QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1072B
1A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
1
LTC3561A
DESCRIPTION
Demonstration circuit 1072B is a step-down con-
verter, using the LTC3561A monolithic synchro-
nous buck regulator. The DC1072B has an input
voltage range of 2.5V to 5.5V, and is capable of
delivering up to 1A of output current. The output
voltage of the DC1072B can be set as low as
0.8V, the reference voltage of the LTC3561A, and
can go as high as 5V. At light load currents, the
DC1072B operates in pulse-skipping mode - ideal
for noise sensitive applications. Of course, in
large load current applications, the DC1072B op-
erates in continuous mode, providing high effi-
ciency - over 95%. The DC1072B consumes less
than 60 uA of quiescent current during sleep op-
eration, and during shutdown, it consumes less
than 1 uA. The DC1072B has a standard operat-
ing frequency of 1 MHz, but can be adjusted to
frequencies as high as 4 MHz. Because of the
high switching frequency of the DC1072B, small,
low profile surface mount components are used in
the circuit. These features, plus the LTC3561A
coming in a small 8-Lead DFN package, make the
DC1072B a perfect match for battery-powered,
hand-held applications.
Design files for this circuit board are avail-
able. Call the LTC factory.
TM
- Burst Mode is a trademark of Linear Technology Corporation
Table 1.
Performance Summary (TA = 25°C)
PARAMETER CONDITIONS VALUE
Minimum Input Voltage 2.5V
Maximum Input Voltage 5.5V
Output Voltage VOUT Regulation VIN = 2.5V to 5.5V, IOUT = 0A to 1A
1.2V ±4% (1.152V to
1.248V)
1.5V ±4% (1.44V to 1.56V)
1.8V ±4% (1.728V to
1.872V)
Typical Output Ripple VOUT VIN = 5V, IOUT = 1A (20 MHz BW) <20mVP–P
Operation Mode Pulse-Skipping - VIN = 3.3V, VOUT = 1.8V <100mA ±0.1A%
Nominal Switching Frequency 1 MHz
QUICK START PROCEDURE
D
emonstration Circuit 1072B is easy to set up
to evaluate the performance of the LTC3561A.
For proper measurement equipment configura-
tion, set up the circuit according to the diagram
in Figure 1. Before proceeding to test, insert
shunts into JP1 OFF position, which connects
the RUN pin to ground (GND), and thus, shuts
down the circuit, and the 1.2V position of JP2.
NOTE:
When measuring the input or output volt-
age ripple, care must be taken to avoid a long
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1072B
1A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
2
ground lead on the oscilloscope probe. Meas-
ure the input or output voltage ripple by touch-
ing the probe tip directly across the Vin or Vout
and GND terminals. See Figure 2 for proper
scope probe technique.
1.
With the DC1072B set up according to the
proper measurement and equipment in Fig-
ure 1, apply 3.3V at Vin (Do not hot-plug Vin
or increase Vin over the rated maximum sup-
ply voltage of 5.5V, or the part may be dam-
aged.). Measure Vout; it should read 0V. If
desired, one can measure the shutdown sup-
ply current at this point. The supply current
will be approximately 10 uA, or less, in shut-
down.
2.
Turn on the circuit by inserting the shunt in
header JP1 into the ON position. The output
voltage should be regulating. Measure Vout -
it should measure 1.2V +/- 2% (1.176V to
1.224V).
3.
Vary the input voltage from 2.5V to 5.5V and
adjust the load current from 0 to 1A. Vout
should read between 1.2V +/- 4% (1.152V to
1.248V).
4.
Measure the output ripple voltage at any out-
put current level; it usually will measure less
than 20 mVAC.
5.
Observe the voltage waveform at the switch
node. Verify the switching frequency is be-
tween 850 kHz and 1.15 MHz (T = 1.176 us
and 0.87 us), and that the switch node wave-
form is rectangular in shape.
Insert jumper JP1 shunt into the OFF position and
move the 1.2V Vout shunt into any of the remain-
ing output voltage options: 1.5V or 1.8V. Just as
in the 1.2Vout test, the output voltage should read
Vout +/- 2% tolerance under static line and load
conditions, and +/- 1% tolerance under dynamic
line and load conditions (+/- 2% total).
When finished, turn off the circuit (connecting
the RUN pin to ground) by inserting the shunt in
header JP1 into the OFF (upper) position.
Connection Diagram
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1072B
1A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
3
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1072B
1A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
4
Figure 3.
Switching Operation
V
IN
= 3.3V, V
OUT
= 1.5V, 1A Output Current
Trace 1: Switch Voltage (2V/div)
Trace 3: Output Voltage (20mV/div AC)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1072B
1A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
5
Figure 4.
Load Step Response
V
IN
= 3.3V, V
OUT
= 1.5V, 900mA Load Step (0.1A <-> 1A)
Trace 3: Output Voltage (100mV/div AC)
Trace 4: Output Current (500 mA/div)
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1072B
1A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
6
Rfb1
249K, 1%
E2
Cout1
22uF
6.3V
VOUT
1.0A
VIN
Rfb5
OPT
Rfilt
0
D1
OPT
2 1
Cout2
0.1uF
+
E3
Rfb3
287K, 1%
L1
2.2uH
1.5V
Rs1
1M
Rfb4
200K, 1%
GND
USER
E1
U1
LTC3561AEDD
5
6
1
2
4
8
7
3
PVIN
SVIN
SD/RT
S G N D
P G N D
ITH
VFB
SW
Cffw
10pF
1.2V
JP2
Cbyp
OPT
C1
OPT
+
2.65V-5.5V Cin3
22uF
16V
Rc
16.2K
GND
Cin2
1uF
OPT
Cin1
10uF
6.3V
Rfb2
499K, 1%
OFF
SELECT
Cout3
OPT
E5
1.8V/1.5V/1.2V
*
Rset
549K, 1%
ON
E4
1 2
VIN
Cc
1000pF 1.8V
JP1
13
2
GND
7 8
5 6
3 4