LTC3452 Linear Technology, LTC3452 Datasheet - Page 9

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

OPERATIO
duty cycle DC
switch region begins is given by:
and the input voltage V
ends is given by:
Boost Mode (V
In boost mode, switch A is always on and switch B is
always off. Referring to Figure 2, when the control voltage
V
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
Forward Current Limit
If the current delivered from V
exceeds 1600mA (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
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
an undervoltage lockout is incorporated on the LTC3452.
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 a few microamperes.
C
V
V
is above voltage V3, switches C and D will alternate
IN
IN
=
=
V
1 150
OUT
–
(
V
4SW
• –
OUT
IN
[
1 150
ns f
U
< V
. The input voltage V
(
•
OUT
)
IN
)
ns f
where the four switch region
•
)
IN
]
OUT
through PMOS switch A
C
backwards through
is above V4.
IN
where the four
DS(ON)
,
Overtemperature Protection
If the junction temperature of the LTC3452 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.
Soft-Start
The LTC3452 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
node and gradually releasing it such that it requires 650µs
to linearly slew from 0.9V to 2.1V. This has the effect of
limiting the rate of duty cycle change as V
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
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 7 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
old at which this occurs is approximately 4.5V.
OUT
runaway. The V
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LTC3452
C
OUT
transitions
C
pin. This
thresh-
9
3452f
C