LTC3453 datasheet(6/12 Pages) LINER

6 / 12 page

LTC3453

3453fa

OPERATIO

Buck-Boost DC-DC Converter

The LTC3453 employs an LTC proprietary buck-boost

DC/DC converter to generate the output voltage required

to drive the LEDs. This architecture permits high-effi-

ciency, 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 opera-

tion of the buck-boost as a function of the control voltage

VC. The output switches are properly phased so transi-

tions between regions of operation are continuous, fil-

tered and transparent to the user. When VIN approaches

VOUT, the buck-boost region is reached where the conduc-

tion 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 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 voltage in-

creases, 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 Four-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

Figure 1. Simplified Diagram of Internal Power Switches

Figure 2. Switch Control vs Control Voltage, VC

SW1

SW2

PMOS A

NMOS B

PVIN

PMOS D

NMOS C

3453 F01

VOUT

75%

DMAX

BOOST

DMIN

BOOST

DMAX

BUCK

DUTY

CYCLE

V4 (

≈2.1V)

V3 (

≈1.65V)

BOOST REGION

BUCK REGION

BUCK/BOOST REGION

V2 (

≈1.55V)

V1 (

≈0.9V)

3453 F02

A ON, B OFF

PWM CD SWITCHES

D ON, C OFF

PWM AB SWITCHES

FOUR SWITCH PWM

CONTROL

VOLTAGE, VC

LTC3453 датащи(PDF) 6 Page - Linear Technology

LTC3453 датащи(HTML) 6 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12

датащи скачать

ссылки URL