Datasheet

LT3014B
7
3014bfa
APPLICATIONS INFORMATION
The LT3014B is a 20mA high voltage, low dropout regu-
lator with micropower quiescent current. The device is
capable of supplying 20mA at a dropout voltage of 350mV.
Operating quiescent current is only 7μA. In addition to the
low quiescent current, the LT3014B incorporates several
protection features which make it ideal for use in bat-
tery-powered systems. The device is 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
LT3014B acts like it has a diode in series with its output
and prevents reverse current fl ow.
Adjustable Operation
The LT3014B has an output voltage range of 1.22V to
60V. 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 adjust 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, 4nA
at 25°C, fl ows through R2 into the ADJ pin. The output
voltage can be calculated using the formula in Figure 1.
The value of R1 should be less than 1.62M to minimize
errors in the output voltage caused by the ADJ pin bias
current.
The device is tested and specifi ed with the ADJ pin
tied to the OUT pin and a 5μA DC load (unless otherwise
specifi ed) for an output voltage of 1.22V. Specifi cations
for output voltages greater than 1.22V will be propor-
tional to the ratio of the desired output voltage to 1.22V
(V
OUT
/1.22V). For example, load regulation for an output
current change of 1mA to 20mA is –13mV typical at V
OUT
= 1.22V. At V
OUT
= 12V, load regulation is:
(12V/1.22V) • (–13mV) = –128mV
Output Capacitance and Transient Response
The LT3014B is designed to be stable with a wide range of
output capacitors. The ESR of the output capacitor affects
stability, most notably with small capacitors. A minimum
output capacitor of 0.47μF with an ESR of 3Ω or less is
recommended to prevent oscillations. The LT3014B is a
micropower device and output transient response will be
a function of output capacitance. Larger values of output
capacitance decrease the peak deviations and provide
improved transient response for larger load current
changes. Bypass capacitors, used to decouple individual
components powered by the LT3014B, will increase the
effective output capacitor value.
Extra consideration must be given to the use of ceramic
capacitors. Ceramic capacitors are manufactured with a
variety of dielectrics, each with different behavior across
temperature and applied voltage. The most common
dielectrics used are specifi ed with EIA temperature char-
acteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
in a small package, but they tend to have strong voltage
and temperature coeffi cients as shown in Figures 2 and 3.
When used with a 5V regulator, a 16V 10μF Y5V capacitor
can 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 more stable
characteristics and are more suitable for use as the output
capacitor. The X7R type has better stability across tempera-
ture, while the
X5R is less expensive and is available in
Figure 1. Adjustable Operation
IN
LT3014B
V
IN
OUT
ADJ
GND
3014B F01
V
OUT
R2
R1
+
R2
R1
V
OUT
= 1.22V
V
ADJ
= 1.22V
I
ADJ
= 4nA AT 25°C
OUTPUT RANGE = 1.22V TO 60V
+ (I
ADJ
)(R2)1 +
 
•
Figure 2. Ceramic Capacitor DC Bias Characteristics
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3014B F02
20
0
–20
–40
–60
–80
–100
0
4
8
10
26
12
14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF