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LTC3454

3454fa

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

Figure 2. Switch Control vs Control Voltage, VC

SW1

SW2

PMOS A

NMOS B

VIN

PMOS D

NMOS C

3454 F01

VOUT

DUTY

CYCLE

DMAX

BUCK

DMIN

BOOST

75%

DMAX

BOOST

V4 (2.1V)

V3 (1.65V)

BOOST REGION

A ON, B OFF

PWM CD SWITCHES

D ON, C OFF

PWM AB SWITCHES BUCK REGION

BUCK/BOOST REGION

V2 (1.55V)

V1 (0.9V)

CONTROL

VOLTAGE, VC

3454 F02

FOUR SWITCH PWM

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)]

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