Datasheet

LTC4089-3
19
40893f
For more information www.linear.com/4089-3
The high voltage regulator output capacitor controls output
ripple, supplies transient load currents, and stabilizes the
regulator control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good value is 10µF. Use X5R or
X7R types, and note that a ceramic capacitor biased with
V
HVOUT
will have less than its nominal capacitance. Table2
lists several capacitor vendors.
Table 2: Capacitor Vendors
VENDOR PHONE URL
PART
SERIES COMMENTS
Panasonic [phone] www.panasonic.com Ceramic,
Polymer,
Tantalum
EEF Series
Kemet
[phone] www.kemet.com Ceramic,
Tantalum
T494,
T495
Sanyo
[phone] www.sanyovideo.com Ceramic,
Polymer,
Tantalum
POSCAP
Murata
[phone] www.murata.com Ceramic
AVX www.avxcorp.com Ceramic,
Tantalum
TPS
Series
Taiyo Yuden
[phone] www.taiyo-yuden.com Ceramic
BOOST Pin Considerations
Capacitor C3 and diode D2 (see Block Diagram) are used
to generate a boost voltage that is higher than the input
voltage. In most cases, a 0.1µF capacitor and fast-switch-
ing diode (such as the 1N4148 or 1N914) will work well.
The BOOST pin must be at least 2.2V above the SW pin
for proper operation.
High Voltage Regulator Soft-Start
The HVEN pin can be used to soft-start the high voltage
regulator and reduce the maximum input current during
start-up. A voltage ramp at the HVEN pin can be created by
driving the pin through an external RC filter (see Figure 6).
By choosing a large RC time constant, the peak start-up
current will not overshoot the current that is required to
regulate the output. Choose the value of the resistor so that
it can supply 20µA when the HVEN pin reaches 2.3V.
Figure 6. Using the HVEN Pin to Soft-Start the
High Voltage Regulator.
APPLICATIONS INFORMATION
RUN
15k
0.1µF
HVEN
GND
40893 F06
LTC4089-3
Alternate NTC Thermistors
The LTC4089-3 NTC trip points were designed to work
with thermistors whose resistance-temperature charac-
teristics follow Vishay Dale’s “R-T Curve 2.” The Vishay
NTHS0603N02N1002J is an example of such a thermis-
tor. However, Vishay Dale has many thermistor products
that follow the “R-T Curve 2” characteristic in a variety of
sizes. Furthermore, any thermistor whose ratio of R
COLD
to
R
HOT
is about 6.0 will also work (Vishay Dale R-T Curve2
shows a ratio of 2.815/0.4839 = 5.82).
Power conscious designs may want to use thermistors
whose room temperature value is greater than 10k. Vishay
Dale has a number of values of thermistor from 10k to 100k
that follow the “R-T Curve 2.” Using these as indicated
in the NTC Thermistor section will give temperature trip
points of approximately 3°C and 42°C, a delta of 39°C.
This delta in temperature can be moved in either direc-
tion by changing the value of R
NOM
with respect to R
NTC
.
Increasing R
NOM
will move both trip points to lower
temperatures. Likewise, a decrease in R
NOM
with respect
to R
NTC
will move the trip points to higher temperatures.
To calculate R
NOM
for a shift to lower temperature, for
example, use the following equation:
R
NOM
=
R
COLD
2.816
•R
NTC
at 25°C