Datasheet

LTC3114-1
23
Rev. C
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APPLICATIONS INFORMATION
applications are generally similar except that voltage ripple
is generally not a concern. Some capacitors exhibit a high
DC leakage current which may preclude their consideration
for applications that require a very low quiescent current
in Burst Mode operation.
Ceramic capacitors are often utilized in switching con
-
verter applications due to their small size, low ESR and
low leakage currents. However, many ceramic capacitors
intended for power applications experience a significant
loss in capacitance from their rated value as the DC bias
voltage on the capacitor increases. It is not uncommon
for a small surface mount capacitor to lose more than
50% of its rated capacitance when operated near its
maximum rated voltage. This effect is generally reduced
as the case size is increased for the same nominal value
capacitor. As a result, it is often necessary to use a larger
value capacitance or a higher voltage rated capacitor than
would ordinarily be required to actually realize the intended
capacitance at the operating voltage of the application. X5R
and X7R dielectric types are recommended as they exhibit
the best performance over the wide operating range and
temperature of the LTC3114-1. To verify that the intended
capacitance is achieved in the application circuit, be sure
to consult the capacitor vendor’s curve of capacitance
versus DC bias voltage.
Programming Custom V
IN
Turn-On and Turn-Off
Thresholds
With the addition of an external resistor divider connected
to the input voltage as shown in Figure3, the RUN pin
can be used to program the input voltage at which the
LTC3114-1 is enabled and disabled.
For a rising input voltage, the LTC3114-1 is enabled when
V
IN
reaches the threshold given by the following equation,
where R1 and R2 are the values of the resistor divider
resistors specified in kΩ:
V
TH(RISING)
= 1.2
R1+R2
R2
⎛
⎝
⎜
⎞
⎠
⎟
Volts
Once the IC is enabled, it will remain so until the input
voltage drops below the comparator threshold by the
hysteresis voltage of approximately 100mV, measured
on the RUN pin. Therefore, the amount of hysteresis is
approximately 8.33% of the programmed turn-on threshold
level given in the previous equation.
Bootstrapping the LDO Regulator
The
high and low side gate drivers are powered through the
PLDO rail, which is generated from the input voltage, V
IN
,
through an internal linear regulator. In some applications,
especially at high input voltages, the power dissipation
in the linear regulator can become a major contributor
to thermal heating of the IC. The Typical Performance
Characteristics section of this data sheet provides data on
the LDO/PLDO current and resulting power loss versus
V
IN
and V
OUT
. A significant performance advantage can
be attained in applications where converter output voltage
(V
OUT
) is programmed to 5V, if V
OUT
is used to power the
LDO/PLDO rails. Powering the LDO/PLDO rails in this
manner is referred to as bootstrapping. This can be done
by connecting a Schottky diode from V
OUT
to LDO/PLDO
as shown in Figure5. With the bootstrap diode installed,
the gate driver currents are supplied by the buck-boost
converter at high efficiency rather than through the inter
-
nal linear regulator. The internal linear regulator contains
reverse blocking cir
cuitry that allows LDO/PLDO pins to
be driven slightly above their nominal regulation level with
only a very slight amount of reverse current. Please note
that the bootstrapping supply (either V
OUT
or a separate
regulator) must be limited to less than 5.7V.
V
OUT
4.7µF
31141 F05
PV
OUT
LTC3114-1
LDO
PLDO
Figure5. Bootstrapping PLDO and LDO
Average Output Current Limit Programming
The LTC3114-1 includes an average output current pro-
gramming feature that transforms the LT
C3114-1 into a
wide voltage compliance range, high efficiency, constant
current source. A resistor from PROG to ground programs
the desired level of average output current up to 1A.
Potential uses include high brightness LED driving and
constant current battery or capacitor charging.
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