Datasheet

LT8619/LT8619-5
18
Rev A
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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 V
IN
if the
shutdown feature is not used.
Adding a resistor divider from V
IN
to EN/UV programs the
LT8619 to regulate the output only when V
IN
is above a
desired voltage (see the Block Diagram). Typically, this
threshold, V
IN(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
V
IN(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)
0
2
4
6
8
10
12
14
16
–100
–80
–60
–40
–20
0
20
CAPACITANCE CHANGE (%)
8619 F09
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