Datasheet LTC3605A (Analog Devices) - 10

LTC3605A
operaTion Multiphase Operation
example, then the output will drop out of regulation. The For output loads that demand more than 5A of current, minimum input voltage to avoid dropout is: multiple LTC3605As can be cascaded to run out of phase tON + tOFF(MIN) to provide more output current. The CLKIN pin allows the VIN(MIN) = VOUT • t LTC3605A to synchronize to an external clock (±30% of ON frequency programmed by RT) and the internal phase- Conversely, the minimum on-time is the smallest dura- locked-loop allows the LTC3605A to lock onto CLKIN’s tion of time in which the top power MOSFET can be in phase as well. The CLKOUT signal can be connected to the its “on” state. This time is typically 40ns. In continuous CLKIN pin of the following LTC3605A stage to line up both mode operation, the minimum on-time limit imposes a the frequency and the phase of the entire system. Tying minimum duty cycle of: the PHMODE pin to INTVCC, SGND or INTVCC/2 generates a phase difference (between CLKIN and CLKOUT) of 180 DCMIN = f • tON(MIN) degrees, 120 degrees, or 90 degrees respectively, which where tON(MIN) is the minimum on-time. As the equation corresponds to 2-phase, 3-phase or 4-phase operation. A shows, reducing the operating frequency will alleviate the total of 12 phases can be cascaded to run simultaneously minimum duty cycle constraint. out of phase with respect to each other by programming the PHMODE pin of each LTC3605A to different levels. In the rare cases where the minimum duty cycle is sur- passed, the output voltage will still remain in regulation, but
Internal/External ITH Compensation
the switching frequency will decrease from its programmed value. This is an acceptable result in many applications, so During single phase operation, the user can simplify the this constraint may not be of critical importance in most loop compensation by tying the ITH pin to INTVCC to en- cases. High switching frequencies may be used in the able internal compensation. This connects an internal 30k design without any fear of severe consequences. As the resistor in series with a 40pF capacitor to the output of sections on inductor and capacitor selection show, high the error amplifier (internal ITH compensation point) while switching frequencies allow the use of smaller board com- also activating output voltage positioning such that the ponents, thus reducing the size of the application circuit. output voltage wil be 1.5% above regulation at no load and 1.5% below regulation at full load. This is a trade-off for
CIN and COUT Selection
simplicity instead of OPTI-LOOP® optimization, where ITH components are external and are selected to optimize the The input capacitance, CIN, is needed to filter the trapezoi- loop transient response with minimum output capacitance. dal wave current at the drain of the top power MOSFET. To prevent large voltage transients from occurring, a low
Minimum Off-Time and Minimum On-Time
ESR input capacitor sized for the maximum RMS current
Considerations
should be used. The maximum RMS current is given by: The minimum off-time, tOFF(MIN), is the smallest amount of V I OUT VIN –1 time that the LTC3605A is capable of turning on the bot- RMS ≅ IOUT(MAX) VIN VOUT tom power MOSFET, tripping the current comparator and turning the power MOSFET back off. This time is generally This formula has a maximum at VIN = 2VOUT, where about 70ns. The minimum off-time limit imposes a maxi- IRMS ≅ IOUT/2. This simple worst-case condition is com- mum duty cycle of tON/(tON + tOFF(MIN)). If the maximum monly used for design because even significant deviations duty cycle is reached, due to a dropping input voltage for do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to further 3605afg 10 For more information www.linear.com/LTC3605A Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Typical Applications Package Description Revision History Typical Application Related Parts