Datasheet

LT3012
9
3012fd
APPLICATIONS INFORMATION
The adjustable 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 1.24V. Speci-
fi cations for output voltages greater than 1.24V will be
proportional to the ratio of the desired output voltage to
1.24V; (V
OUT
/1.24V). For example, load regulation for an
output current change of 1mA to 250mA is –7mV typical at
V
OUT
= 1.24V. At V
OUT
= 12V, load regulation is:
(12V/1.24V) • (–7mV) = –68mV
Output Capacitance and Transient Response
The LT3012 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 3.3μF with an ESR of 3Ω or less is
recommended to prevent oscillations. The LT3012 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
Figure 1. Adjustable Operation
improved transient response for larger load current
changes. Bypass capacitors, used to decouple individual
components powered by the LT3012, 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 tempera-
ture 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
temperature, while the X5R is less expensive and is avail-
able in higher values. Care still must be exercised when
using X5R and X7R capacitors; the X5R and X7R codes
only specify operating temperature range and maximum
capacitance change over temperature. Capacitance change
due to DC bias with X5R and X7R capacitors is better than
Y5V and Z5U capacitors, but can still be signifi cant enough
to drop capacitor values below appropriate levels. Capaci-
tor DC bias characteristics tend to improve as component
case size increases, but expected capacitance at operating
voltage should be verifi ed.
Figure 2. Ceramic Capacitor DC Bias Characteristics Figure 3. Ceramic Capacitor Temperature Characteristics
V
IN
3012 F01
V
OUT
R2
R1
+
R2
R1
V
OUT
= 1.24V
V
ADJ
= 1.24V
I
ADJ
= 30nA AT 25°C
OUTPUT RANGE = 1.24V TO 60V
+ (I
ADJ
)(R2)1 +
IN
LT3012
OUT
ADJ
GND
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3012 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
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100
25 75
3012 F03
–25 0
50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF