LT3954 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LT3954

40VIN LED Converter with Internal PWM Generator

Linear Technology 

LT3954

APPLICATIONS INFORMATION

Inductor Selection

The inductor used with the LT3954 should have a saturation

current rating appropriate to the maximum switch current

of 7.2A. Choose an inductor value based on operating

frequency, input and output voltage to provide a current

mode signal of approximately 0.8A magnitude. The follow-

ing equations are useful to estimate the inductor value for

continuous conduction mode operation (use the minimum

value for VIN and maximum value for VLED):

LBUCK

=

VLED(VIN – VLED)

VIN • 0.8A • fOSC

( ) LBUCK-BOOST =

VLED • VIN

VLED + VIN • 0.8A • fOSC

LBOOST

=

VIN(VLED – VIN)

VLED • 0.8A • fOSC

Use the equation for Buck-Boost when choosing an in-

ductor value for SEPIC – if the SEPIC inductor is coupled,

then the equation’s result can be used as is. If the SEPIC

uses two uncoupled inductors, then each should have a

inductance double the result of the equation.

Table 5 provides some recommended inductor vendors.

Table 5. Recommended Inductor Manufacturers

MANUFACTURER

WEB

Coilcraft

www.coilcraft.com

Cooper-Coiltronics

www.cooperet.com

Würth-Midcom

www.we-online.com

Vishay

www.vishay.com

Loop Compensation

The LT3954 uses an internal transconductance error

amplifier whose VC output compensates the control loop.

The external inductor, output capacitor and the compen-

sation 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

response and stability. For typical LED applications, a 4.7nF

compensation capacitor at VC is adequate, and a series

resistor should always be used to increase the slew rate

on the VC pin to maintain tighter regulation of LED current

during fast transients on the input supply to the converter.

Disconnect Switch Selection

An NMOS in series with the LED string at the cathode is

recommended in most LT3954 applications to improve

the PWM dimming. The NMOS BVDSS rating should be as

high as the open LED regulation voltage set by the FB pin,

which is typically the same rating as the power switch of the

converter. The maximum continuous drain current ID(MAX)

rating should be higher than the maximum LED current.

A PMOS high side disconnect is needed for buck mode,

buck-boost mode or an output short circuit protected

boost. A level shift to drive the PMOS switch is shown

in the application schematic Boost LED Driver with Out-

put Short Circuit Protection. In the case of a high side

disconnect follow the same guidelines as for the NMOS

regarding voltage and current ratings. It is important to

include a bypass diode to GND at the drain of the PMOS

switch to ensure that the voltage rating of this switch is

not exceeded during transient fault events.

The DC-Coupling Capacitor Selection for SEPIC

LED Driver

The DC voltage rating of the DC-coupling capacitor CDC

connected between the primary and secondary inductors of

a SEPIC should be larger than the maximum input voltage:

VCDC > VIN(MAX)

CDC has nearly a rectangular current waveform. During

the switch off-time, the current through CDC is IVIN, while

approximately –ILED flows during the on-time. The CDC

voltage ripple causes current distortions on the primary

and secondary inductors. The CDC should be sized to limit

its voltage ripple. The power loss on the CDC ESR reduces

3954fa

20

For more information www.linear.com/3954

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