DEMO CIRCUIT 1572A LTC3615 QUICK START GUIDE LTC3615 DUAL 4 MHz, 3A SYNCHRONOUS STEP-DOWN DC/DC CONVERTER DESCRIPTION Demonstration circuit 1572 is a dual output regulator ther circuit. The LTC3615 comes in a 24 Pin QFN or consisting of two constant-frequency step-down convert- leaded package, which each having an exposed pad on ers, based on the LTC3615 dual channel monolithic syn- the bottom-side of the IC for better thermal performance. chronous buck regulator. The DC1572 has an input volt- These features, plus a programmable operating frequency age range of 2.25V to 5.5V, with each regulator capable of range from 400 kHz to 4 MHz (2.25 MHz switching fre- delivering up to 3A of output current. The DC1572 can quency with the RT pin connected to SV ), make the IN operate in Burst Mode, pulse-skip or forced continuous DC1572 demo board an ideal circuit for use industrial or mode. In shutdown, the DC1572 requires less than 100 distributed power applications. Gerber files for this cir- uA total. The DC1572 is very efficient: over 90% for ei- cuit are available. Call the LTC Factory. QUICK START PROCEDURE The DC1572 is easy to set up to evaluate the per- and J4, shunt XJ7 into the forced continuous mode formance of the LTC3615. For a proper measure- (FCM) position of MODE header JP7, shunt XJ6 into ment equipment configuration, set up the circuit ac- the 2.25 MHz (RT) position of the frequency cording to the diagram in Fig. 1. (RT/SYNC) header JP6, shunts XJCOMP1 and XJCOMP2 into the external compensation (ECMP) NOTE: When measuring the input or output voltage positions of headers JITH1 and JITH2, and shunt ripple, care must be taken to avoid a long ground XJP1 into the Vout1 voltage option of choice of lead on the oscilloscope probe. Measure the input or header JP1-4: 1.8V, 2.5V, 3.3V, or user select, and output voltage ripple by touching the probe tip di- shunt XJP5 into the Vout2 voltage option of choice rectly across the Vin or Vout and GND terminals. See of header JP5-8: 1.2V, 1.5V, 1.8V, or user select. the proper scope probe technique in figure 2. 2. Apply 5.5V at Vin. Measure both Vouts they Please follow the procedure outlined below for should read 0V. If desired, one can measure the proper operation. shutdown supply current at this point. The supply current will be less than 100 uA in shutdown. 1. Connect the input power supply to the Vin and 3. Turn on Vout1 and Vout2 by shifting shunts XJ1 GND terminals. Connect the loads between the Vout and XJ2 from the OFF positions to the ON positions. and GND terminals. Refer to figure 1 for the proper Both output voltages should be within a tolerance of measurement equipment setup. +/- 2%. Before proceeding to operation, insert jumper 4. Vary the input voltage from the min. Vin (which shunts XJ1 and XJ2 into the OFF positions of head- is dependent on Vout) to 5.5V, and the load currents ers J1 and J2, shunt XJ5 into the 180 out-of-phase from 0 to 3A. Both output voltages should be within position of PHASE header J5, shunts XJ3 and XJ4 +/- 4% tolerance. into the soft-start (EXT SS) positions of headers J3 1 LTC3615 5. Set the load current of both outputs to 3A and the input voltage to 5.5V, and then measure each output ripple voltage (refer to figure 2 for proper measurement technique) they should each measure less than 30 mVAC. Also, observe the voltage waveform at either switch node of each regulator. The switching fre- quencies should be between 1.85 MHz and 2.65 MHz (T = 540 ns and 374 ns). In all cases, both switch node waveforms should be rectangular in shape, and 180 out-of-phase with each other. Change the shunt position on header J5 to set the switch waveforms in phase with respect to each other. To operate the ckt.s in Burst Mode or pulse-skip mode, change the shunt in header J7. When finished, insert shunts XJ1 and XJ2 to the OFF position(s) and disconnect the power. Warning - If the power for the demo board is carried in long leads, the input voltage at the part could ring, which could affect the operation of the circuit or even exceed the maximum voltage rating of the IC. To eliminate the ringing, a small tantalum capacitor (for instance, AVX part TPSY226M035R0200) is inserted on the pads between the input power and return terminals on the bottom of the demo board. The (greater) ESR of the tantalum will dampen the (possible) ringing voltage due to the use of long input leads. On a normal, typical PCB, with short traces, this capacitor is not needed. Table 1. Performance Summary (T = 25C) A PARAMETER CONDITIONS VALUE Minimum Input Voltage 2.25V Maximum Input Voltage 5.5V RUN Pin = GND Shutdown Run RUN Pin = V Operating IN V = 2.5V to 5.5V, I = 0A to 3A 1.8V 4% (1.728V 1.872V) IN OUT1 V = 3.3V to 5.5V, I = 0A to 3A 2.5V 4% (2.4V 2.6V) Output Voltage V IN OUT1 OUT1 V = 3.9V to 5.5V, I = 0A to 3A 3.3V 4% (3.168V 3.432V) IN OUT1 Typical Output Ripple V V = 5V, I = 3A (20 MHz BW) < 30mV OUT1 IN OUT1 PP Line 1% Output Regulation V OUT1 Load 1% 1.2V 4% (1.152V 1.248V) V = 2.25V to 5.5V, I = 0A to 3A IN OUT2 Output Voltage V OUT2 1.5V 4% (1.44V - 1.56V) V = 2.5V to 5.5V, I = 0A to 3A 1.8V 4% (1.728V 1.872V) IN OUT2 Typical Output Ripple V V = 5V, I = 3A (20 MHz BW) < 30mV OUT2 IN OUT2 PP Line 1% Output Regulation V OUT2 Load 1% RT Pin connected to 178k 2.25 MHz Nominal Switching Frequencies RT Pin = SV 2.25 MHz IN Channel 1: V = 5V, Vout1 = 3.3V Iout1 = 600 mA IN Burst Mode Operation Channel 2: V = 5V, Vout2 = 1.8V Iout2 = 700 mA IN Channel 1: V = 5V, Vout1 = 3.3V Iout1 = 600 mA IN Pulse-Skip Operation Channel 2: V = 5V, Vout2 = 1.8V Iout2 = 700 mA IN Phase Pin = SV 180Out-of-Phase IN Phase Phase Pin OPEN 90Out-of-Phase Phase Pin = GND In Phase 2