LT3010/LT3010-5
9
30105fe
APPLICATIONS INFORMATION
The LT3010 is a 50mA high voltage low dropout regulator
with micropower quiescent current and shutdown. The
device is capable of supplying 50mA at a dropout voltage
of 300mV. The low operating quiescent current (30µA)
drops to 1µA in shutdown. In addition to the low quies-
cent current, the LT3010 incorporates several protection
features which make it ideal for use in battery-powered
systems. The device is protected against both reverse
input and reverse output voltages. In battery backup ap-
plications where the output can be held up by a backup
battery when the input is pulled to ground, the LT3010 acts
like it has a diode in series with its output and prevents
reverse current flow.
Adjustable Operation
The adjustable version of the LT3010 has an output voltage
range of 1.275V to 60V. The output voltage is set by the
ratio of two external resistors as shown in Figure 2. The
device servos the output to maintain the voltage at the
adjust pin at 1.275V referenced to ground. The current
in R1 is then equal to 1.275V/R1 and the current in R2 is
the current in R1 plus the ADJ pin bias current. The ADJ
pin bias current, 50nA at 25°C, flows through R2 into the
ADJ pin. The output voltage can be calculated using the
formula in Figure 2. 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.
A small capacitor (C1) placed in parallel with the top resistor
(R2) of the output divider is necessary for stability and tran-
sient performance of the adjustable LT3010. The impedance
of C1 at 10kHz should be less than the value of R1.
Figure 2. Adjustable Operation
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 1.275V. Speci-
fications for output voltages greater than 1.275V will be
proportional to the ratio of the desired output voltage to
1.275V; (V
OUT
/1.275V). For example, load regulation for an
output current change of 1mA to 50mA is –10mV typical
at V
OUT
= 1.275V. At V
OUT
= 12V, load regulation is:
(12V/1.275V)•(–10mV)=–94mV
Output Capacitance and Transient Response
The LT3010 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 1µF with an ESR of 3Ω or
less is recommended to prevent oscillations. The LT3010
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 LT3010, 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 specified 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 coefficients as shown in Figures 3 and 4.
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
IN
LT3010
V
IN
OUT
ADJ
GND
30105 F02
V
OUT
R2 C1
R1
+
R2
R1
V
OUT
= 1.275V
V
ADJ
= 1.275V
I
ADJ
= 50nA AT 25°C
OUTPUT RANGE = 1.275V TO 60V
+ (I
ADJ
)(R2)1 +
( )
