LTC3454EDD View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC3454EDD

1A Synchronous Buck-Boost High Current LED Driver

Linear Technology 

LTC3454

TYPICAL APPLICATIONS

diode such as an MBRM120T3 or equivalent. Do not use

ordinary rectifier diodes, since the slow recovery times

will compromise efficiency.

In applications in which VIN is greater than 4V and VOUT to

GND short-circuit protection is needed, a Schottky diode

such as MBRM120T3 or equivalent may be used from

GND to SW1 and/or a 2Ω/1nF series snubber from SW1

to GND. The Schottky diode should be added as close

to the pins as possible. Neither of these is required for

shorted LED protection.

In applications in which VIN is greater than 4.5V, a Schottky

diode such as MBRM120T3 or equivalent may be required

from SW1 to VIN if the LTC3454 is enabled with an output

voltage already present. The Schottky diode should be

added as close to the pins as possible.

Closing the Feedback Loop

The LTC3454 incorporates voltage mode PWM control.

The control to output gain varies with operation region

(buck, boost, buck/boost), but is usually no greater than

15. The output filter exhibits a double pole response

given by:

fFILTER_POLE = 2 • π •

1

Hz

L • COUT

where COUT is the output filter capacitor.

The output filter zero is given by:

fFILTER_

ZERO

=

2

•

π

•

1

RESR

•

COUT

Hz

where RESR is the capacitor equivalent series resistance.

A troublesome feature in boost mode is the right-half plane

zero (RHP), and is given by:

fRHPZ

=

VIN2

2 • π •IOUT • L• VOUT

Hz

The loop gain is typically rolled off before the RHP zero

frequency.

A simple Type I compensation network can be incorporated

to stabilize the loop but at a cost of reduced bandwidth

and slower transient response. To ensure proper phase

margin, the loop is required to be crossed over a decade

before the LC double pole.

The unity-gain frequency of the error amplifier with the

Type I compensation is given by:

fUG

=

2

•

gm

π • CVC

where gm is the error amp transconductance (typically

1/5.2k) and CVC is the external capacitor to GND at the

VC pin. For the white LED application, a 0.1μF or greater

capacitor value is recommended.

Maximum LED Current

As described in the Operation section, the output LED

current with both enable pins logic high is equal to

ILED = 3850 [0.8V/(RISET1 || RISET2)]

Since the maximum continuous output current is limited to

1A, this sets a minimum limit on the parallel combination

of RISET1 and RISET2 equal to

RMIN = (RISET1 || RISET2)|MIN = 3850(0.8V/1A)

= 3080Ω

Although the LTC3454 can safely provide this current

continuously, the external LED may not be rated for this

high a level of continuous current. Higher current levels

are generally reserved for pulsed applications, such as

LED camera flash. This is accomplished by programming

a high current with one of the RISET resistors and pulsing

the appropriate enable pin.

Varying LED Brightness

Continuously variable LED brightness control can be

achieved by interfacing directly to one or both of the ISET

pins. Figure 3 shows four such methods employing a

voltage DAC, a current DAC, a simple potentiometer or a

PWM input. It is not recommended to control brightness

by PWMing the enable pins directly as this will toggle

the LTC3454 in and out of shutdown and result in erratic

operation.

3454fa

10

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