LTC3555-1 AND LTC3555-3
1
DESCRIPTION
Demonstration circuits 1376A-A and 1376A-B are High
Efficiency USB Power M Triple Step Down
DC/DC featuring the LTC3555-1 and LTC3555-3 respec-
tively. The LTC
®
3555-1/LTC3555-3 are highly integrated
power management and battery charger ICs for Li-
Ion/polymer battery applications. It includes a high effi-
ciency current limited switching PowerPath manager
with automatic charge current reduction, a battery
charger, an ideal diode and three general purpose syn-
chronous step-down switching regulators. Designed
specifically for USB applications, the LTC3555-
1/LTC3555-3’s switching power manager automatically
limits input current to a maximum of either 100mA or
500mA for USB applications or 1A for AC-powered appli-
cations. Unlike linear PowerPath controllers, the
LTC3555-1/LTC3555-3’s switching input stage transmits
nearly all of the 2.5W available from the USB port to the
system load with minimal power wasted as heat. This
feature allows the LTC3555-1/LTC3555-3 to provide
more power to the application and eases the constraint
of thermal budgeting in small spaces. Two of the three
general purpose switching regulators can provide up to
400mA and the third can deliver 1A. The entire product
can be started from an external push button and subse-
quently controlled via I
2
C. The LTC3555-1/LTC3555-3 is
available in the 28-pin (4mm × 5mm × 0.75mm) QFN
surface mount package.
Design files for this circuit board are available. Call
the LTC factory.
L
, LTC, LTM, LT, Burst Mode, are registered trademarks of Linear Technology Corporation.
PowerPath are trademarks of Linear Technology Corporation. Other product names may be
trademarks of the companies that manufacture the products.
PERFORMANCE SUMMARY
Specifications are at T
A
= 25°C
SYMBOL PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
VBUS
Input Supply Range
4.35 5.5
V
VOUT
VOUT Output Voltage
Mode and load dependant
3.4
Bat + 0.3
4.7
V
LDO3V3
LDO3V3 Output Voltage
Load and VOUT dependant
3.3
V
V
FLOAT
BAT Regulated Output Voltage
LTC3555-1
LTC3555-3
4.179
4.079
4.2
4.1
4.221
4.121
V
V
I
BAT
Constant Current Mode Charge Current
R
PROG
= 2K
Ω
460
511
532
mA
VOUT1
VOUT1 Output Voltage
3.3
V
VOUT2
VOUT2 Output Voltage
Servo Voltage = 0.8
3.0
V
VOUT3
VOUT3 Output Voltage
Servo Voltage = 0.8
1.5
V
QUICK START PROCEDURE
Demonstration circuits 1376A-A and 1376A-B are easy
to set up to evaluate the performance of the LTC3555-1
and LTC3555-3. These circuits are best evaluated using
a Li-Ion or Li-Polymer battery. When using a battery
simulator for evaluation, oscillations must be verified
with a real battery at the same conditions because it is
difficult to match the impedance of a real battery. Ca-
pacitor C15 in combination with R9, R10, and R11 were
added to simulate the low impedance of a real battery.
It will allow proper operation of the demo circuit with or
without a battery, or a battery simulator consisting of a
power supply in parallel with a 3.6 Ohm resistor.
Demo Circuit1376A-A/-B
QUICK START GUIDE
LTC3555-1 and LTC3555-3
High Efficiency USB Power Manager
+ Triple Step Down DC/DC
