Datasheet

LTC3577/LTC3577-1
38
3577fa
OPERATION
To keep the average steady-state inductor current below
300mA the maximum output current is reduced as pro-
grammed output voltage increases. The output current
available is given by:
I
BOOST(MAX)
= 300mA •
V
OUT(MIN)
V
BOOST
Note that the maximum boost output current must be
set by the minimum V
OUT
operating voltage. If the boost
converter is allowed to operate down to the V
OUT
UVLO
then 2.5V must be assumed as the minimum operating
V
OUT
voltage.
Inductor Selection
The LED boost converter is designed to work with a 10H
inductor. The inductor must be able to handle a peak
current of 1A and should have a low ESR value for good
effi ciency. Table 7 shows several inductors that work
well with the LED boost converter. These inductors offer
a good compromise in current rating, DCR and physical
size. Consult each manufacturer for detailed information
on their entire selection of inductors.
Diode Selection
When boosting to increasingly higher voltages, parasitic
capacitance at the switch pin becomes an increasing
large component of the switching loses. For this reason
it is important to minimize the capacitance on the switch
node. The diode selected should be sized to handle the
peak inductor current and the average output current.
At high boost voltages a diode with the lowest possible
junction capacitance will often result in a more effi cient
solution than one with a lower forward drop.
I
2
C OPERATION
I
2
C Interface
The LTC3577 may communicate with a bus master using
the standard I
2
C 2-wire interface. The Timing Diagram
shows the relationship of the signals on the bus. The two
bus lines, SDA and SCL, must be high when the bus is
not in use. External pull-up resistors or current sources,
such as the LTC1694 SMBus accelerator, are required
on these lines. The LTC3577 is both a slave receiver and
slave transmitter. The I
2
C control signals, SDA and SCL
are scaled internally to the DV
CC
supply. DV
CC
should be
connected to the same power supply as the bus pull-up
resistors.
The I
2
C port has an undervoltage lockout on the DV
CC
pin.
When DV
CC
is below approximately 1V , the I
2
C serial port
is cleared, the LTC3577 is set to its default confi guration
of all zeros.
Table 7. Recommended Inductors for Boost Switching Regulators
INDUCTOR TYPE L (H) MAX I
DC
(A) MAX DCR () SIZE in mm (L × W × H) MANUFACTURER
DB62LCB 10 1.22 0.118
6.2 × 6.2 × 2
Toko
www.toko.com
CDRH4D16NP-100M 10 10.5 0.155
4.8 × 4.8 × 1.8
Sumida
www.sumida.com
SD18-100-R 10 1.28 0.158*
5.2 × 5.2 × 1.8
Cooper
www.cooperet.com
LPS4018-103 10 1.1 0.200
4.0 × 4.0 × 1.8
Coil Craft
www.coilcraft.com
*Typical