LTC3452 Linear Technology, LTC3452 Datasheet - Page 8

• Manufacturer Part Number: LTC3452
• Description: Synchronous Buck-Boost MAIN/CAMERA White LED Driver
• Manufacturer: Linear Technology

OPERATIO
LTC3452
Buck-Boost DC-DC Converter
The LTC3452 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 V
the V
frequencies well below the factory trimmed switching
frequency of 1MHz. The low R
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, V
= PV
operation of the buck-boost as a function of the control
voltage V
transitions between regions of operation are continuous,
filtered and transparent to the user. When V
V
tion time of the four switch region is typically 150ns.
Referring to Figures 1 and 2, the various regions of
operation encountered as V
described.
8
OUT
Figure 1. Simplified Diagram of Internal Power Switches
C
, the buck-boost region is reached where the conduc-
IN
C
pin determines the duty cycle of the switches. Since
, V
pin is a filtered signal, it provides rejection of
C
OUT
. The output switches are properly phased so
PV
20
IN
and GND. Figure 2 shows the regions of
PMOS A
NMOS B
U
SW1
19
C
SW2
17
DS(ON)
increases will now be
V
OUT
16
, low gate charge
3452 F01
PMOS D
NMOS C
IN
approaches
IN
Buck Mode (V
In buck mode, switch D is always on and switch C is always
off. Referring to Figure 2, when the control voltage V
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 DC
where DC
switch” range.
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 (V
Referring to Figure 2, when the control voltage V
voltage V2, switch pair AD continue to operate for duty
cycle DC
in. As switch pair AC phases in, switch pair BD phases out
accordingly. When the V
buck-boost range at voltage V3, switch pair AC completely
phases out switch pair BD and the boost region begins at
DC
DC
BOOST
BOOST
CYCLE
D
D
BUCK
DUTY
D
75%
MAX
MAX
MIN
0%
BUCK
4SW
Figure 2. Switch Control vs Control Voltage, V
BUCK
FOUR SWITCH PWM
PWM CD SWITCHES
PWM AB SWITCHES
= (150ns • f) • 100%
4SW
|max = 100% – DC
BUCK
A ON, B OFF
D ON, C OFF
|max, and the switch pair AC begins to phase
IN
equals the duty cycle in % of the “four
|max given by:
> V
OUT
BOOST REGION
BUCK REGION
)
C
voltage reaches the edge of the
4SW
BUCK/BOOST REGION
www.DataSheet4U.com
IN
≈ V
3452 F02
OUT
V4 (≈2.1V)
V3 (≈1.65V)
V2 (≈1.55V)
V1 (≈0.9V)
CONTROL
VOLTAGE, V
)
C
C
is above
C
3452f
C
is