DEMO MANUAL DC1897B LTC3605A 20V, 5A Monolithic Synchronous Step-Down Regulator DESCRIPTION Demonstration circuit 1897 is a step-down converter, using DC1897B can also track another voltage with the LTC3605A the LTC 3605A monolithic synchronous buck regulator. track function. Because of the high switching frequency of The DC1897B has a maximum input voltage of 20V, and the LTC3605A, which is programmable up to 4MHz, the is capable of delivering up to 5A of output current at a DC1897B uses low profile surface mount components. All minimum input voltage of 4V. The output voltage of the these features make the DC1897B an ideal circuit for use DC1897B can be set as low as 0.6V, the reference volt- in industrial applications and distributed power systems age of the LTC3605A. At low load currents, the DC1897B Design files for this circuit board are available at operates in discontinuous mode, and during shutdown, DEMO MANUAL DC1897B QUICK START PROCEDURE Demonstration Circuit 1897 is easy to set up to evaluate the Vary the input voltage from 4V to 20V and adjust the load performance of the LTC3605A. For proper measurement current from 0 to 5A. V should regulate around 2.5V OUT equipment configuration, set up the circuit according to 3% (2.425V to 2.575V). Measure the output ripple volt- the diagram in Figure 1. Before proceeding to test, check ageit should measure less than 30mV AC. that the shunts are inserted into these positions: the Observe the voltage waveform at the switch pins (the 2.5V output voltage header JP1, the 180 out-of-phase other side of the inductor from the output). Verify the (2-PHASE) position of the phase mode (PHMODE) header switching frequency is between 800kHz and 1.2MHz JP5, the forced continuous mode (FCM) position of mode (t = 1.25ns and 833ns), and that the switch node waveform header JP6, and the on position of run header JP7. is rectangular in shape. When measuring the input or output voltage ripple, care Change the shunt position on the MODE header from FCM must be taken to avoid a long ground lead on the oscil- to DCM (discontinuous mode). Set the input voltage to loscope probe. Measure the input or output voltage ripple 12V and the output current to any current less than 1A. by touching the probe tip directly across the VIN or VOUT Observe the discontinuous mode of operation at the switch and GND terminals. See Figure 2 for proper scope probe node, and measure the output ripple voltage. It should measurement technique. measure less than 100mV AC. With the DC1897B set up according to the proper measure- Insert the JP7 shunt into the OFF position and move the ment configuration and equipment in Figure 1, apply 6.3V shunt in the 2.5V output JP1 header into any of the two at VIN (do not increase V over the rated maximum supply IN remaining output voltage option headers: 3.3V (JP2) or voltage of 20V, or the part may be damaged). Measure 5V (JP3). Just as in the 2.5V VOUT test, the output voltage V it should read 2.5V (If desired, the quiescent current OUT should read VOUT 1% tolerance under static line and of the circuit can be monitored now by swapping the shunt load conditions and 1% tolerance under dynamic line in header JP7 into the OFF position). The output voltage and load conditions (2% total). Also, the circuit operation should be regulating. Measure V it should measure OUT in discontinuous mode will be the same. When finished, 2.5V 2% (2.45V to 2.55V). turn off the circuit by inserting the shunt in header JP7 into the OFF position. dc1897bf 2