LT3498 Просмотр технического описания (PDF) - Linear Technology

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LT3498

20mA LED Driver and OLED Driver with Integrated Schottky in 3mm x 2mm DFN

Linear Technology 

LT3498

OPERATION— OLED DRIVER

The low noise boost of the LT3498 uses a novel control

scheme to provide high efficiency over a wide range

of output current. In addition, this technique keeps the

switching frequency above the audio band over all load

conditions.

The operation of the part can be better understood by

referring to the Block Diagram. The part senses the

output voltage by monitoring the voltage on the FB2 pin.

The user sets the desired output voltage by choosing the

value of the external topside feedback resistor. The part

incorporates a precision 182kΩ bottom-side feedback

resistor. Assuming that output voltage adjustment is not

used (CTRL2 pin is tied to 1.5V, or greater), the internal

reference (VREF = 1.215V) sets the voltage to which FB2

will servo during regulation.

The Switch Control block senses the output of the ampli-

fier and adjusts the switching frequency, as well as other

parameters to achieve regulation. During the start-up of

the circuit, special precautions are taken to ensure that

the inductor current remains under control.

Because the switching frequency is never allowed to fall

below approximately 50kHz, a minimum load must be

present to prevent the output voltage from drifting too

high. This minimum load is automatically generated within

the part via the Shunt Control block. The level of this current

is adaptable, removing itself when not needed to improve

efficiency at higher load levels.

The low-noise boost of the LT3498 also has an integrated

Schottky diode and PMOS output disconnect switch. The

PMOS switch is turned on when the part is enabled. When

the part is in shutdown, the PMOS switch turns off, allowing

the VOUT2 node to go to ground. This type of disconnect

function is often required in power supplies.

APPLICATIONS INFORMATION— LED DRIVER

Inductor Selection

A 15μH inductor is recommended for most applications

for the LED driver of the LT3498. Although small size and

high efficiency are major concerns, the inductor should

have low core losses at 2.3MHz and low DCR (copper

wire resistance). Some small inductors in this category

are listed in Table 1. The efficiency comparison of different

inductors is shown in Figure 2.

Table 1: Recommended Inductors

PART

MAX CURRENT

L DCR RATING

(μH) (Ω)

(mA) VENDOR

LQH32CN150K53 15 0.58

LQH2MCN150K02 15 1.6

LQH32CN100K53 10 0.3

LQH2MCN100K02 10 1.2

300 Murata

200 www.murata.com

450

225

SD3110-150

15 0.764

380 Cooper

www.cooperet.com

1001AS-150M

(TYPE D312C)

D03314-153ML

15 0.80

15 0.86

360 Toko

www.toko.com

680 Coilcraft

www.coilcraft.com

80

75

70

65

60

55

15uH Murata LQH32CN150K53

15uH Murata LQH2MCN150K02

15uH Cooper SD3110-150

50

15uH Toko D312C

15uH Coilcraft DO3314-153ML

45

0

5

10

15

20

LED CURRENT (mA)

3498 F02

Figure 2. Efficiency Comparison of Different Inductors

Capacitor Selection

The small size of ceramic capacitors makes them ideal for

LT3498 LED driver applications. Use only X5R and X7R

types, because they retain their capacitance over wider

temperature ranges than other types, such as Y5V or

Z5U. A 4.7μF input capacitor and a 1μF output capacitor

are sufficient for most applications.

3498fa

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