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LTM8045 Datasheet(PDF) 13 Page - Linear Technology |
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LTM8045 Datasheet(HTML) 13 Page - Linear Technology |
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13 / 30 page  LTM4649 13 4649fc For more information www.linear.com/LTM4649 APPLICATIONS INFORMATION PLL and Frequency Synchronization The LTM4649 device operates over a range of frequen- cies to improve power conversion efficiency. The nominal switchingfrequencyis450kHz.Itcanalsobesynchronized from 400kHz to 800kHz with an input clock that has a high level above 2V and a low level below 0.8V at the CLKIN pin. Once the LTM4649 is synchronizing to an external clock frequency, it will always be running in Forced Continuous operation.Althoughsynchronizationto250kHzispossible, 400kHz is the lowest recommended operating frequency to limit inductor ripple current. Multiphase Operation For outputs that demand more than 10A of load current, multiple LTM4649 devices can be paralleled to provide more output current and reduced input and output volt- age ripple. The CLKOUT signal together with CLKIN pin can be used tocascadeadditionalpowerstagestoachieveamultiphase power supply solution. Tying the PHMODE pin to INTVCC, GND, or leaving it floating generates a phase difference (between CLKIN and CLKOUT) of 180°, 120°, or 90° respectively as shown in Table 2. A total of 4 phases can be cascaded to run simultaneously with respect to each other by programming the PHMODE pin of each LTM4649 channel to different levels. Figure 3 shows a 3-phase de- sign and 4-phase design example for clock phasing with the PHMODE table. Table 2. PHMODE and CLKOUT Signal Relationship PHMODE GND FLOAT INTVCC CLKOUT 120° 90° 180° The LTM4649 device is an inherently current mode con- trolled device, so parallel modules will have good current sharing. This will balance the thermals in the design. Tie the COMP, VFB,TRACK/SSandRUNpinsofeachLTM4649 together to share the current evenly. Figures 19 and 20 each show a schematic of a parallel design. 4649 F03 4-PHASE DESIGN 3-PHASE DESIGN 90 DEGREE 90 DEGREE 90 DEGREE 0 PHASE FLOAT FLOAT FLOAT FLOAT CLKIN VOUT PHMODE CLKOUT 90 PHASE CLKIN VOUT PHMODE CLKOUT 180 PHASE CLKIN VOUT PHMODE CLKOUT 270 PHASE CLKIN VOUT PHMODE CLKOUT 120 DEGREE 120 DEGREE 0 PHASE GND GND GND CLKIN VOUT PHMODE CLKOUT 120 PHASE CLKIN VOUT PHMODE CLKOUT 240 PHASE CLKIN VOUT PHMODE CLKOUT Figure 3. Examples of 3-Phase, 4-Phase Operation with PHMODE Table FREQ PIN VOLTAGE (V) 0 900 800 600 400 100 200 700 500 300 0 2 4649 F02 2.5 1 1.5 0.5 Figure 2. Operating Frequency vs FREQ Pin Voltage |
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