LTC3453_15 Datasheet, PDF (7/12 Pages) Linear Technology – Synchronous Buck-Boost High Power White LED Driver

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OPERATIO
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)]
Boost Mode (V_IN < V_OUT)
In boost mode, switch A is always on and switch B is
always off. Referring to Figure 2, when the control voltage
V_C is above voltage V3, switches C and D will alternate
conducting similar to a typical synchronous boost regula-
tor. The maximum duty cycle of the converter is limited to
88% typical and is reached when V_C is above V4.
Forward Current Limit
If the current delivered from V_IN through PMOS switch A
exceeds 1612mA (typical), switch A is shut off immedi-
ately. Switches B and D are turned on for the remainder of
the cycle in order to safely discharge the forward inductor
current at the maximum rate possible.
Reverse Current Limit
If the current delivered from V_OUT backwards through
PMOS switch D exceeds 200mA (typical), switch D is shut
off immediately. Switches A and C are turned on for the
remainder of the cycle in order to safely discharge the
reverse inductor current at the maximum rate possible.
Undervoltage Lockout
To prevent operation of the power switches at high R_DS(ON),
an undervoltage lockout is incorporated on the LTC3453.
When the input supply voltage drops below approximately
1.9V, the four power switches and all control circuitry are
turned off except for the undervoltage block, which draws
only several microamperes.
Overtemperature Protection
If the junction temperature of the LTC3453 exceeds 130°C
for any reason, all four switches are shut off immediately.
The overtemperature protection circuit has a typical hys-
teresis of 11°C.
LTC3453
Soft-Start
The LTC3453 includes an internally fixed soft-start which
is active when powering up or coming out of shutdown.
The soft-start works by clamping the voltage on the V_C
node and gradually releasing it such that it requires
0.65ms to linearly slew from 0.9V to 2.1V. This has the
effect of limiting the rate of duty cycle change as V_C
transitions from the buck region through the buck-boost
region into the boost region. Once the soft-start times out,
it can only be reset by entering shutdown, or by an
undervoltage or overtemperature condition.
Main Error Amp
The main error amplifier is a transconductance amplifier
with source and sink capability. The output of the main
error amplifier drives a capacitor to GND at the V_C pin. This
capacitor sets the dominant pole for the regulation loop.
(See the Applications Information section for selecting the
capacitor value.) The error amp gets its feedback signal
from a proprietary circuit which monitors all 4 LED current
sources to determine which LED to close the regulation
loop on.
Safety Error Amp
The safety error amplifier is a transconductance amplifier
with sink only capability. In normal operation, it has no
effect on the loop regulation. However, if any of the LED
pins open-circuits, the output voltage will keep rising, and
safety error amp will eventually take over control of the
regulation loop to prevent V_OUT runaway. The V_OUT thresh-
old at which this occurs is approximately 4.5V.
LED Current Setting Amplifiers and Enable Circuit
The LTC3453 includes two LED current setting amplifiers
that work in conjunction with dual external current setting
resistors and dual enable pins to program LED current to
one of four levels (including shutdown). All four LED
current source outputs are programmed to the same level.
When both enable inputs are logic low, the LTC3453 is in
shutdown, the buck-boost is disabled and all LED currents
are zero. In shutdown, the input supply current is typically
6µA. If either enable input is logic high, the buck-boost will
regulate the output voltage such that the LEDs are biased
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