QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 463
STEP-DOWN REGULATOR WITH LDO
LTC3700
DESCRIPTION
Demonstration circuit 463 is a step-down (buck) regula-
tor and a Low Drop Out (LDO) regulator using the
LTC3700 in the 10-lead MSOP package. The buck regu-
lator is capable of providing 1A at an output voltage of
3.3V from an input supply of 5V to 9V. The LDO can be
powered from either the buck regulator output or a
separated DC input, and produces an output up to 2.5V
at 150mA. The combination of a buck regulator and a
LDO, and the use of surface mount components result in
a highly efficient application circuit in a very small board
space. The demo circuit highlights the capabilities of the
LTC3700. A constant operating frequency of 550kHz
makes the LTC3700 attractive for noise-sensitive appli-
cations while allowing for smaller external components
such as output filter inductor and capacitor. In addition,
high efficiency operation over a wide load current range
makes the LTC3700 ideal for battery-powered applica-
tions.
Design files for this circuit board are available. Call
the LTC factory.
Table 1.
Performance Summary
PARAMETER CONDITION VALUE
Input Voltage Range of Buck regulator 5V to 9V
Input Voltage Range of LDO 3.3V to 5V
V
OUT1
V
IN
= 5V to 9V, I
OUT1
= 0A to 1A 3.3V ±3%
V
OUT2
V
IN
= 3.3V to 5V, I
OUT1
= 0A to 150mA 2.5V ±4%
Typical Output Ripple V
OUT1
V
IN
= 5V, I
OUT1
= 1A 30mV
P–P
Typical Output Ripple V
OUT2
V
IN
= 3.3V, I
OUT2
= 150mA 5mV
P–P
Nominal Switching Frequency Buck Regulator 550kHz
QUICK START PROCEDURE
Demonstration circuit 463 is easy to set up to evaluate
the performance of the LTC3700. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
NOTE:
When measuring the input or output voltage rip-
ple, care must be taken to avoid a long ground lead on
the oscilloscope probe. Measure the input or output
voltage ripple by touching the probe tip directly across
the input or output and ground terminals. See Figure 2
for proper scope probe technique.
1.
Connect a 5V to 9V power source to the Vin1 and
GND terminals.
2.
To operate the LDO circuit with a separate, second
input, set JP1 to the Vin2 position. Connect a second
3.3V to 5V power source to the Vin2 and GND termi-
nals.
3.
Connect the loads.
4.
For single source operation, set the input source se-
lector Jumper JP1 to Vout1 position. Remove the
connection of power source Vin2. The LDO will be
powered from Vout1.
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 463
STEP-DOWN REGULATOR WITH LDO
+
–
+
–
+
–
+
–
LOAD
+–
+–
+–
+–
LOAD
+
–
+
–
+–
+–
VIN
GND
Figure 1.
Proper Measurement Equipment Setup Figure 2.
Measuring Input or Output Ripple
20mV/DIV
10mV/DIV
1µs/DIV 1µs/DIV
Figure 3.
3.3V Buck Output Ripple
(V
IN
= 5V, I
OUT
= 1A)
Figure 4.
2.5V LDO Output Ripple
(V
IN
= 3.3V, I
OUT
= 150mA)
2
A
A
B
B
C
C
D
D
E
E
4 4
3 3
2 2
1 1
8
1 10
9
5
2
3
4 7
6
SENSE-
VIN2 ITH/RUN
VFB
GND
LDO
VFB2
PGOOD VIN
PGATE
