Datasheet

LT3070
20
3070fb
For more information www.linear.com/LT3070
APPLICATIONS INFORMATION
Stability and Input Capacitance
The LT3070 is stable with a minimum capacitance of
47µF connected to its IN pins. Use low ESR capacitors to
minimize instantaneous voltage drops under large load
transient conditions. Large V
IN
droops during large load
transients may cause the regulator to enter dropout with
corresponding degradation in load transient response.
Increased values of input and output capacitance may be
necessary depending on an application’s requirements.
Sufficient input capacitance is critical as the circuit is
intentionally operated close to dropout to minimize power.
Ideally, the output impedance of the supply that powers
IN should be less than 10mΩ to support a 5A load with
large transients.
In cases where wire is used to connect a power supply
to the input of the LT3070 (and also from the ground of
the LT3070 back to the power supply ground), large input
capacitors are required to avoid an unstable application.
This is due to the inductance of the wire forming an LC
tank circuit with the input capacitor and not a result of the
LT3070 being unstable. The self inductance, or isolated
inductance, of a wire is directly proportional to its length.
However, the diameter of a wire does not have a major
influence on its self inductance. For example, one inch of
18-AWG, 0.04 inch diameter wire has 28nH of self induc-
tance. The self inductance of a 2-AWG isolated wire with
a diameter of 0.26 inch is about half the inductance of a
18-AWG wire. The overall self inductance of a wire can
be reduced in two ways. One is to divide the current flow-
ing towards the LT3070 between two parallel conductors
which flows in the same direction in each. In this case,
the farther the wires are placed apart from each other, the
more inductance will be reduced, up to a 50% reduction
when placed a few inches apart. Splitting the wires basi-
cally connects two equal inductors in parallel. However,
when placed in close proximity from each other, mutual
inductance is added to the overall self inductance of the
wires. The most effective way to reduce overall inductance
is to place the forward and return-current conductors (the
wire for the input and the wire for the return ground) in
very close proximity. Two 18-AWG wires separated by
0.05 inch reduce the overall self inductance to about one-
fourth of a single isolated wire. If the LT3070 is powered
by a battery mounted in close proximity with ground and
power planes on the same circuit board, a 47µF input
capacitor is sufficient for stability. However, if the LT3070
is powered by a distant supply, use a low ESR, large value
input capacitor on the order of 330µF. As power supply
output impedance varies, the minimum input capacitance
needed for application stability also varies.
Bias Pin Capacitance Requirements
The BIAS pin supplies current to most of the internal
control circuitry and the output stage driving the pass
transistor. The LT3070 requires a minimum 2.2µF by-
pass capacitor for stability and proper operation. To
ensure proper operation, the BIAS voltage must sat-
isfy the following conditions: 2.2V ≤ V
BIAS
≤ 3.6V and
V
BIAS
≥ (1.25 • V
OUT
+ 1V). For V
OUT
≤ 0.95V, the
minimum BIAS voltage is limited to 2.2V.
DC BIAS VOLTAGE (V)
0
–100
CHANGE IN VALUE (%)
–80
–60
–40
–20
4 8
12
16
3070 F04
0
20
2 6
10
X5R
Y5V
14
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
TEMPERATURE (°C)
–50
–20
0
40
25 75
X5R
Y5V
3070 F05
–40
–60
–25 0
50 100 125
–80
–100
20
CHANGE IN VALUE (%)
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
Figure 4. Ceramic Capacitor DC Bias Characteristics
Figure 5. Ceramic Capacitor Temperature Characteristics