2-PHASE DUAL STEP-DOWN CONVERTER FOR DDR/QDR MEMORY TERMINATION
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 829
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
(V
out2
) tracks ½ V
out1
(V
ref
) and is capable of providing
symmetrical source/sink output current. The constant
frequency current mode architecture with MOSFET
V
DS
sensing eliminates the need for sense resistors
and improves efficiency. Out of phase operation sig-
nificantly reduces input ripple current as well as the
input capacitor size.
Table 1. Performance Summary (T
A
= 25°C)
PARAMETER
Input Voltage Range
V
OUT1
V
IN
= 2.75-8V, I
OUT1
= 0A to 5A,
I
OUT2
= 0A to 5A
V
OUT2
V
IN
= 2.75-8V, I
OUT1
= 0A to 5A,
I
OUT2
= 0A to 5A
Maximum load current I
OUT1
Maximum load current I
OUT2
Typical Output Ripple V
OUT1
Typical Output Ripple V
OUT2
Typical Switching Frequency
Typical Spread Spectrum Frequency Range
V
IN
= 5V
V
IN
= 5V
V
IN
= 3.3V, I
OUT1
= 3A (20MHz BW)
V
IN
= 3.3V, I
OUT2
= 3A (20MHz BW)
PLLLPF Pin Floating; JP1:300K/550K/750KHz
JP1: Spread Spectrum
5A
+/-5A
20mV
P–P
7mV
P–P
550kHz
450kHz- 580kHz
1.25V ±2.5%
CONDITION
VALUE
2.75V to 8V
2.5V ±2.5%
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 V
IN
selects 750KHz operation. The
switching frequency can also be externally synchro-
nized 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 signifi-
cantly reduced peak switching noise.
Design files for this circuit board are available. Call
the LTC factory.
QUICK START PROCEDURE
Demonstration circuit 829 is easy to set up to evalu-
ate 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 out-
put voltage ripple by touching the probe tip directly
1
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
5.
Once the proper output voltages are established,
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 829
+Vin (2.75V-8V) and GND (input return).
2.
Connect the 2.5V load (Load 1 in Figure 1) between
adjust the loads within the operating range and ob-
serve the output voltage regulation, ripple voltage
and other parameters.
6.
To test sink current operation of Vout2 (1.25V), the
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.
4.
Turn on the input power supply and check for 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.
proper output voltages. Vout1 should be 2.5V+/-
2.5%. Vout2 should be 1.25V+/-2.5%.
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
VIN
GND
2
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