LT3932 datasheet(17/28 Pages) AD

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LT8386

Rev. 0

For more information www.analog.com

Adaptive Peak Switch Current Limit

The LT8386 peak switch current limit is 3.6A (typical)

when the VIN pin voltage is 9V and above. This means the

inductor peak current can rise up to 3.6A to deliver the

programmed LED current to the output. When the VIN pin

voltage decreases, the current limit is adaptively increased

so that the part can provide more power to the output

compared to the fixed 3.6A input current limit case. The

current limit increases up to 5A when the VIN pin voltage

is 5V and below. Figure 5 illustrates this adaptive peak

switch current limit over VIN pin voltages.

APPLICATIONS INFORMATION

the inductor current ripple amplitude to the average induc-

tor current value, both values of which are determined

by the input and output voltages, switching frequency,

efficiency and the inductor values. Hence, the minimum

inductor value LMIN that ensures the peak inductor cur-

rent below 3A is:

LMIN =

VIN(MIN) •(VOUT – VIN(MIN))

2 • VOUT • fSW

⎛

⎝

⎜

⎞

⎠

⎟

3 –

VOUT •ILED

VIN(MIN) •EFFICIENCY

⎛

⎝

⎜

⎞

⎠

⎟

(2)

Using this equation gives an inductance of about 8.1µH

assuming 90% efficiency for the given conditions.

With this minimum inductor value guideline, choose

an inductor with low core loss and low DC resistance.

Inductor 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 4.7µF ceramic capacitor should be sufficient

to provide these non-steady state currents. Place the input

capacitor close to the inductor. If possible, place an addi-

tional 1µF ceramic capacitor close to the VIN pin for better

noise immunity. Use X7R ceramic capacitors as they typi-

cally retain their capacitance better than other capacitor

types over wide voltage and temperature ranges.

If the input power source has high impedance, or there is

significant inductance due to long wires or cables, addi-

tional bulk electrolytic capacitance may be necessary. A

low ESR ceramic input capacitor combined with parasitic

Figure 5. Adaptive Peak Switch Current Limit

Selecting an Inductor

The LT8386 limits the inductor peak current to an input

current limit ILIM, which is a function of the VIN pin volt-

age, over the duty cycle without subharmonic oscillations.

This current limit will override the CTRL pin input com-

mand if the programmed LED current demands higher

inductor peak current than ILIM. Therefore, it is important

to select the inductor value to ensure the peak inductor

current is below the limit over the desired input voltage

range. The following is an example of inductor value deci-

sion process for the application where we want 500mA

LED current at 45V output, while the input ranges from

9V to 25V and the switching frequency is 2MHz. The ILIM

in this example will be 3A, which is the minimum ILIM that

LT8386 guarantees with 9V input voltage. The maximum

peak inductor current can be derived by adding the half of

VIN PIN VOLTAGE (V)

8386 F05

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