DC774A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 774
HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH MARGINING AND
TRACKING
LTC3770EUH
DESCRIPTION
Demonstration circuit 774 is a high efficiency synchronous step-down DC/DC converter with 5V to 28V input
range. It can provide 2.5V output with 10A maximum
current. The demo board features the LTC3770EUH controller. The LTC3770 has a high accuracy +/-0.67% 0.6V
reference voltage and provides many advanced functions
including output tracking, programmable voltage margining, programmable current limit, output over voltage
protection and input under voltage lockout, power good
monitor, programmable soft start, selectable discontinuous operation mode or forced continuous mode at light
load, and adjustable dead time between the top gate and
bottom gate signals to optimize the efficiency.
Design files for this circuit board are available. Call
the LTC factory.
Table 1. Performance Summary (TA = 25°C)
PARAMETER
CONDITION
VALUE
Input Voltage Range
5V to 28V
Output Voltage
VIN = 5-28V, IOUT = 0A to 10A,
2.5V ±2%
Maximum load current
VIN = 5-28V
10A
Typical Output Ripple
VIN = 12V, IOUT = 10A (20MHz BW)
60mVP–P
Typical Switching Frequency
VIN = 12V, IOUT = 10A
250kHz
OPERATING PRINCIPLES
The LTC3770 uses a constant on-time, valley current
mode control architecture able to deliver very low duty
cycles with fast transient response without requiring a
sense resistor. The switching frequency of the converter
can be selected by an external resistor Ron and is compensated for variations on input supply voltage. An internal phase-lock loop allows the controller to be synchronized to an external clock. Figure 3 gives the efficiency curves (JP1: DCM).
The LTC3770 has the output voltage up and down tracking capability. Using the TRACK/SS pin, the converter
output can track another supply’s output voltage. To select tracking function, place JP2 at TRACK, take off the
soft start cap Css and connect a resistor divider to the
external voltage to be tracked. The tracking can be coincident or ratiometric, as shown in figure 4.
Voltage margining is the dynamic adjustment of the output voltage to its worst case operating range during test
to screen the components, verify some control/protection functionality of the board and improve the
system reliability. The LTC3770 has two logic control
pins, MARGIN1 and MARGIN0, to enable and set up
margin up (higher output voltage) or margin down
(lower output voltage). Table 2 is a summary of the configurations.
When the margining function is enabled, the error amplifier reference voltage will be adjusted to: VREFIN=0.6V +/1.18VxR5/R6
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 774
HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH MARGINING AND
TRACKING
By selecting the ratio of R5 to R6, the margin voltage is
programmable and the converter output can be set to a
desirable testing range.
QUICK START PROCEDURE
Demonstration circuit 774 is easy to set up to evaluate
the performance of LTC3770. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
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.
1.
Initial jumper positions:
JP2: SS; JP1: FC
2.
With power off, connect the input power supply to
VIN (5V-28V) and GND (input return).
3.
Connect the 2.5V load between VOUT and GND. (Initial load: 0 A)
4.
Connect the DVMs to the input and output.
5.
Turn on the input power supply and check for the
proper output voltages. VOUT should be 2.5V+/-2%.
6.
Once the proper output voltage is established, adjust
the load within the operating range and observe the
output voltage regulation, ripple voltage and other parameters.
Figure 1. Proper Measurement Equipment Setup
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 774
HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH MARGINING AND
TRACKING
GND
VIN
Figure 2. Measuring Input or Output Ripple
100
98
96
EFFICIENCY (%)
94
92
90
88
86
84
V in=5V
82
V in=12V
80
0
2
4
6
8
10
LO A D C U R R E N T (A )
Figure 3. Efficiency vs load current
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 774
HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH MARGINING AND
TRACKING
(a) Coincident tracking
(b) Ratiometric tracking
Figure 4. (a)(b) Up/down output tracking. Upper waveform: external voltage; lower waveform: output voltage of the LTC3770 converter
Table 2. Margining Function
MARGIN1
MARGIN0
MODE
Low
Low
No margining
Low
High
Margining up
High
Low
Margining down
High
High
No margining
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 774
HIGH EFFICIENCY, HIGH POWER BUCK CONVERTER WITH MARGINING AND
TRACKING
5
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