DC686A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC686
HIGH EFFICIENCY STEP-UP/DOWN DC/DC CONVERTER
LTC3780
DESCRIPTION
Demonstration circuit 686, featuring the
LTC3780, is a high efficiency step-up/down
DC/DC converter that can regulate its output
from input voltages both above and below the
output. This board delivers 12V at 5 Amps from
an input range of 5V to 32V.
The LTC3780 is a high performance buck-boost
switching regulator controller designed to
regulate outputs using input voltages above,
below or equal to the output voltage. The
constant frequency current mode architecture
allows phase-lockable frequency of up to
400kHz. A wide input and output range with
continuous transfer function through all
operation modes, makes the product ideal for
automotive, telecom and battery systems.
Table 1.
The LTC3780 is equipped with fault protection
provided by an output overvoltage comparator
and internal foldback current limiting. Soft-start
control is accomplished with an external
capacitor that controls the slew rate of the
reference voltage for the error amplifier, which
in turn controls the output slew rate of the
regulator, and optional timed short-circuit
shutdown. A Power Good output pin indicates
when the output is within 7.5% of designed set
point.
Design files for this circuit board are
available. Call the LTC factory.
LTC is trademark of Linear Technology Corporation
Performance Summary (TA = 25°C)
PARAMETER
CONDITION
VALUE
Load Regulation
Vin = 32V, Vout = 12V, 0 – 5A load
+2%
Line Regulation
Vin = 5 to 32V, Vout = 12V, 5A load
+2%
Maximum Output Current
Vin = 5 - 32V, Vout = 12V
5.0A
Full Load Efficiency @ 5Vin
Vin = 5V, Vout=12V, Io=5A
90%
Full Load Efficiency @ 12Vin
Vin=12V, Vout=12V, Io=5A
98%
Full Load Efficiency @ 32Vin
Vin = 32V, Vout=12V, Io=5A
90%
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC686
HIGH EFFICIENCY STEP-UP/DOWN DC/DC CONVERTER
QUICK START PROCEDURE
Demonstration circuit 686 is easy to set up to
evaluate the performance of the LTC3780.
For proper equipment setup, follow the
procedure below, referring to figure 1:
1. With the power source turned off, connect
the input power supply to the board
through the VIN+ (J1) and GND (J2)
terminals.
2. Connect the load to the VOUT (J3) and
GND (J4) terminals.
3. Set the RUN jumper at JP1 to the OFF
position.
4. Position the STBY jumper at JP4 to the
ON position for initial testing.
5. Set the FSET jumper at JP3 to the INT
PROG position.
6. Position the switching mode jumper at JP2
to position A (BURST) for initial testing.
7. Turn on the input power source to at least
6V, but below 32V.
8. Change the RUN jumper at JP1 to the ON
position.
9. Verify that the output voltage is 12V. If
there is no output, temporarily disconnect
the load to make sure that the load is not
too high.
10. Once the proper output voltage is
established, vary the input voltage and
load within the given operating ranges and
observe the output voltage regulation,
ripple voltage, efficiency and other
parameters.
OPERATION
Demonstration circuit 686, featuring the
LTC3780, is able to regulate loads for input
voltages below, equal to or above the output
voltage. If the input voltage is below the
output voltage, the LTC3780 acts as a boost
controller. If the input is above the output, the
LTC3780 acts as a buck controller. If the
input voltage is near the output, the LTC3780
behaves as a buck-boost. For a more
detailed description of this operation and how
the chip transitions from mode to mode,
please refer to the LTC3780 data sheet.
Demonstration circuit 686 is configured to
operate the LTC3780 at a switching frequency
of 200kHz and regulate its output at 12VDC to
a maximum load of 5A. It is possible to
synchronize the switching frequency to an
external source between 200kHz and 400kHz.
To do so, move the jumper at JP3 to the EXT
SYNC position and apply the external signal,
preferably a square wave, to TP10 (SYNC). If
synchronization is not required, the user can
still vary the frequency by applying the JP3
jumper to the EXT SYNC position and
applying 0.5V to 2V directly to the FSET
terminal (TP9). The user should bear in mind
that increasing the switching frequency also
increases the switching losses and thus
reduces the efficiency of the demonstration
circuit.
The user may also configure the board to
operate in one of three control modes – burst
mode, discontinuous mode and constant
current mode. These modes are activated by
setting the JP2A, 2B, or 2C positions, labeled
BURST for burst mode, DCM for
discontinuous mode and CCM for constant
current mode. These modes are discussed in
detail in the LTC3780 data sheet. The user is
cautioned, however, that continuous current
mode allows the output to either sink or
source current, which may raise the input
voltage to undesirable levels.
TP3 is for monitoring only. Do not apply the
load here, as damage may occur. Only apply
the output load to the J3 and J4 power jacks.
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC686
HIGH EFFICIENCY STEP-UP/DOWN DC/DC CONVERTER
Other terminals are provided for monitoring.
TP4 is used to examine the open drain
PGOOD signal, which indicates that the
output is within regulation. For purposes of
demonstration, the PGOOD signal is pulled up
through a 10K resistor to the on-board 6V,
which may not be suitable for all systems.
TP5 is attached to the soft start pin of the
LTC3780. Please refer to the data sheet for
detailed information on these two functions.
When power is applied, the LTC3780
becomes operational once the voltage
exceeds 5.1V. If the full rated load of DC686
is present at the output when power is
applied, the input will ramp up to almost 14A.
If the output impedance of the power source
or the wiring resistance is too high, the input
voltage at the demonstration board will fall
below the UVLO trip point on the LTC3780
and cause it to shut down the output. This will
in turn cause the input current to fall and the
input voltage to rise, turning on the LTC3780
again, resulting in a “motor-boat” behavior.
Be careful to use a power supply with
sufficient output current and connect it to
DC686 with sufficiently low gauge wire.
Furthermore, as the LTC3780 must provide
essentially 100% duty cycle to either Q3 or Q4
in boost or buck mode, the charge pump that
provides voltage to these MOSFET gates
must be refreshed. This refresh event occurs
every 10 or so switching cycles and has the
net effect of injecting a small delay between
adjacent PWM pulses. The proper way to
examine the switching waveforms is by
synchronizing the oscilloscope to the refresh
pulse
For example, if you want to examine the
switching waveform at the inductor, apply
probes to both sides. On one side, you will
see the 200kHz switching waveform. On the
other, you will see the refresh pulse at about
1/10th the frequency.
Synchronize the oscilloscope to the side of
the inductor switching at the refresh pulse
frequency.
When measuring switching waveforms on
DC686, the user must keep the operating
modes in mind. Suppose the user measures
the waveform at the inductor. Depending on
whether the operating mode is boost, buckboost or buck, the 200kHz switching may
appear on one or the other or both sides of
the inductor.
3
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT DC686
HIGH EFFICIENCY STEP-UP/DOWN DC/DC CONVERTER
Figure 1. DC686 Test and Measurement Setup Diagram
4
5
4
3
2
1
D
D
1
PGOOD
2
3
4
SENSE-
5
ITH
6
VOSENSE
INTVCC
19
7
SGND
BG1
18
8
RUN
PGND
17
C
1
2
9
3
BOOST1
24
SS
TG1
23
SENSE+
SW1
22
VIN
21
EXTVCC
20
C
FCB
BG2
16
10
PLLFLTR
SW2
15
11
PLLIN
TG2
14
12
STBYMD
BOOST2
13
B
B
1
2
3
1
2
3
A
A
5
4
3
2
1
5
4
3
2
1
D
D
78
56
2
C
4
C
1
3
56
78
4
2
3
1
B
B
A
A
5
4
3
2
1
Similar pages