DC957A - Demo Manual

DEMO CIRCUIT 957
QUICK STARTLTC3785
GUIDE
LTC3785
3.3V, 3.0A Synchronous
Buck-Boost Controller
DESCRIPTION
Demonstration circuit 957 is a wide input range,
3.3V, 3.0A Synchronous Buck-Boost Converter
featuring the LTC3785.
This circuit was designed to demonstrate the high
levels of performance, efficiency, and small solution size attainable using this part in a buck-boost
power supply. It operates at 500kHz and produces
a regulated 3.3V, 3.0A output from an input voltage
range of 2.7 to 10V. It is suitable for portable applications, and has a footprint area that is 0.36
square inches.
Synchronous rectification helps to attain efficiency
up to 96%, depending on line and load. The
PERFORMANCE SUMMARY
SYMBOL
VIN
VOUT
IOUT
FSW
VOUT P-P
IREG
POUT/PIN
PARAMETER
Input Supply Range
Output Voltage
Output Current Range
Switching (Clock) Frequency
Output Ripple
Output Regulation
Efficiency (see Figure 2)
LTC3785 provides current limit and shutdown disconnect. It offers Burst Mode operation for high
efficiency at light loads.
Design files for this circuit board are available.
Call the LTC factory.
, LTC, LTM, LT, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are
registered trademarks of Linear Technology Corporation. Adaptive Power, C-Load,
DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView, µModule, Micropower
SwitcherCAD, Multimode Dimming, No Latency , No Latency Delta-Sigma, No
RSENSE, Operational Filter, PanelProtect, PowerPath, PowerSOT, SmartStart,
SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, UltraFast and VLDO are
trademarks of Linear Technology Corporation. Other product names may be trademarks of the companies that manufacture the products.
Specifications are at TA = 25°C
CONDITIONS
MIN
2.7
VIN = 2.7 –10V
0
TYP
MAX
10
3.3
VIN = 4.2V, IOUT = 3.0A (20MHz BW)
Line and Load (2.7-10V, 0-3.0A)
VIN =4.2V, IOUT = 3.0A
3.0
500
20
±0.36
92.5
UNITS
V
V
A
kHz
mVP–P
%
%
OPERATING PRINCIPLES
The LTC3785 Synchronous Buck-Boost Controller
is employed to produce a 3.3V output from an input ranging from 2.7V to 10V.
The converter utilizes a proprietary 4-switch topology to provide an output that is within the input
voltage range. It seamlessly transitions from buck,
to buck-boost, to boost modes of operation to
maintain the output as the input voltage goes from
maximum to minimum values.
1
LTC3785
QUICK START PROCEDURE
Demonstration circuit 957 is easy to set up to
evaluate the performance of the LTC3785. Refer to
Figure 1 for proper measurement equipment setup
and follow the procedure below:
NOTE. When measuring the output voltage ripple, care must be
taken to avoid a long ground lead on the oscilloscope probe.
Measure the output voltage ripple by touching the probe tip and
ground ring directly across the last output capacitor (C15) as shown
in Figure 1.
1. Set an input power supply that is capable of
2.7V to 10V to 4.2V. Turn off the supply.
NOTE. Make sure that the input voltage never exceeds 10V.
4. Check for the proper output voltage of 3.3V.
Turn off the power at the input.
5. Once the proper output voltages are established, connect a variable load capable of sinking 3.0A at 3.3V to the output terminals +Vout
and –Vout. Set the current for 0A.
a. If efficiency measurements are desired, an
2. With power off, connect the supply to the input
terminals +Vin and –Vin.
ammeter or a resistor shunt that is capable
of handling 3.0Adc can be put in series with
the output load in order to measure the
DC957’s output current.
a. Input voltages lower than 2.7V can keep the
b. A voltmeter with a capability of measuring at
converter from turning on due to the undervoltage lockout feature of the LTC3785.
b. If efficiency measurements are desired, an
ammeter capable of measuring 5Adc or a resistor shunt can be put in series with the input supply in order to measure the DC957’s
input current.
c. A voltmeter with a capability of measuring at
least 10V can be placed across the input
terminals in order to get an accurate input
voltage measurement.
3. Turn on the power at the input.
2
least 3.3V can be placed across the output
terminals in order to get an accurate output
voltage measurement.
6. Turn on the power at the input.
NOTE. If there is no output, temporarily disconnect the load to
make sure that the load is not set too high.
7. Once the proper output voltage is again established, adjust the load within the operating range
and observe the output voltage regulation, ripple
voltage, efficiency and other desired parameters.
LTC3785
Figure 1. Proper Measurement Equipment Setup
3
LTC3785
DC957A Ltc3785 3.3V Efficiency
100
90
80
Vin=2.7V
Vin=3V
Vin=4.2V
Vin=10V
Vin=2.7V (BURST)
Vin=3V (BURST)
Vin=4.2V
Vin=10 (BURST)
Efficiency (%)
70
60
50
40
30
20
10
0
1.00E-03
1.00E-02
1.00E-01
1.00E+00
1.00E+01
Iout (A)
Figure 2. Efficiency – Note 90%+ efficiency over a wide current range.
Figure 3. Output Ripple at 4.2Vin and 3.0Aout (20MHz) - 10uS and 20mV / div
4
LTC3785
Figure 4. Output Ripple at 2.7Vin and 3.0Aout (20MHz) - 10uS and 50mV / div
Figure 5. Transient Response Waveform at 4.2Vin and 1.5 - 3.0Aout – 500uS and 50mV / div
5
LTC3785
Figure 6. Complete Board Schematic
6
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