DC785A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 785
HIGH EFFICIENCY, 8A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
LTC3418
DESCRIPTION
DC785 is an 8A high efficiency, high frequency buck
converter, incorporating the LTC3418 monolithic synchronous regulator. The DC785 has an input voltage
range of 2.25V to 5.5V and an output voltage range from
0.8V to 5V. The operating frequency range of the DC785
is either set with an external resistor or synchronized to
an external clock, with a range between 300 kHz and 4
MHz. The DC785 can deliver high power – up to 8A of
output current – in a relatively small circuit, because of
the high current power switches (30 mΩ of on-state resistance) on the LTC3418. The LTC3418 also incorporates OPTI-LOOP compensation, so that the DC785 can
be optimized to provide fast transient response over a
wide range of line and load conditions. Extra features
include tracking, for easy power supply sequencing, a
1.25V external reference, available for customer use, and
TM
Burst-Mode capability, for those circuits that operate
during intervals of low output power. All these features
make the DC785 perfectly suited for high current, high
power applications, such as desktop computers.
Design files for this circuit board are available. Call
the LTC factory.
TM - Burst Mode is a trademark of Linear Technology Corporation
Table 1.
Performance Summary (TA = 25°C)
PARAMETER
Minimum Input Voltage
CONDITION
VALUE
2.25V
Maximum Input Voltage
Output Voltage VOUT
Typical Output Ripple VOUT
Output Regulation
5.5V
VIN = 2.25V to 5.5V, IOUT1 = 0A to 8A
1.8V ±4%
VIN = 5V, IOUT1 = 8A (20 MHz BW)
20mVP–P
Line
±1%
Load
±1%
Nominal Switching Frequency
1 MHz
QUICK START PROCEDURE
The DC785 demonstration board is easy to set up to
evaluate the performance of the LTC3418. For proper
measurement equipment configuration, set up the
circuit according to the diagram in Figure 1. Before
proceeding to test, insert jumper JP2 shunt into the
off (lower) position, connecting the RUN pin to
ground (GND), which shuts down the circuit.
NOTE: When measuring the input or output voltage
ripple, 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.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 785
HIGH EFFICIENCY, 8A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
1.
Connect the input power supply and the load to the
board. Do not hot-plug Vin or increase Vin over
the rated maximum supply voltage of 5.5V, or the
part may be damaged. Refer to figure 1 for the
proper measurement equipment setup.
2.
Insert the shunts in the TRACK OFF position of
jumper JP1, in the Force Continuous position of
jumper JP3, and in the 1.2V output voltage position.
3.
Apply 3.3V at Vin. Measure Vout; it should read
0V. If desired, one can measure the shutdown
supply current at this point. The supply current
will be approximately 1 uA in shutdown.
4.
Turn on the circuit by inserting the shunt in jumper
JP2 into the ON (upper) position. The output voltage should be regulating. Measure Vout - it should
measure 1.2V +/- 2%.
5.
Vary the input voltage from 2.25V to 5.5V and adjust the load current from 0 to 8A. Vout should
read between 1.2V +/- 4%.
6.
Measure the output ripple voltage at any output
current level; it usually will measure less than 20
mVAC.
7.
Observe the voltage waveform at the switch node
(one pin of the inductor). Verify the switching frequency is between 0.88 MHz and 1.12 MHz (T =
1.13 us and 0.893 us), and that the switch node
waveform is rectangular in shape.
Insert jumper JP2 shunt into the OFF position and move
the 1.2V Vout shunt into any of the remaining output
voltage options: 1.5V or 1.8V. Re-insert jumper JP2 into
the ON position. Just as in the 1.2Vout test, the output
voltage should read Vout +/- 2% tolerance under static
line and load conditions, and +/- 2% tolerance under
TM
dynamic line and load conditions. The Burst-Mode capability of the LTC3418 can also be observed now by
changing the JP3 jumper shunt from the Force Continuous (upper) position to the Burst-ModeTM (lower)
position.
When finished, turn off the circuit (connecting the
RUN pin to ground) by inserting the shunt in jumper
JP2 into the OFF (lower) position.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 785
HIGH EFFICIENCY, 8A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 785
HIGH EFFICIENCY, 8A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
Figure 3. Load Step Response
Vin – 3.3V
Vout – 1.8V
Iout Step – 7A
Trace 1: Output Voltage (50 mV/div AC)
Trace 2: Output Current (5A/div)
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 785
HIGH EFFICIENCY, 8A MONOLITHIC SYNCHRONOUS BUCK REGULATOR
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