LTM8024 datasheet(14/26 Pages) AD

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LTM4709

Rev. 0

For more information www.analog.com

range. The X5R and X7R dielectrics result in more sta-

ble characteristics and are more suitable for use as the

output capacitor.

DC BIAS VOLTAGE (V)

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

–100

–80

4709 F04

–60

–40

X5R

Y5V

–20

Figure 4. Ceramic Capacitor DC Bias Characteristics

The X7R type has better stability across temperatures,

while the X5R is less expensive and is available in higher

values. Care still must be taken when using X5R and

X7R capacitors; the X5R and X7R codes only specify the

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 significant enough to drop

capacitor values below appropriate levels. Capacitor DC

bias characteristics tend to improve as component case

size increases but expected capacitance at operating

voltage should be verified. Voltage and temperature coeffi-

cients are not the only source of problems. Some ceramic

capacitors have a piezoelectric response. A piezoelectric

device generates a voltage across its terminals due to

mechanical stress, like the way a piezoelectric micro-

phone works. For a ceramic capacitor, the stress can be

induced by vibrations in the system or thermal transients.

Stability and Input Capacitance

The LTM4709 is stable with a minimum capacitance of

4.7μF connected to the VIN pins. Use low ESR capac-

itors to minimize instantaneous voltage drops under

large load step conditions. Large VIN droop during

transient may cause the regulator to enter dropout with

the corresponding degradation in load transient response.

Additional 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 VIN should be

less than 20mΩ to support a large change in the load step.

In case where wire is used to connect a power supply to

the input of the LTM4709 (and from the ground of the

LTM4709 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

LTM4709 being unstable. The self-inductance is directly

proportional to its length. The width of a routing trace

does not have a major influence on its self-inductance as

compared to its length. For example, 1 inch of 18-AWG,

0.04-inch-wide routing trace has 28nH of self-inductance.

The self-inductance of a 2-AWG trace with a width of 0.26-

inch is about half the inductance of the 18-AWG wire.

The overall self-inductance of a trace can be reduced in

two ways. One is to divide the current flow toward the

LTM4709 between two parallel conduction paths. In this

case, the farther the wires are placed apart from each

other, the more the inductance is reduced, up to a 50%

reduction when placed a few inches apart. Splitting the

wires basically 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 returning conductors (the trace for the

input and the trace for the return ground) in very close

proximity. In this case, two 18-AWG wires separated by

0.05-inch reduces the overall self-inductance to about

one-fourth of a single trace. If the LTM4709 is powered

by a battery mounted in proximity with ground and power

planes on the same circuit board, a 10μF input capacitor

is sufficient for stability. If, however, the LTM4709 is pow-

ered by a distant supply, use a low ESR, large value input

capacitor in the order of 220μF.

Bias Requirement

The BIAS1,2,3 pin supplies current to the internal control

circuitry and the output stage driving the pass transistor.

APPLICATIONS INFORMATION

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