LT3755 datasheet(16/30 Pages) LINEAR

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LT3922

3922f

For more information www.linear.com/LT3922

applicaTions inForMaTion

Using this equation gives an inductance of about 2µH as-

suming 90% efficiency for the given conditions.

With this minimum inductor value guideline, choose an

inductor with low core loss and low DC resistance. In-

ductor must be able to handle the peak inductor current

without saturation. To minimize the radiated noise, use a

shielded inductor. The manufacturers featured in Table 2

are recommended sources of inductors.

Table 2. Inductor Manufacturers

MANUFACTURER

WEBSITE

Wurth Electronics

www.we-online.com

Coilcraft

www.coilcraft.com

Vishay Intertechnology

www.vishay.com

Selecting an Input Capacitor

The input capacitor supplies the inductor ripple current

and the transient current that occurs in PWM dimming

operations. A 10µF ceramic capacitor should be sufficient

to provide these non-steady state currents. Place the

input capacitor close to the inductor. If possible, place

an additional 1µF ceramic capacitor close to the VIN pin

for better noise immunity. Use X7R ceramic capacitors

as they typically retain their capacitance better than other

capacitortypesoverwidevoltageandtemperatureranges.

If the input power source has high impedance, or there

is significant inductance due to long wires or cables, ad-

ditional bulk electrolytic capacitance may be necessary.

A low ESRlow ESR ceramic input capacitor combined

with parasitic inductances in the current paths can form a

high-Q LC tank circuit which can ring the capacitor voltage

up to twice the input voltage. A higher ESR electrolytic

capacitor, on the other hand, minimizes this ringing. Refer

to the Linear Technology Application Note 88 for more

information. Sources of quality ceramic and electrolytic

capacitors are listed in Table 3.

Table 3. Capacitor Manufacturers

MANUFACTURER

WEBSITE

Murata Manufacturing

www.murata.com

Garrett Electronics

www.garrettelec.com

Panasonic

www.industrial.panasonic.com

Nippon Chemi-Con

www.chemi-con.co.jp

Stabilizing the Regulation Loop

The LT3922 uses internal error amplifiers to regulate the

LEDcurrentandtheoutputvoltagetotheuserprogrammed

values. The output impedance of the error amplifiers

and the external compensation capacitor, CC, connected

to VC pin create the dominant pole of the control loop.

The compensation resistor, RC, in series with CC forms

a left-half-plane (LHP) zero. This LHP zero allows better

regulation of LED current and output voltage during tran-

sient operations. For most LED applications, 1nF and 10k

would be good starting values for CC and RC, respectively.

Refer to the Linear Technology Application Note 76 for

more information.

Selecting and Placing Output Capacitors

The output capacitors need to have very low ESR to reduce

the output ripple. Placing several low ESR ceramic capaci-

tors in parallel is an effective way to reduce ESR. These

output capacitors in a boost converter should have a ripple

current rating greater than the half of the maximum SW

pin current. Use X7R ceramic capacitors as they typically

retain their capacitance better than other capacitor types

over wide voltage and temperature ranges.

The LT3922 utilizes a proprietary architecture to reduce

EMI noise generated by switching. To best utilize this

feature, VOUT should be bypassed with three capacitors.

Figure 5 shows the VOUT capacitor placements for the

QFN package. COUT1 and COUT2 are 0402-0.47µF ceramic

capacitors placed as close as possible to the LT3922’s

VOUT and GND pins. COUT3 should be larger in size and

value. A 1206-4.7µF ceramic capacitor is recommended

for typical applications.

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