LTC3453 Datasheet, PDF (8/12 Pages) Linear Technology – Synchronous Buck-Boost High Power White LED Driver

LTC3453
U
OPERATIO
at the current programmed by resistors R_ISET1 and/or
R_ISET2. Individually enabled, each LED current setting
amplifier programs the output LED current to
I_LED = 384 (0.8V/R_ISET1,2)
If both enable inputs are logic high, the setting currents are
summed internally and the output LED current will be
given by
I_LED = 384 [0.8V/(R_ISET1 || R_ISET2) ]
Thus three different (nonzero) current levels are program-
mable, optimal for low current LED torch and high current
LED camera flash applications.
LED Current Sources
Each LED pin is driven by a current source specifically
designed for low dropout. The LTC3453 employs a propri-
etary architecture that determines which of the four LEDs
requires the largest forward voltage drop at its pro-
grammed current, and then generates a feedback voltage
based on this one for closing the buck-boost regulation
loop. This results in the lowest output voltage required for
regulating all of the LEDs and thus the highest LED power
efficiency. The voltage present at the LED pin of the
“controlling LED” will be typically 130mV at 75mA of
current.
LED Detect Circuit
If fewer than four LED outputs are required, unused ones
should be connected to V_OUT. Each LED pin has an internal
LED detect circuit that disables the output current source
to save power if an output is not needed. A small 30µA
current is employed to detect the presence of an LED at
startup.
APPLICATIO S I FOR ATIO
Component Selection
Inductor Selection
The high frequency operation of the LTC3453 allows the
use of small surface mount inductors. The inductor cur-
rent ripple is typically set to 20% to 40% of the maximum
inductor current. For a given ripple the inductance terms
are given as follows:
( ) _L
>
V_IN(MIN)2 • V_OUT – V_IN(MIN)
f • I_OUT(MAX) • %Ripple •
• 100%
V_OUT2 ^,
( ) L > VOUT • VIN(MAX) – VOUT • 100%
f • I_OUT(MAX) • %Ripple • V_IN(MAX)
where f = operating frequency, Hz
%Ripple = allowable inductor current ripple, %
V_IN(MIN) = minimum input voltage, V
V_IN(MAX) = maximum input voltage, V
V_OUT = output voltage, V
I_OUT(MAX) = maximum output load current
8
For high efficiency, choose an inductor with a high fre-
quency core material, such as ferrite, to reduce core loses.
The inductor should have low ESR (equivalent series
resistance) to reduce the I²R losses, and must be able to
handle the peak inductor current without saturating. Molded
chokes or chip inductors usually do not have enough core
to support peak inductor currents >1A. To minimize radi-
ated noise, use a toroid, pot core or shielded bobbin
inductor. For the white LED application, a 4.7µH inductor
value is recommended. See Table 1 for a list of component
suppliers.
Table 1. Inductor Vendor Information
SUPPLIER
WEB SITE
Coilcraft
www.coilcraft.com
Cooper/Coiltronics
www.cooperet.com
Murata
www.murata.com
Sumida
www.japanlink.com/sumida
Vishay-Dale
www.vishay.com
3453fa