LTC4089 datasheet(18/24 Pages) LINER

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LTC4089/LTC4089-5

40895fb

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

VHVOUT will have less than its nominal capacitance. Table

2 lists several capacitor vendors.

Table 2: Capacitor Vendors

VENDOR

PHONE

URL

PART

SERIES

COM-

MENTS

Panasonic (714) 373-7366

www.panasonic.com

Ceramic,

Polymer,

Tantalum

EEF Series

Kemet

(864) 963-6300

www.kemet.com

Ceramic,

Tantalum

T494,

T495

Sanyo

(408) 749-9714

www.sanyovideo.com

Ceramic,

Polymer,

Tantalum

POSCAP

Murata

(404) 436-1300

www.murata.com

Ceramic

AVX

www.avxcorp.com

Ceramic,

Tantalum

TPS

Series

Taiyo

Yuden

(864) 963-6300

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.

RUN

15k

0.1

µF

HVEN

GND

4089 F06

LTC4089

Alternate NTC Thermistors

The LTC4089/LTC4089-5 NTC trip points were designed

to work with thermistors whose resistance-temperature

characteristics follow Vishay Dale’s “R-T Curve 2.” The

Vishay NTHS0603N02N1002J is an example of such a

thermistor. 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 RCOLD to RHOT is about 7.0 will also work (Vishay Dale

R-T Curve 2 shows a ratio of 2.815/0.4086 = 6.89).

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 47°C, a delta of 44°C.

This delta in temperature can be moved in either direc-

tion by changing the value of RNOM with respect to RNTC.

APPLICATIO S I FOR ATIO

Figure 6. Using the HVEN Pin to Soft-Start the

High Voltage Regulator.

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