Datasheet

LTC3454
7
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-
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 V
C
pin determines the duty cycle of the switches.
Since the V
C
pin is a fi ltered signal, it provides rejection
of frequencies well below the factory trimmed switching
frequency of 1MHz. The low R
DS(ON)
, low gate charge
synchronous switches provide high frequency pulse width
modulation control at high effi ciency. Schottky diodes
across synchronous rectifi er switch B and synchronous
rectifi er 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 effi ciency by typi-
cally 1% to 2% at higher loads.
Figure 1 shows a simplifi ed diagram of how the four internal
power switches are connected to the inductor, V
IN
, V
OUT
and GND. Figure 2 shows the regions of operation of the
buck-boost as a function of the control voltage V
C
. The
output switches are properly phased so transitions between
regions of operation are continuous, fi ltered and transpar-
ent to the user. When V
IN
approaches V
OUT
, 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 V
C
increases will now be described.
Buck Mode (V
IN
> V
OUT
)
In buck mode, switch D is always on and switch C is
always off. Referring to Figure 2, when the control
voltage V
C
is above voltage V1, switch A begins to turn on
Figure 2. Switch Control vs Control Voltage, V
C
10
SW1
6
SW2
PMOS A
NMOS B
9
V
IN
PMOS D
NMOS C
3454 F01
7
V
OUT
0%
DUTY
CYCLE
D
MAX
BUCK
D
MIN
BOOST
75%
D
MAX
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, V
C
3454 F02
FOUR SWITCH PWM
Figure 1. Simplifi ed Diagram of Internal Power Switches
each cycle. During the off time of switch A, synchronous
rectifi er 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 DC
BUCK
|Max given by:
DC
BUCK
|Max = 100% – DC
4SW
where DC
4SW
equals the duty cycle in % of the “four
switch” range.
DC
4SW
= (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 (V
IN
≈ V
OUT
)
Referring to Figure 2, when the control voltage V
C
is above
voltage V2, switch pair AD continue to operate for duty
cycle DC
BUCK
|max, and the switch pair AC begins to phase
in. As switch pair AC phases in, switch pair BD phases out
accordingly. When the V
C
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 DC
4SW
. The input voltage V
IN
where the four
switch region begins is given by:
V
IN
= V
OUT
/[1 – (150ns • f)]
and the input voltage V
IN
where the four switch region
ends is given by
V
IN
= V
OUT
• (1 – DC
4SW
) = V
OUT
• [1 – (150ns • f)]