DC829A Analog Devices

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 Switching frequency is internally set at 550KHz. The dual synchronous step-down DC/DC converter for frequency can be adjusted using PLLLPF pin. Tying DDR/QDR memory termination applications featuring PLLLPF pin to GND selects 300KHz operation tying the LTC3776EUF controller. The input voltage range of PLLLPF pin to V selects 750KHz operation. The IN the board is from 2.75V to 8V. It has two outputs: switching frequency can also be externally synchro- 2.5V (5A maximum at 5V input) and 1.25V (+/-5A nized from 250KHz to 850KHz using SYNC pin (JP1: maximum at 5V input). The channel 2 output voltage SYNC). (V ) tracks V (V ) and is capable of providing out2 out1 ref The demo board can be selected to operate in spread symmetrical source/sink output current. The constant spectrum mode (JP1: Spread Spectrum) with signifi- frequency current mode architecture with MOSFET cantly reduced peak switching noise. V sensing eliminates the need for sense resistors DS and improves efficiency. Out of phase operation sig- nificantly reduces input ripple current as well as the Design files for this circuit board are available. Call input capacitor size. the LTC factory. Table 1. Performance Summary (T = 25C) A PARAMETER CONDITION VALUE Input Voltage Range 2.75V to 8V V V = 2.75-8V, I = 0A to 5A, 2.5V 2.5% OUT1 IN OUT1 I = 0A to 5A OUT2 V V = 2.75-8V, I = 0A to 5A, 1.25V 2.5% OUT2 IN OUT1 I = 0A to 5A OUT2 Maximum load current I V = 5V 5A OUT1 IN Maximum load current I V = 5V +/-5A OUT2 IN Typical Output Ripple V V = 3.3V, I = 3A (20MHz BW) 20mV OUT1 IN OUT1 PP Typical Output Ripple V V = 3.3V, I = 3A (20MHz BW) 7mV OUT2 IN OUT2 PP 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 evalu- NOTE: When measuring the input or output voltage ate the performance of LTC3776. Refer to Figure 1 for ripple, care must be taken to avoid a long ground lead proper measurement equipment setup and follow the on the oscilloscope probe. Measure the input or out- procedure below: put voltage ripple by touching the probe tip directly 1 GND VIN 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 5. Once the proper output voltages are established, Figure 2 for proper scope probe technique. adjust the loads within the operating range and ob- serve the output voltage regulation, ripple voltage 1. With power off, connect the input power supply to and other parameters. +Vin (2.75V-8V) and GND (input return). 6. To test sink current operation of Vout2 (1.25V), the 2. Connect the 2.5V load (Load 1 in Figure 1) between Load 2 can be connected from Vout1 (load +) to Vout1 and GND connect the 1.25V load (Load 2 in Vout2 (load-). Please note Vout1 provides the Figure 1) between Vout2 and GND. (Initial loads: 0 source current for Load 2 with this set up. At 5V A) input, the total load current at Vout1 should not be 3. Connect the DVMs to the input and outputs. 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 Figure 2. Measuring Input or Output Ripple 2