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LT8619/LT8619-5

8619f

For more information www.linear.com/LT8619

APPLICATIONS INFORMATION

stay in sleep mode longer between each pulse. This can

be achieved by using a larger value inductor, and should

be considered independent of switching frequency when

choosing an inductor. For example, while a lower inductor

value would typically be used for a high switching fre-

quency application, if high light load efficiency is desired,

a higher inductor value should be chosen.

The optimum inductor for a given application may differ

from the one indicated by this design guide. A larger value

inductor provides a higher maximum load current and

reduces the output voltage ripple. For applications requir-

ing smaller load currents, the value of the inductor may

be lower and the LT8619 may operate with higher ripple

current. This allows you to use a physically smaller induc-

tor, or one with a lower DCR resulting in higher efficiency.

Be aware that low inductance may result in discontinuous

mode operation, which further reduces maximum load

current.

For details of maximum output current and discontinuous

operation, see Analog Devices’s Application Note 44.

Input Capacitor

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage rip-

ple at the LT8619 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

In continuous mode, the input capacitor RMS current is

given by:

IRMS(MAX) ≈ILOAD(MAX)

VOUT VIN – VOUT

(

)

VIN

This equation has a maximum RMS current at VIN =

2VOUT, where IRMS(MAX) = ILOAD(MAX)/2.

Bypass the input of the LT8619 circuit with a 2.2μF to

10μF ceramic capacitor of X7R or X5R type placed as

close as possible to the VIN and GND pin. Y5V types have

poor performance over temperature and applied voltage,

and should not be used. Note that larger input capacitance

is required when a lower switching frequency is used.

If the input power source has high impedance, or there

is significant inductance due to long wires or cables, a

ceramic input capacitor combined with the trace or cable

inductance forms a high quality (underdamped) tank cir-

cuit. If the LT8619 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the LT8619’s voltage rating. This situation is

easily avoided (see Analog Devices Application Note 88),

by adding a lossy electrolytic capacitor in parallel with the

ceramic capacitor.

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated

by the LT8619 to produce the DC output. In this role it

determines the output ripple, thus low impedance at the

switching frequency is important. The second function

is to store energy in order to satisfy transient loads and

stabilize the LT8619’s control loop. The current slew rate

of a regulator is limited by the inductor and feedback loop.

When the amount of current required by the load changes,

the initial current deficit must be supplied by the output

capacitor until the feedback loop reacts and compensates

for the load changes. Ceramic capacitors have very low

equivalent series resistance (ESR) and provide the best

ripple performance. For good starting values, see the

Typical Applications section.

Transient performance can be improved with a higher

value capacitor and the addition of a feedforward capaci-

tor placed between VOUT and FB. Increasing the output

capacitance will also decrease the output voltage ripple. A

lower value of output capacitor can be used to save space

and cost but transient performance will suffer and may

cause loop instability. See the Typical Applications in this

data sheet for suggested capacitor values.

Ceramic Capacitors

When choosing a capacitor, special attention should be

given to the manufacturer’s data sheet in order to accu-

rately calculate the effective capacitance under the rel-

evant bias voltage and operating temperature conditions.

Ceramic dielectrics can offer near ideal performance as

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