitance decrease

the peak deviations and provide improved transient re-

sponse for larger load current changes. For output capaci-

tor values greater than 22

µF a small feedforward capacitor

with a value of 300pF across the upper divider resistor (R2

in Figure 1) is required. Under extremely low output

current conditions (ILOAD < 30µA) a low frequency small

signal oscillation (200Hz/8mVP-P at 1.2V output) can

occur. A minimum load of 100

µA is recommended to

prevent this instability.

Give extra consideration to the use of ceramic capacitors.

Manufacturers make ceramic capacitors with a variety of

dielectrics, each with a different behavior across tempera-

ture and applied voltage. The most common dielectrics are

Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics

provide high C-V products in a small package at low cost,

but exhibit strong voltage and temperature coefficients.

The X5R and X7R dielectrics yield highly stable

characterisitics and are more suitable for use as the output

capacitor at fractionally increased cost. The X5R and X7R

dielectrics both exhibit excellent voltage coefficient char-

acteristics. The X7R type works over a larger temperature

range and exhibits better temperature stability whereas

X5R is less expensive and is available in higher values.

Figures 2 and 3 show voltage coefficient and temperature

coefficient comparisons between Y5V and X5R material.

Voltage and temperature coefficients are not the only

sources of problems. Some ceramic capacitors have a

piezoelectric response. A piezoelectric device generates

voltage across its terminals due to mechanical stress, simi-

lar to the way a piezoelectric accelerometer or microphone

works. For a ceramic capacitor, the stress can be induced

by vibrations in the system or thermal transients. The re-

sulting voltages produced can cause appreciable amounts

SHDN

3021 F01

OUT

VIN

ADJ

GND

LT3021

VOUT

VOUT = 200mV

VADJ = 200mV

IADJ = 20nA AT 25°C

OUTPUT RANGE = 0.2V TO 9.5V

1 +

– IADJ (R2)

()

Figure 1. Adjustable Operation

Specifications for output voltages greater than 200mV are

proportional to the ratio of desired output voltage to

200mV; (VOUT/200mV). For example, load regulation for

an output current change of 1mA to 500mA is typically

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