LT3757 datasheet(15/22 Pages) AD

15 / 22 page

LT3935

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

The DA limit is relevant only when the output is shorted to

GND. When the LED string is shorted to GND, the voltage

across is high enough that the required on-time is greater

than the minimum on-time. This means that, in spite of a

shortedLEDstring,theinductorcurrentremainsinregula-

tion even at the highest switching frequency.

Selecting an Inductor

The inductor must be rated for the current limit regardless

of the intended application. Its value, in most applications,

should be selected such that the inductor current ripple is

not more than 25% of the maximum output current. When

that current is 4A, for example, the minimum inductance

can be calculated using the following equation:

L = 1µH •

VOUT

VIN(MAX)

•

VIN(MAX) – VOUT

• 1MHz

fSW

However,forhighoutputvoltageseventheaboveequation

would suggest an inductance value that is too small. For

stability, the LT3935 requires an inductance greater than:

L =

1µH

•

VOUT

• 1MHz

fSW

Choose the larger of the values given by these equations.

The manufacturers featured in Table 2 are recommended

sources of inductors.

Table 2. Inductor Manufacturers

MANUFACTURER

WEBSITE

Wurth Elektronik

www.we-online.com

Coilcraft

www.coilcraft.com

Selecting an Output Capacitor

SomeapplicationsaresensitivetoripplecurrentintheLED

string. In those cases, a capacitor at the output will absorb

part of the inductor current ripple and thereby reduce the

LED current ripple. Typically, the value of this capacitance

is inversely proportional to the switching frequency and

the output voltage as shown below:

COUT = 100µF •

VOUT

•

1MHz

fSW

However, applications may still be stable with more or

less capacitance, and more capacitance may improve LED

current waveforms.

Use X7R or X5R ceramic capacitors as they retain their

capacitance better than other capacitor types over a wide

voltageandtemperaturerange.Sourcesofqualityceramic

and electrolytic capacitors are listed in Table 3.

Table 3. Capacitor Manufacturers

MANUFACTURER

WEBSITE

Murata Manufacturing

www.murata.com

Garrett Electronics

www.garrettelec.com

AVX

www.avx.com

Nippon Chemi-Con

www.chemi-con.co.jp

Stabilizing the Regulation Loop

Stabilizing the regulation loop typically requires only a

capacitor CC connected from the VC pin to GND. For most

designs, values between 1nF and 10nF are suitable. When

using an output capacitor COUTlargerthan10µF,aresistor

RC in series with CC is also necessary. Larger valves of

COUT require larger values of RC. See Typical Applications

for some examples.

Selecting and Placing the Input Capacitors

Although they do not impact stability, several capacitors

are necessary between VIN and GND to properly bypass

the input supply voltage. At least 10μF is required in total,

althoughitdoesnothavetobecomposedentirelyofceramic

capacitors placed very close to the VIN pins. However, it

is important that a ceramic capacitor be placed as close

Figure 5. Extended Off-Time at Current Limit

OUTPUT SHORTED

DA LIMIT

100µs/DIV

INDUCTOR

CURRENT

2A/DIV

3935 F05

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