QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 565
3A SYNCHRONOUS BOOST CONVERTER WITH OUTPUT DISCONNECT
LT3421
DESCRIPTION
Demonstration Circuit 565 is a constant-frequency syn-
chronous boost converter using the LTC3421. Guaran-
teed to start-up at 1V input voltage, this converter pro-
vides up to 95% efficiency. For 1.8V minimum input
voltage, this converter can provide up to 1A load cur-
rent.
This converter has true output disconnect by eliminating
body diode conduction of the internal PMOS rectifier.
This feature reduces the inrush current during start-up
and isolates the load from input source in shutdown to
eliminate any leakage current. With the V
REF
output and
low-battery comparator disabled, the IC only draws less
than 1µA in shutdown.
The output voltage is set at 3.3V. A different output volt-
age in the range of 2.4V to 5.25V can be obtained by
changing one of the feedback resistors. The switching
frequency is set at 1MHz, which is a good trade-off be-
tween efficiency and size. The frequency can be ad-
justed by selecting R13 at R
T
pin. It can also be syn-
chronized to an external clock for noise-sensitive appli-
cations.
Table 1. Performance Summary (T
A
= 25°C unless otherwise noted)
PARAMETER
Minimum Input Voltage
Output Voltage V
OUT
Maximum Output Current
Typical Output Ripple V
OUT
Typical Switching Frequency
Efficiency
V
IN
= 2.4V, I
OUT
= 0.2A
V
IN
= 2.4V, I
OUT
= 1A
Logic Low Voltage-Off
On/Off Control
Logic High Voltage-On V
OUT
= 0V
V
OUT
> 2.4V
V
IN
= 1.8V to 3.2V, I
OUT
= 0A to 1A
V
IN
= 1.8V to 3.2V
V
IN
= 2.4V, I
OUT
= 1A (20MHz BW)
CONDITION
VALUE
1V
3.3V ±3%
1A
30mV
P–P
1MHz
95% Typical
88% Typical
0.25V MAX
1V MIN
0.6V MIN
Long wires from input sources (such as wall adaptors)
can cause large voltage spikes during initial plug-in.
C12 is installed on DC565 to damp the possible voltage
spikes. C12 is not required for applications where input
source is close to the regulator. Please refer to Applica-
tion Note 88 for details.
For V
OUT
> 4.3V applications, D1, Q1, R18 and C10 have
to be installed to limit the peak switch voltage below the
absolute maximum rating. Instead of using a schottky
diode connected directly from SW pin to the output, this
active snubber keeps the LTC3421 true output discon-
nect feature. R18 can be used to limit the current going
through Q1 in short-circuit conditions.
The optional circuit at ILIM pin can be added to provide
current limit foldback for extended short-circuit condi-
tions. Refer to Short Circuit section in the datasheet for
details.
Design files for this circuit board are available. Call
the LTC factory.
1
QUICK START PROCEDURE
Demonstration circuit 565 is easy to set up to evaluate
the performance of the LTC3421. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
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 Vin or Vout and GND terminals. See Figure 2 for
proper scope probe technique.
1.
NOTE:
When
2.
3.
With power off, connect the input power supply to Vin
and GND.
Turn on the power at the input.
NOTE:
Make
sure that the input voltage does not ex-
ceed 3.3V.
4.
Check for the proper output voltage. Vout = 3.2V to
3.4V.
NOTE:
If
there is no output, temporarily disconnect the
load to make sure that the load is not set too high.
Place jumpers in the following positions:
JP1
JP2
JP3
ON or OFF
RUN
AUTO ON
5.
Once the proper output voltage is established, adjust
the load within the operating range and observe the
output voltage regulation, ripple voltage, efficiency
and other parameters.
2
Embedded System
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