DEMO MANUAL DC326B NO-DESIGN SWITCHER OPERATION How to Measure Voltage Regulation and Efficiency When measuring voltage regulation or efficiency, voltage measurements should be made directly across the VOUT and GND terminals, not at the end of test leads at the load. Similarly, input voltage should be measured directly at the VIN and GND terminals of the LT1766 demo board. Input and output current should be measured by placing an ammeter in series with the input supply and load. Refer to figure 2 for proper monitoring equipment setup. How to Measure Output Voltage Ripple When measuring output voltage ripple, care must be taken to avoid a long ground lead on the oscilloscope probe. A sturdy wire should be soldered to the output side of the GND terminal. The other end of the wire is looped around the ground side of the probe and should be kept as short as possible. The tip of the probe is touched directly to VOUT (see Figure 3). Bandwidth is generally limited to 20MHz for ripple measurements. Also, if multiple pieces of line-powered test equipment are used, be sure to use isolation transformers on their power lines to prevent ground loops, which can cause erroneous results. Figure 4 shows the output voltage ripple with a steady-state load of 1A for the LT1766. Heat Dissipation Issues Since the LT1766 includes a 1.5A onboard power switch, care must be taken not to exceed the 125c maximum operating junction temperature for the part. A simple technique is to use the PC board as a heat sink. On the LT1766 demo board, the power IC is surrounded by ground plane on both sides of the PC board. The two sides are connected through vias to better handle the power dissipation. If the LT1766 is laid out on a multilayer board, there should be metal on the inner layers directly underneath the LT1766. This helps in spreading heat and improves the power dissipation capability of the PCB. Note: See Thermal Calculations section in the Applications Information of the LT1766 datasheet.DEMO MANUAL DC326B NO-DESIGN SWITCHER OPERATION Efficiency vs. Load Current Introduction The LT1766 is a 1.5A 200kHz Step-Down switching regulator capable of 100 Vout = 5V L = 47uH operation at input voltages as high as 60V. The demonstration circuit shown Vin=12V 90 in the schematic allows for output selection of 3.3V or 5V using the jumper J1. The board comes equipped with input (VIN), output (VOUT), GND, SYNC and 80 S/D terminals to simplify bench testing. The demonstration circuit highlights the Vin=42V 70 ability of the LT1766 to achieve excellent efficiencies at both high and low input voltages. The efficiency curves in Figure 1 illustrate both 42V to 5V and 60 12V to 5V conversions with peak efficiencies of greater than 80% and 90%. 50 0.00 0.25 0.50 0.75 1.00 1.25 Load Current (A) Figure 1. LT1766 efficiency vs. Load Current Shutdown Pin For normal operation, the S/D pin can be left floating. S/D has two output-disable modes, lockout and shutdown. When the pin is taken below the 2.38V lockout threshold, switching is disabled. This is typically used for input undervoltage lockout. Grounding the S/D pin places the LT1766 in shutdown mode. This reduces total board supply current to typically 25uA. Synchronization Pin To synchronize switching to an external clock, apply a logic-level signal to the SYNC pin. Amplitude must be from a logic low level to greater than 2.2V with a duty cycle from 10% to 90%. Synchronization frequency is possible from 228kHz up to 700kHz. Quick Start Guide A list of procedures for getting started, including the basic set-up for measurement equipment, are provided in the quick start guide attached. NOTE: The LT1766 datasheet should be read in conjunction with the demonstration board information provided. Efficiency (%)