13

LT3020/LT3020-1.2/

LT3020-1.5/LT3020-1.8

3020fc

APPLICATIO S I FOR ATIO

and the output voltage clamps at a Schottky diode above

ground. Please note that the behavior described above

applies to the LT3020 only. If a resistor divider is con-

nected under the same conditions, there will be additional

V/R current.

In circuits where a backup battery is required, several

different input/output conditions can occur. The output

voltage may be held up while the input is either pulled to

ground, pulled to some intermediate voltage or is left open

circuit. In the case where the input is grounded, there is

less than 1

µA of reverse output current.

If the LT3020 IN pin is forced below the OUT pin or the OUT

pin is pulled above the IN pin, input current drops to less

than 10

µA typically. This occurs if the LT3020 input is

connected to a discharged (low voltage) battery and either

a backup battery or a second regulator circuit holds up the

output. The state of the SHDN pin has no effect on the

reverse output current if OUT is pulled above IN.

Input Capacitance and Stability

The LT3020 is designed to be stable with a minimum

capacitance of 2.2

µF placed at the IN pin. Ceramic capaci-

tors with very low ESR may be used. However, in cases

where a long wire is used to connect a power supply to the

input of the LT3020 (and also from the ground of the

LT3020 back to the power supply ground), use of low

value input capacitors combined with an output load

current of 20mA or greater may result in 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 LT3020 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, the self inductance of a 2-AWG

isolated wire with a diameter of 0.26 in. is about half the

inductance of a 30-AWG wire with a diameter of 0.01 in.

One foot of 30-AWG wire has 465nH of self inductance.

The overall self-inductance of a wire can be reduced in two

ways. One is to divide the current flowing towards the

LT3020 between two parallel conductors. 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 ground) in very close

proximity. Two 30-AWG wires separated by 0.02 in. re-

duce the overall self-inductance to about one-fifth of a

single isolated wire.

If the LT3020 is powered by a battery mounted in close

proximity on the same circuit board, a 2.2

µF input capaci-

tor is sufficient for stability. However, if the LT3020 is

powered by a distant supply, use a larger value input

capacitor following the guideline of roughly 1

µF (in addi-

tion to the 2.2

µF minimum) per 8 inches of wire length. As

power supply output impedance may vary, the minimum

input capacitance needed to stabilize the application may

also vary. Extra capacitance may also be placed directly on

the output of the power supply; however, this will require

an order of magnitude more capacitance as opposed to

placing extra capacitance in close proximity to the LT3020.

Furthermore, series resistance may be placed between the

supply and the input of the LT3020 to stabilize the appli-

cation; as little as 0.1

Ω to 0.5Ω will suffice.