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

will give better dimming control since for turning on the

switch, the state of the PWM pin is sampled only during a

narrow time slot at the beginning of each switch period.

Inductor Selection

The inductors used with the LT3476 should have a satura-

tion current rating of 2.5A or greater. For best loop stability

results, the inductor value selected should provide a ripple

current of 350mA or more. For buck (step-down) or boost

(step-up) configurations, and using a 21kΩ resistor on

RT (TSW ~ 1μs), inductor values from 4.7μH to 10μH are

recommended for most applications. In the buck mode,

the inductor value can be estimated using the formula:

L(μH) =

DBUCK

• TSW (μS) • (VCAP

ΔI

−

VLED ),

DBUCK

=

VL E D

VCAP

VLED is the voltage across the LED string and VCAP is the

input voltage to the converter. In the boost mode, the

inductor value can be estimated using the formula:

L(μH) = DBOOST • TSW (μS) • VIN ,

ΔI

DBOOST

=

VCAP − VIN

VCAP

VIN is the input voltage and VCAP is the voltage across

the LED string. Table 2 below provides some suggested

components and vendors.

Table 2. Inductors

PART NUMBER

VALUE

(μH)

Sumida

CDRH6D38-100

10

CDRH5D28-5R3

5.3

CDRH73-100

10

Toko

D63CB

10

D63CB

4.7

Cooper-ET

SD25-4R7

4.7

IRMS

(A)

2.0

1.90

1.68

1.49

2.08

1.80

DCR

HEIGHT

(Ω)

(mm)

0.028

4.0

0.028

3.0

0.072

3.4

0.042

3.5

0.026

3.5

0.047

2.5

8

Input Capacitor Selection

For proper operation, it is necessary to place a bypass

capacitor to GND close to the VIN pin of the LT3476. A

1μF, or greater, capacitor with low ESR should be used.

A ceramic capacitor is usually the best choice.

In the buck configuration, the capacitor at the input to the

power converter has large pulsed currents due to the cur-

rent returned through the Schottky diode when the switch

is off. For best reliability, this capacitor should have low

ESR and ESL and meet the ripple current requirement,

IRMS = ISW • ((1− D) • D)

where D is the switch duty cycle. A 2.2μF ceramic type

capacitor placed close to the Schottky and the ground

plane is usually sufficient for each channel.

Output Capacitor Selection

The selection of output filter capacitor depends on the load

and the converter configuration, i.e., step-up or step-down.

For LED applications, the equivalent resistance of the LED

is typically low, and the output filter capacitor should be

sized to attenuate the current ripple from the inductor to

35mA or less. The following equation is useful to estimate

the required capacitor value:

CFILT

=

2

•

TSW

RLED

A typical filter capacitor value for RLED = 5Ω and TSW =

1μs is 0.47μF. For loop stability, consider the output pole

is at the frequency where closed loop gain should be

unity, so the dominant pole for loop compensation will

be established by the capacitor at the VC input.

For the LED boost applications, to achieve the same LED

ripple current the required filter capacitor value is about

five times larger than the value calculated above due to

the pulsed nature of the source current. A 2.2μF ceramic

type capacitor placed close to the Schottky and the ground

plane of the IC is usually sufficient for each channel.

As the output capacitor is subject to high ripple current,

ceramic capacitors are recommended due to their low

ESR and ESL at high frequency.

3476fa

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