LT3922 datasheet(18/28 Pages) AD

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LT8386

Rev. 0

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APPLICATIONS INFORMATION

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 capaci-

tor, on the other hand, minimizes this ringing. Refer to the

Linear Technology Application Note 88 for more informa-

tion. Sources of quality ceramic and electrolytic capaci-

tors are listed in Table 3.

Table 3. Capacitor Manufacturers

MANUFACTURER

WEBSITE

Murata Manufacturing

www.murata.com

TDK

www.global.tdk.com

Panasonic

www.industrial.panasonic.com

Stabilizing the Regulation Loop

The LT8386 uses internal error amplifiers to regulate

the LED current and the output voltage to the user pro-

grammed values. The output impedance of the error

amplifiers and the external compensation capacitor, CC,

connected to VC pin create the dominant pole of the con-

trol 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 dur-

ing transient 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 capacitor needs to have very low ESR to

reduce the output ripple. Placing several low ESR ceramic

capacitors 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 maxi-

mum SW pin current. Use X7R ceramic capacitors as they

typically retain their capacitance better than other capaci-

tor types over wide voltage and temperature ranges.

The LT8386 utilizes a proprietary architecture to reduce

EMI noise generated by switching. Figure 6 shows the

VOUT capacitor placement for the LQFN package.

Figure 6. Placement of Output Capacitors

VOUT

COUT

10F

1206

GND

24 23

8386 F06

COUT1

0.1F

0402/0603

COUT2

0.1F

0402/0603

Selecting a MOSFET for PWM Dimming

Pulse-width-modulation (PWM) dimming of the LED cur-

rent is an effective way to control the brightness of the

light without varying its color. The brightness can also be

adjusted with finer resolution this way than by varying

the current level.

The LT8386 features a high-side PWMTG driver that

allows a high voltage PMOS switch to be positioned at

the top of the LED string after the output capacitor and

the current sense resistor. The high-side PWMTG driver is

similar to a low-side NMOS driver because in both cases

the LED current will be zero when the switch is open and

the string is disconnected, but in contrast the high-side

PMOS feature eliminates the need for a dedicated return

path for the LED current in automotive applications or

other grounded chassis systems.

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