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LTC3454EDD

1A Synchronous Buck-Boost High Current LED Driver

Linear Technology 

LTC3454

OPERATION

Buck-Boost DC/DC Converter

The LTC3454 employs an LTC proprietary buck-boost

DC/DC converter to generate the output voltage required to

drive a high current LED. This architecture permits high-

efficiency, low noise operation at input voltages above,

below or equal to the output voltage by properly phasing

four internal power switches. The error amp output voltage

on the VC pin determines the duty cycle of the switches.

Since the VC pin is a filtered signal, it provides rejection

of frequencies well below the factory trimmed switching

frequency of 1MHz. The low RDS(ON), low gate charge

synchronous switches provide high frequency pulse width

modulation control at high efficiency. Schottky diodes

across synchronous rectifier switch B and synchronous

rectifier switch D are not required, but if used do provide

a lower voltage drop during the break-before-make time

(typically 20ns), which improves peak efficiency by typi-

cally 1% to 2% at higher loads.

Figure 1 shows a simplified diagram of how the four internal

power switches are connected to the inductor, VIN, VOUT

and GND. Figure 2 shows the regions of operation of the

buck-boost as a function of the control voltage VC. The

output switches are properly phased so transitions between

regions of operation are continuous, filtered and transpar-

ent to the user. When VIN approaches VOUT, the buck-boost

region is reached where the conduction time of the four

switch region is typically 150ns. Referring to Figures 1

and 2, the various regions of operation encountered as VC

increases will now be described.

Buck Mode (VIN > VOUT)

In buck mode, switch D is always on and switch C is

always off. Referring to Figure 2, when the control

voltage VC is above voltage V1, switch A begins to turn on

VIN

VOUT

9

7

PMOS A

SW1

10

PMOS D

SW2

6

NMOS B

NMOS C

3454 F01

Figure 1. Simplified Diagram of Internal Power Switches

each cycle. During the off time of switch A, synchronous

rectifier switch B turns on for the remainder of the cycle.

Switches A and B will alternate conducting similar to a

typical synchronous buck regulator. As the control volt-

age increases, the duty cycle of switch A increases until

the maximum duty cycle of the converter in buck mode

reaches DCBUCK|Max given by:

DCBUCK|Max = 100% – DC4SW

where DC4SW equals the duty cycle in % of the “four

switch” range.

DC4SW = (150ns • f) • 100%

where f is the operating frequency in Hz.

Beyond this point the “four switch” or buck-boost region

is reached.

Buck-Boost or 4-Switch Mode (VIN ≈ VOUT)

Referring to Figure 2, when the control voltage VC is above

voltage V2, switch pair AD continue to operate for duty

cycle DCBUCK|max, and the switch pair AC begins to phase

in. As switch pair AC phases in, switch pair BD phases out

accordingly. When the VC voltage reaches the edge of the

buck-boost range at voltage V3, switch pair AC completely

phases out switch pair BD and the boost region begins at

duty cycle DC4SW. The input voltage VIN where the four

switch region begins is given by:

VIN = VOUT/[1 – (150ns • f)]

and the input voltage VIN where the four switch region

ends is given by

VIN = VOUT • (1 – DC4SW) = VOUT • [1 – (150ns • f)]

75%

DMAX

BOOST

DMIN

BOOST

DMAX

BUCK

A ON, B OFF

PWM CD SWITCHES

BOOST

REGION

FOUR SWITCH PWM

BUCK/BOOST REGION

D ON, C OFF

PWM AB SWITCHES BUCK REGION

V4 (2.1V)

V3 (1.65V)

V2 (1.55V)

0%

V1 (0.9V)

DUTY

CYCLE

CONTROL

3454 F02 VOLTAGE, VC

Figure 2. Switch Control vs Control Voltage, VC

3454fa

7

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