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LT8619/LT8619-5

8619f

For more information www.linear.com/LT8619

APPLICATIONS INFORMATION

an output capacitor, i.e. high volumetric efficiency with

extremely low equivalent resistance. There is a downside

however; the high K dielectric material exhibits a substan-

tial temperature and voltage coefficient, meaning that its

capacitance varies depending on the operating tempera-

ture and applied voltage. X7R capacitors provide a range

intermediate capacitance values which varies only ±15%

over the temperature range of –55°C to 125°C. The Y5V

capacitance can vary from 22% to –82% over the –30°C

to 85°C temperature range and should not be used for the

LT8619 application.

Figure 7 shows the voltage coefficient of four different

ceramic 22μF capacitors, all of which are rated for 16V

operation. Note that with the exception of the X7R in the

1210 and 1812 package, the capacitors lose more than

30% of their capacitance when biased at more than half of

the rated voltage. Typically, as the package size increases,

the bias voltage coefficient decreases. If the voltage coef-

ficient of a big ceramic capacitor in a particular pack-

age size is not acceptable; multiple smaller capacitors

with less voltage coefficient can be placed in parallel as

an effective means of meeting the capacitance require-

ment. Not All Capacitors are Interchangeable. A wrong

capacitor selection can degrade the circuit performance

considerably.

Ceramic capacitors can also cause problems due to their

piezoelectric nature. During Burst Mode operation, the

switching frequency depends on the load current, and at

very light loads the LT8619 can excite the ceramic capaci-

tor at frequencies that may generate audible noise. Since

the LT8619 operates at a lower inductor current during

Burst Mode operation, the noise is typically very quiet

to a casual ear. If this is unacceptable, consider using

a high performance tantalum or electrolytic capacitor at

the output instead. Low noise ceramic capacitors are also

available.

Ceramic capacitors are also susceptible to mechanical

stress which can result in significant loss of capacitance.

The most common sources of mechanical stress includes

bending or flexure of the PCB, contact pressure during in

circuit parameter testing, and direct contact by a solder-

ing iron tip. Consult the manufacturer’s application notes

for additional information regarding ceramic capacitor

handling.

Enable Pin

The LT8619 is in shutdown when the EN/UV pin is low

and active when the pin is high. The power-on threshold

of the EN comparator is 1.0V, with 40mV of hysteresis,

once EN/UV drops below this power-on threshold, the

MOSFETs are disabled, but the internal biasing circuit

stays alive. When forced below 0.56V, all the internal

blocks are disabled and the IC is put into a low current

shutdown mode. The EN/UV pin can be tied to VIN if the

shutdown feature is not used.

Adding a resistor divider from VIN to EN/UV programs the

LT8619 to regulate the output only when VIN is above a

desired voltage (see the Block Diagram). Typically, this

threshold, VIN(EN/UV), is used in situations where the input

supply is current limited, or has a relatively high source

resistance. A switching regulator draws constant power

from the source, so source current increases as source

voltage drops. This looks like a negative resistance load

to the source and can cause the source to current limit

or latch low under low source voltage conditions. The

VIN(EN/UV) threshold prevents the regulator from operating

at source voltages where the problems might occur. This

Figure 7. Ceramic Capacitor Voltage Coefficient

X7R, 1210

X7R, 1812

X5R, 1206

X5R, 0805

C3225X7R1C226K250

C4532X7R1C226M200

C3216X5R1C226M160

C2012X5R1C226K125

DC BIAS VOLTAGE (V)

–100

–80

–60

–40

–20

8619 F09

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