Data Sheet

LT3077

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Rev. 0

29 of 38

In addition to the DC bias and temperature variation noted above, variation in effective capacitance with applied AC

voltage needs to be considered as well. Ceramic capacitors are typically specified for an AC ripple of 1V. Effective

capacitance decreases with lower AC ripple voltages. Effective capacitance will decrease by 30% or more with the

very low AC ripple at the output of the LT3077. Contact the ceramic capacitor vendor for more information on

temperature, DC bias voltage, and AC ripple voltage effects when selecting a capacitor to meet the minimum

capacitance requirements of the LT3077.

Figure 67. Ceramic Capacitor DC Bias Characteristics

Figure 68. Ceramic Capacitor Temperature Characteristics

Stability and Input Capacitance

The LT3077 is stable with a minimum capacitance of 4.7μF connected to the IN pins. Use low ESR capacitors to

minimize instantaneous voltage drops under large-load transient conditions. Large VIN droops during large-load

transients may cause the regulator to enter dropout with the corresponding degradation in load transient response.

Therefore, increased input and output capacitance values 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 20mΩ to support a

3A load with large transients.

In cases where a wire is used to connect a power supply to the input of the LT3077 (and also from the ground of the

LT3077 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 LT3077

being unstable. A wire's self-inductance, or isolated inductance, is directly proportional to its length. However, the

diameter of a wire does not have a significant influence on its self-inductance. For example, one inch of 18-AWG, 0.04

inch diameter wire has 28nH of self-inductance. The self-inductance of a 2-AWG isolated wire with a diameter of 0.26

inch is about half the inductance of the 18-AWG wire. The overall self-inductance of a wire can be reduced in two

ways. One is to divide the current flowing toward the LT3077 between two parallel conductors. In this case, the

farther the wires are placed apart, the more the inductance is reduced, up to a 50% reduction when set a few inches

apart. Splitting the wires connects two equal inductors in parallel. However, when placed near 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. In this case, two 18-AWG wires separated by 0.05 inches reduce the overall self-inductance to

about one-fourth of a single isolated wire. If the LT3077 is powered by a battery mounted near the ground and power

planes on the same circuit board, a 10μF input capacitor is sufficient for stability. If a distant supply powers the

LT3077, use a low ESR, large value input capacitor on the order of 220μF. As power supply output impedance varies,

the minimum input capacitance needed for application stability also varies.

20

0

–20

DC BIAS VOLTAGE (V)

0

2

4

6

8

10

12

Y5V

X5R

14

16

–40

–60

–80

–100

BOTH CAPACITORS ARE 16V

1210 CASE SIZE, 10µF

40

20

0

–20

TEMPERATURE (°C)

–50

–25

0

25

50

75

100

Y5V

X5R

125

–40

–60

–80

–100

BOTH CAPACITORS ARE 16V

1210 CASE SIZE, 10µF

A N A L O G D E V I C E S

C O N F I D E N T I A L