LT3476EUHF(RevA) Просмотр технического описания (PDF) - Linear Technology

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LT3476EUHF
(Rev.:RevA)

High Current Quad Output LED Driver

Linear Technology 

LT3476

APPLICATIO S I FOR ATIO

Ceramic type capacitors using X7R dielectric are best for

temperature and DC bias stability of the capacitor value.

All ceramic capacitors exhibit loss of capacitance value

with increasing DC voltage bias, so it may be necessary to

choose a higher value capacitor or larger case size to get

the required capacitance at the operating voltage. Always

check that the voltage rating of the capacitor is sufficient.

Table 3 shows some recommended capacitor vendors.

Table 3. Low-ESR Surface Mount Capacitors

VENDOR

TYPE

Taiyo-Yuden

Ceramic

AVX

Ceramic

Murata

Ceramic

SERIES

X5R, X7R

X5R, X7R

X5R, X7R

Compensation Design

The LT3476 uses an internal transconductance error

amplifier whose VC output compensates the control loop.

The external inductor, output capacitor, and compensa-

tion resistor and capacitor determine the loop stability.

The inductor and output capacitor are chosen based on

performance, size and cost. The compensation resistor

and capacitor at VC are selected to optimize control loop

stability. The component values shown in the typical ap-

plications circuits yield stable operation over the given

range of input-to-output voltages and load currents. For

most buck applications, a small filter capacitor (1μF or

less) across the load is desirable. In this case, a 10nF

compensation capacitor at VC is usually quite adequate.

A compensation resistor of 5kΩ placed between the VC

output and the compensation capacitor minimizes chan-

nel-to-channel interaction by reducing transient recovery

time. The boost configuration will have a larger output

capacitor, 2.2μF to 10μF.

The following circuit techniques involving the compensa-

tion pin may be helpful where there is a large variation in

programmed LED current, or a large input supply range is

expected. At low duty cycles (TON less than 350ns) and low

average inductor current (less than 500mA), the LT3476

may start to skip switching pulses to maintain output

regulation. Pulse-skipping mode is usually less desirable

because it leads to increased ripple current in the LED.

To improve the onset of pulse-skipping behavior, place a

capacitor between the SW node and the compensation

capacitor that is 1:1000 the value of the compensation

capacitor. In the buck configuration, an additional tech-

nique is available. The filter capacitor between the CAP

node and the LED bottom (see the Typical Application on

the first page) can be moved to between the LED top and

the LED bottom. This circuit change places the inductor

ripple current through the sense resistor, which improves

pulse-skipping behavior. There is usually less than 1%

impact to the current regulation point.

Diode Selection

The Schottky rectifier conducts current during the interval

when the switch is turned off. Select a diode with VR rated

for the maximum SW voltage. For boost circuits that may

use the output disconnect feature, the diode should be

rated for at least 40V. It is not necessary that the forward

current rating of the diode equal the switch current limit.

The average current IF through the diode is a function

of the switch duty cycle, so select a diode with forward

current rating of IF = 1.5A • (1-D). If using the PWM fea-

ture for dimming, it may also be important to consider

diode leakage from the output (especially at hot) during

the PWM low interval. Table 4 has some recommended

component vendors.

Table 4. Schottky Diodes

VR

PART NUMBER

(V)

On Semiconductor

MBRM140

40

Diodes Inc.

DFLS140L

40

B140 HB

40

Philips Semiconductor

PMEG4010EJ

40

IAVE

VF AT 1A

(A)

(mV)

1

550

1

550

1

530

1

540

3476fa

9

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