DC829A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 829
2-PHASE DUAL STEP-DOWN CONVERTER FOR DDR/QDR MEMORY TERMINATION
LTC3776EUF
DESCRIPTION
Demonstration circuit 829 is a high efficiency 2-phase
dual synchronous step-down DC/DC converter for
DDR/QDR memory termination applications featuring
the LTC3776EUF controller. The input voltage range of
the board is from 2.75V to 8V. It has two outputs:
2.5V (5A maximum at 5V input) and 1.25V (+/-5A
maximum at 5V input). The channel 2 output voltage
(Vout2) tracks ½ Vout1 (Vref) and is capable of providing
symmetrical source/sink output current. The constant
frequency current mode architecture with MOSFET
VDS sensing eliminates the need for sense resistors
and improves efficiency. Out of phase operation significantly reduces input ripple current as well as the
input capacitor size.
Switching frequency is internally set at 550KHz. The
frequency can be adjusted using PLLLPF pin. Tying
PLLLPF pin to GND selects 300KHz operation; tying
PLLLPF pin to VIN selects 750KHz operation. The
switching frequency can also be externally synchronized from 250KHz to 850KHz using SYNC pin (JP1:
SYNC).
The demo board can be selected to operate in spread
spectrum mode (JP1: Spread Spectrum) with significantly reduced peak switching noise.
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
VOUT1
2.75V to 8V
VIN = 2.75-8V, IOUT1 = 0A to 5A,
2.5V ±2.5%
IOUT2 = 0A to 5A
VOUT2
VIN = 2.75-8V, IOUT1 = 0A to 5A,
1.25V ±2.5%
IOUT2 = 0A to 5A
Maximum load current IOUT1
VIN = 5V
5A
Maximum load current IOUT2
VIN = 5V
+/-5A
Typical Output Ripple VOUT1
VIN = 3.3V, IOUT1 = 3A (20MHz BW)
20mVP–P
Typical Output Ripple VOUT2
VIN = 3.3V, IOUT2 = 3A (20MHz BW)
7mVP–P
Typical Switching Frequency
PLLLPF Pin Floating; JP1:300K/550K/750KHz
550kHz
Typical Spread Spectrum Frequency Range
JP1: Spread Spectrum
450kHz- 580kHz
QUICK START PROCEDURE
Demonstration circuit 829 is easy to set up to evaluate the performance of LTC3776. 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
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 829
2-PHASE DUAL STEP-DOWN CONVERTER FOR DDR/QDR MEMORY TERMINATION
across the Vin or Vout and GND terminals. See
Figure 2 for proper scope probe technique.
1. With power off, connect the input power supply to
+Vin (2.75V-8V) and GND (input return).
2. Connect the 2.5V load (Load 1 in Figure 1) between
Vout1 and GND; connect the 1.25V load (Load 2 in
Figure 1) between Vout2 and GND. (Initial loads: 0
A)
3. Connect the DVMs to the input and outputs.
5. Once the proper output voltages are established,
adjust the loads within the operating range and observe the output voltage regulation, ripple voltage
and other parameters.
6. To test sink current operation of Vout2 (1.25V), the
Load 2 can be connected from Vout1 (load +) to
Vout2 (load-). Please note Vout1 provides the
source current for Load 2 with this set up. At 5V
input, the total load current at Vout1 should not be
higher than 5A.
4. Turn on the input power supply and check for the
proper output voltages. Vout1 should be 2.5V+/2.5%. Vout2 should be 1.25V+/-2.5%.
Figure 1. Proper Measurement Equipment Setup
GND
VIN
Figure 2. Measuring Input or Output Ripple
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 829
2-PHASE DUAL STEP-DOWN CONVERTER FOR DDR/QDR MEMORY TERMINATION
100
Vo=2.5V
90
80
Vo=1.25V
70
60
50
40
Vin=3.3V
30
10
100
1k
10k
LOA D CURRENT (mA )
Figure 3. Efficiency vs load current (550KHz)
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 829
2-PHASE DUAL STEP-DOWN CONVERTER FOR DDR/QDR MEMORY TERMINATION
4
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