QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1020
HIGH EFFICIENCY USB POWER MANAGER + TRIPLE STEP-DOWN DC/DC
LTC3555
1
DESCRIPTION
Demonstration Circuit 1020 is a High Efficiency
USB Power Manager + Three Step-Down DC/DC
Converters featuring the LTC®3555. The
LTC®3555 is a highly integrated power manage-
ment and battery charger IC for Li-Ion/polymer bat-
tery applications. It includes a high efficiency cur-
rent limited switching PowerPath manager with
automatic load prioritization, a battery charger, an
ideal diode and three general purpose synchronous
step-down switching regulators. Designed specifi-
cally for USB applications, the LTC3555’s switch-
ing power manager automatically limits input cur-
rent to a maximum of either 100mA or 500mA for
USB applications or 1A for AC-powered applica-
tions. Unlike linear PowerPath controllers, the
LTC3555’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 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 subsequently controlled via I2C. The LTC3555
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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
PowerPath is a trademark of Liner Technology Corporation.
TABLE 1. TYPICAL SPECIFICATIONS (25°C)
Input Voltage Range VBUS 4.35V to 5.5V
VOUT 3.4V to 4.7V (Mode and load dependent)
LDO3V3 3.3V (Load and VOUT dependent)
Output Float Voltage VBAT (constant voltage mode) 4.2V
Output Charge Current IBAT (constant current mode) 0.5A, (RPROG = 2.00K)
VOUT1 3.3V, 400mA
VOUT2 3.0V-1.6V, 400mA
VOUT3 1.5V-0.8V, 1A
QUICK START PROCEDURE
Demo Circuit 1020 is best evaluated using a Li-Ion
or Li-Polymer battery. When using a battery simu-
lator 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 bat-
tery. Capacitor C15 was added to simulate the low
impedance of a real battery. It will allow proper
operation of the demo circuit with or without a
battery and even with a battery simulator consist-
ing of a power supply in parallel with a 3.6 Ohm
resistor.
Complete the Quick Start Procedure outlined in
the Quick Start Guide for Demo Circuit 590 avail-
able from the Linear Technology Web Site, prior
to proceeding.
Refer to Figure 2 for the proper measurement
equipment setup and jumper settings and follow
the procedure below.
NOTE:
When measuring the input or output voltage ripple,
care must be taken to avoid a long ground lead on the oscil-
loscope probe. Measure the input or output voltage ripple by
touching the probe tip directly across the VBUS or VOUT(x)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1020
HIGH EFFICIENCY USB POWER MANAGER + TRIPLE STEP-DOWN DC/DC
LTC3555
2
and GND terminals. See Figure 3 for proper scope probe
technique.
1. After following the software installation pro-
cedure in the DC590 Quick Start Guide, con-
nect the DC1020, the DC590 and the Host PC
as shown in Figure 1. The following demon-
stration software should pop-up as soon as the
DC1020 board is connected to the Host PC.
FIGURE 1: Graphical User Interface Demon-
stration Software for DC1020.
2. If using an external supply, connect a 0 to 6V,
1.5A supply with voltage adjusted to 0V be-
tween the VBUS and GND terminals. Connect
a voltmeter across the VBUS and GND termi-
nals. If using the USB input option, have cable
ready to plug in for when all loads and jumpers
are set. Connect the positive terminal of a
voltmeter to CLPROG and the negative termi-
nal to GND to measure the average input cur-
rent. The voltage on CLPROG will reach 1.188
Volts when the average input current limit is
reached. Do not use the USB input and an
external supply at the same time.
3. Set JP1, NTC jumper to INT to use the demo
board resistor network. To use an external
NTC resistor, connect the external NTC resis-
tor between the J2-3 terminal and the GND
terminal. Then set the NTC jumper to EXT.
4. Set JP2 and JP3 (ILIM0 and ILIM1 respec-
tively) to “LO”. At Power-up, the average in-
put current setting will be equal to the value set
by the position of JP2 and JP3 until an I2C
command is received and over-writes that de-
fault value.
TABLE 2. Input Current Limit Settings
ILIM1 ILIM0 CURRENT LIMIT
0 0 100mA
(
1X
)
0 1 1A
(
10X
)
1 0 0.5mA
(
SUSP
)
1 1 500mA
(
5X
)
5. Connect a 0 to 25mA adjustable load in series
with an ammeter between the LDO3V3 and
GND terminals. Connect a voltmeter between
the LDO3V3 and GND terminals.
6. Connect a 0 to 2A adjustable load in series
with an ammeter between the VOUT terminal
and the GND terminal. Connect a voltmeter
between the VOUT and GND terminals.
7. Connect a 0 to 400mA adjustable load in series
with an ammeter between the VOUT1 and
GND terminals. Connect a voltmeter between
the VOUT1 and GND terminals.
8. Connect a 0 to 400mA adjustable load in series
with an ammeter between the VOUT2 and
GND terminals. Connect a voltmeter between
the VOUT2 and GND terminals.
9. Connect a 0 to 1.0A adjustable load in series
with an ammeter between the VOUT3 and
GND terminals. Connect a voltmeter between
the VOUT3 and GND terminals.
10. Connect a Li-Ion or Li-Polymer battery with
the positive lead to the BAT terminal and the
negative lead to a GND terminal. Connect the
positive lead of a voltmeter to the BAT termi-
nal and the negative lead to the GND terminal.
Connect the positive terminal of a voltmeter to
PROG and the negative terminal to GND to
measure the current flowing into the battery.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1020
HIGH EFFICIENCY USB POWER MANAGER + TRIPLE STEP-DOWN DC/DC
LTC3555
3
The voltage on PROG will read 1 Volt when
the maximum charge current is reached. Ob-
serve VOUT and the LDO3V3 output.
11. If using the USB input, plug in the USB cable
and observe the charge current, and the various
output voltages. If using an external power
supply, slowly increase the supply and observe
that the LTC3555 starts to use the VBUS input
power as VBUS reaches 4.35V.
12. Discharge the battery voltage to 2.5 volts. If
Charger mode is on, observe it is charging in
trickle charge mode and the charge current is
50mA. As the battery voltage increase above
2.85V observe that the charger goes into full
constant current mode. Observe that VOUT
stays above 3.6V when the input current limit
is not exceeded and BAT drops below 3.3
Volts. Remove the NTC jumper on JP1. Ob-
serve the CHRG LED slow blinking rate
(1.5Hz at 50% Duty Cycle). Also observe the
35KHz pulse-width varying duty cycle be-
tween 6.25% and 93.75% on the CHRG ter-
minal with an oscilloscope. Reinstall the NTC
jumper.
13. Increase the VOUT load and observe when the
input current starts to limit. When input current
limit is reached, additional load smoothly tran-
sitions to the battery through the ideal diode.
Due to the high efficiency of the switching
regulator, the sum of the charge and load cur-
rent may be significantly above the input cur-
rent, depending on the voltage on the BAT pin.
14. Repeat step 13 with different current limit
modes.
15. Set the VBUS input supply to 5V. Observe the
battery current as the battery voltage increases
above 4.1V. The Charge current starts to de-
crease as the battery voltage increases towards
4.2V.
See the LTC3555 data sheet for more information
on how this part performs.
APPLICATION INFORMATION
This demo circuit is designed to demonstrate the
full capability of the device. Not all components
are required in all applications. The critical circuit
components are on the top of the board near the
IC.
The input capacitor network of C18 and R26 is
used to dampen input source inductances that
commonly occur in laboratory setups with twisted
leads and a bench power supply. When using a
USB cable or adaptor cable this input damping
network will likely not be required. Please note
that the in-circuit capacitance of the specified
10uF, 0805 ceramic capacitor for C18 and C1 is
approximately 5uF each at applied voltage.
VOUT Capacitor C5 is only required when using
the 10X mode. The LTC3555 requires a minimum
of 10uF on the VOUT pin in 10X mode for stabil-
ity. As an alternative to using two smaller X5R
capacitors, one larger X5R ceramic capacitor
would be acceptable, provided that the actual in-
circuit capacitance with 4.5 Volts applied is
greater than 10uF.
Capacitor C15 is included to simulate a low im-
pedance battery. It is especially helpful when test-
ing the demo circuit with a battery simulator com-
prised of a power supply with a 3.6 Ohm power
resistor across it. The leads connecting the power
supply to the demo circuit should be a twisted pair
to minimize lead inductance; although, even
twisted pairs can introduce enough inductance into
the circuit to cause instability in the battery
charger section without the presence of C15. Ca-
pacitor C15 will also provide proper operation of
the circuit without a battery present. It is possible
to replace C15 with a ceramic capacitor in series
with a resistor to stabilize the battery charger when
no battery is present. Please refer to the datasheet
for the suggested capacitor and resistance range.
When not using the 3.3 Volt LDO (LDO3V3) con-
nect VOUT to LDO3V3 by installing R9, which
eliminates the need for capacitor C3.
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1020
HIGH EFFICIENCY USB POWER MANAGER + TRIPLE STEP-DOWN DC/DC
LTC3555
4
FIGURE 2: Proper Measurement Equipment Setup for DC1020
VIN
GND
FIGURE 3: Measuring Input or Output Ripple Voltage
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1020
HIGH EFFICIENCY USB POWER MANAGER + TRIPLE STEP-DOWN DC/DC
LTC3555
5
FIGURE 4: Circuit Schematic
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1020
HIGH EFFICIENCY USB POWER MANAGER + TRIPLE STEP-DOWN DC/DC
LTC3555
6
FIGURE 5: Bill of Materials
