LTC3419

10

3419f

deviations do not offer much relief. Note that capacitor

manufacturer’s ripple current ratings are often based on

only 2000 hours lifetime. This makes it advisable to further

derate the capacitor, or choose a capacitor rated at a higher

temperature than required. Several capacitors may also be

paralleled to meet the size or height requirements of the

design. An additional 0.1μF to 1μF ceramic capacitor is

when not using an all-ceramic capacitor solution.

series resistance (ESR). Typically, once the ESR require-

Δ≈ Δ

+

⎛

⎝⎜

⎞

⎠⎟

VI

ESR

fC

OUT

L

O OUT

1

8

voltage, the output ripple is highest at maximum input

If tantalum capacitors are used, it is critical that the capaci-

tors are surge tested for use in switching power supplies.

An excellent choice is the AVX TPS series of surface mount

tantalum. These are specially constructed and tested for low

ESR so they give the lowest ESR for a given volume. Other

capacitor types include Sanyo POSCAP, Kemet T510 and

T495 series, and Sprague 593D and 595D series. Consult

the manufacturer for other specific recommendations.

Using Ceramic Input and Output Capacitors

Higher values, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high

ripple current, high voltage rating and low ESR make

them ideal for switching regulator applications. Because

the LTC3419 control loop does not depend on the output

capacitor’s ESR for stable operation, ceramic capacitors

can be used freely to achieve very low output ripple and

small circuit size.

However, care must be taken when ceramic capacitors are

used at the input. When a ceramic capacitor is used at the

input and the power is supplied by a wall adapter through

long wires, a load step at the output can induce ringing at

and be mistaken as loop instability. At worst, a sudden

inrush of current through the long wires can potentially

part. For more information, see Application Note 88.

When choosing the input and output ceramic capacitors,

choose the X5R or X7R dielectric formulations. These

dielectrics have the best temperature and voltage charac-

teristics of all the ceramics for a given value and size.

Setting the Output Voltage

during regulation. Thus, the output voltage is set by a

resistive divider according to the following formula:

VV

R

R

⎛

⎝⎜

⎞

⎠⎟

06

1

2

1

2

.( )

Keeping the current small (< 10μA) in these resistors

maximizes efficiency, but making it too small may allow

stray capacitance to cause noise problems or reduce the

phase margin of the error amp loop.

To improve the frequency response of the main control

sources, such as the inductor or the SW line.

Fixed output versions of the LTC3419 (e.g. LTC3419-1)

include an internal resistive divider, eliminating the need

for external resistors. The resistor divider is chosen such

Table 2 lists the fixed output voltages available for the

LTC3419.

APPLICATIONS INFORMATION

Table 2. Fixed Output Voltage Versions

PART NUMBER

LTC3419

Adjustable

Adjustable

LTC3419-1

1.575V

1.8V