advertisement Dual Output Monolithic Synchronous Buck Regulator Operates from 3.6V to 15V Inputs and Delivers 3A Per Channel – Design Note 1020 Tom Gross Introduction The LTC3633 is a high efficiency, dual output monolithic synchronous step-down switching regulator, capable of delivering 3A of output current per channel from input voltages between 3.6V and 15V. Its compact 4mm × 5mm QFN package has very low thermal impedance eliminating the need for an external heat sink. Its high input voltage capability makes the LTC3633 ideal for 12V distributed power systems. 1.8V and 3.3V 2MHz Buck Regulator Figure 1 shows a LTC3633 12VIN to 1.8VOUT and 3.3VOUT, 3A, buck regulator. With the frequency setting pin, RT, connected to the internal 3.3V regulator rail (INTVCC), this circuit runs at 2MHz —the LTC3633 switching frequency is programmable from 500kHz to as high as 4MHz, and can also be synchronized to an external clock—reducing the value and size of the output capacitors and inductor. Even with this high switching frequency, the efficiency of this circuit is good: 87% at full load for the 3.3V regulator and 82% at full load for the 1.8V regulator, at 12VIN (see Figure 2). VIN 3.6V TO 15V CIN3 22µF 16V 1206 VISHAY IHLP-2020BZER1R0M01 VOUT1 1.8V 3A 20 BOOST1 CBST1 0.1µF 23 SW1 R3 20k 1% COUT3 22µF 6.3V 1206 COUT5 10µF 6.3V 0805 CIN1 47µF 16V 1210 L1 1µH VOUT1 COUT1 22µF 6.3V 1206 CIN2 47µF 16V 1210 24 25 10 CFFW1 10pF 28 27 R5 10k 1% 94 92 VOUT = 3.3V EFFICIENCY (%) 90 88 86 VOUT = 1.8V 84 82 80 VIN = 12V FSW = 2MHz Burst Mode® OPERATION 78 76 0 0.5 DN1020 F02 By connecting the phase mode pin (PHMODE) to INTVCC, the two internal regulators operate 180 degrees out-of-phase with respect to each other, reducing the input current ripple, and thus, the number and value of input capacitors. Due to a controlled on-time design, the LTC3633 achieves very fast load transient response L, LT, LTC, LTM, Linear Technology, Burst Mode and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 6 RUN2 15 16 VIN2 VIN2 17 BOOST2 SW2 SW1 SW2 LTC3633EUFD VON2 TRACKSS2 VFB2 VFB1 PGND TRACKSS1 SGND 14 13 POWER GROUND RPG1 100k RPG2 100k VOUT2 VISHAY IHLP-2020BZER1R0M01 12 COUT2 22µF 6.3V 1206 R4 61.9k 1% CFFW2 10pF 9 COUT4 22µF 6.3V 1206 29 R6 13.7k 1% 7 COUT6 10µF 6.3V 0805 PGOOD2 Figure 1. LTC3633: 1.8VOUT and 3.3VOUT, 2MHz Buck Regulator 11/10/1020 SIGNAL GROUND L2 CBST2 1µH 1µF CSS1 4700pF PGOOD1 ITH1 RT PHMODE INTVCC MODE/SYNC V2P5 ITH2 PGOOD2 1 26 5 2 19 4 18 11 8 PGOOD1 3.5 3 Figure 2. High Efficiency from Each Channel for the Circuit in Figure 1 21 22 3 VIN1 VIN1 RUN1 VON1 1 1.5 2 2.5 LOAD CURRENT (A) VOUT2 3.3V 3A DN1020 F01 1.2VOUT, 6A, Out-of-Phase Supply The LTC3633’s two internal regulators can be combined form a single output circuit, operating out-of-phase to deliver high total output current with minimal input and output capacitance—useful for distributed power systems. This design also generates lower high-frequency EMI noise than a single-phase design, due to the lower input ripple current caused by the out-of-phase switching currents with respect to each other (see Figure 4). The 12VIN to 1.2VOUT dual-phase LTC3633 regulator is shown in Figure 4, with Figure 5 showing its load step response. with minimal output capacitance, as shown in Figure 3. With only 50µF of output capacitance, it takes only 10µs to recover from a 3A load step that results in less VOUT 100mV/DIV AC-COUPLED IOUT 1A/DIV 20µs/DIV DN1020 F03a VOUT1: 12VIN TO 1.8VOUT LOAD STEP 3A LOAD STEP, 2MHz SWITCHING FREQUENCY FORCED CONTINUOUS MODE, INTERNAL COMPENSATION Conclusion The dual 3A output LTC3633 provides a high performance compact regulator solution for powering low voltage loads in 12V intermediate bus systems. Configured as a two-phase, single output regulator, it can deliver up to 6A with minimum input ripple and an effective switching frequency up to 8MHz. VOUT 100mV/DIV AC-COUPLED IOUT 1A/DIV 20µs/DIV DN1020 F03b VOUT2: 12VIN TO 3.3VOUT LOAD STEP 3A LOAD STEP, 2MHz SWITCHING FREQUENCY FORCED CONTINUOUS MODE, INTERNAL COMPENSATION VOUT 100mV/DIV AC-COUPLED Figure 3. Load Step Responses for the Figure 1 Circuit IOUT 2A/DIV than 100mV of output voltage deviation. Note that compensation is internal, set up by tying the compensation pin (ITH) to INTVCC. This connects an internal series RC to the compensation point of the loop, eliminating the need for external compensation components. To further optimize the loop, an external RC circuit can be applied to the ITH pin. VIN 3.6V TO 15V CIN3 22µF 16V 1206 PGOOD1 CIN2 47µF 16V 1210 CIN1 47µF 16V 1210 RPG1 100k 21 22 3 VIN1 VIN1 RUN1 1 26 RITH 6.49k 1% 27 CSS1 4700pF CITH 470pF 19 CVCC 1µF 18 5 11 10 SIGNAL GROUND 15 16 VIN2 VIN2 PGOOD1 PGOOD2 ITH1 BOOST1 TRACKSS1 SW1 INTVCC V2P5 CSS2 4700pF LTC3633EUFD SW1 VON1 VFB1 PHMODE VFB2 RT BOOST2 8 20 23 24 SW2 TRACKSS2 SW2 7 PGND 29 VON2 RPG2 100k PGOOD2 L1 CBST1 0.47µH 0.1µF VOUT1 VISHAY IHLP-2525CZERR47M01 CFFW1 10pF 25 R1 20k 1% 28 9 17 ITH2 SGND DN1020 F05 Figure 5. Load Step Response 4 MODE/SYNC 2 POWER GROUND 6 RUN2 20µs/DIV DUAL VOUT: 12VIN TO 1.2VOUT LOAD STEP 6A LOAD STEP, 2MHz SWITCHING FREQUENCY FORCED CONTINUOUS MODE, EXTERNAL COMPENSATION 14 13 CFFW2 10pF CBST2 L2 0.1µF 0.47µH R2 20k 1% VOUT2 VISHAY IHLP-2525CZERR47M01 COUT2 22µF 6.3V 1206 12 COUT4 22µF 6.3V 1206 COUT1 22µF 6.3V 1206 COUT3 22µF 6.3V 1206 COUT5 10µF 6.3V 0805 COUT6 10µF 6.3V 0805 VOUT 1.2V 6A DN1020 F04 Figure 4. 6A Out-of-Phase Buck Regulator Using the LTC3633 Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 3229 dn1020f LT/TP 1110 • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2010