LTC3453 Datasheet, PDF (6/12 Pages) Linear Technology – Synchronous Buck-Boost High Power White LED Driver

LTC3453
U
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 V_C pin determines the duty cycle of the switches. Since
the V_C pin is a filtered 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 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_IN, V_OUT and GND. Figure 2 shows the regions of opera-
tion of the buck-boost as a function of the control voltage
V_C. The output switches are properly phased so transi-
tions between regions of operation are continuous, fil-
tered and transparent to the user. When V_IN approaches
V_OUT, 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 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 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_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 Four-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
PV_IN
16
PMOS A
SW1
15
NMOS B
V_OUT
13
PMOS D
SW2
14
NMOS C
3453 F01
Figure 1. Simplified Diagram of Internal Power Switches
75%
D_MAX
BOOST
D_MIN
BOOST
D_MAX
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}
0%
V4 (≈2.1V)
V3 (≈1.65V)
V2 (≈1.55V)
V1 (≈0.9V)
DUTY
CYCLE
CONTROL
3453 F02 _{VOLTAGE, VC}
Figure 2. Switch Control vs Control Voltage, V_C
3453fa
6