Datasheet LTM4625 (Analog Devices) - 10

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APPLICATIONS INFORMATION Discontinuous Current Mode (DCM)
The programmable operating frequency range is from In applications where low output ripple and high efficiency 800kHz to 4MHz. at intermediate current are desired, discontinuous current
Frequency Synchronization and Clock In
mode (DCM) should be used by connecting the MODE pin to SGND. At light loads the internal current comparator The power module has a phase-locked loop comprised may remain tripped for several cycles and force the top of an internal voltage controlled oscillator and a phase MOSFET to stay off for several cycles, thus skipping cycles. detector. This allows the internal top MOSFET turn-on to be The inductor current does not reverse in this mode. locked to the rising edge of the external clock. The external clock frequency range must be within ±30% around the
Forced Continuous Current Mode (CCM)
resistor set operating frequency. A pulse detection circuit In applications where fixed frequency operation is more is used to detect a clock on the CLKIN pin to turn on the critical than low current efficiency, and where the lowest phase-locked loop. The pulse width of the clock has to output ripple is desired, forced continuous operation should be at least 100ns. The clock high level must be above 2V be used. Forced continuous operation can be enabled by and clock low level below 0.3V. During the start-up of tying the MODE pin to INTV the regulator, the phase-locked loop function is disabled. CC. In this mode, inductor current is allowed to reverse during low output loads, the COMP
Multiphase Operation
voltage is in control of the current comparator threshold throughout, and the top MOSFET always turns on with each For output loads that demand more than 5A of current, oscillator pulse. During start-up, forced continuous mode multiple LTM4625s can be paralleled to run out of phase is disabled and inductor current is prevented from revers- to provide more output current without increasing input ing until the LTM4625’s output voltage is in regulation. and output voltage ripples. The CLKOUT signal can be connected to the CLKIN pin of
Operating Frequency
the following LTM4625 stage to line up both the frequency The operating frequency of the LTM4625 is optimized to and the phase of the entire system. Tying the PHMODE pin achieve the compact package size and the minimum out- to INTVCC, SGND or INTVCC/2 generates a phase differ- put ripple voltage while still keeping high efficiency. The ence (between CLKIN and CLKOUT) of 180°, 120°, or 90° default operating frequency is 1MHz. In most applications, respectively, which corresponds to 2-phase, 3-phase or no additional frequency adjustment is required. 4-phase operation. A total of 12 phases can be cascaded If an operating frequency other than 1MHz is required by to run simultaneously out of phase with respect to each the application, the operating frequency can be increased other by programming the PHMODE pin of each LTM4625 by adding a resistor, R to different levels. Figure 2 shows a 4-phase design and FSET, between the FREQ pin and SGND, as shown in Figure 22. The operating frequency a 6-phase design example for clock phasing. can be calculated as:
Table 2. PHMODE Pin Status and Corresponding Phase Relationship (Relative to CLKIN)
1.6e11
PHMODE INTV
f(Hz) =
CC SGND INTVCC
/2 CLKOUT 180° 120° 90° 162k ||RFSET (Ω) The operating frequency can also be decreased by adding A multiphase power supply significantly reduces the a resistor between the FREQ pin and INTVCC, calculated as: amount of ripple current in both the input and output ca- pacitors. The RMS input ripple current is reduced by, and 2.8e11 f(Hz) = 1MHz – the effective ripple frequency is multiplied by, the number RFSET (Ω) Rev D 10 For more information www.analog.com Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Decoupling Requirements Operation Applications Information Package Description Revision History Package Photo Design Resources Related Parts