Datasheet LTM4626 (Analog Devices) - 10

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APPLICATIONS INFORMATION
capacitor or tantalum-polymer capacitor is required due by adding a resistor, RFSET, between the FREQ pin and to variation of actual capacitance over bias voltage and GND, as shown in Figure 24. The operating frequency temperature. Table 7 shows a matrix of different output can be calculated as: voltages and output capacitors to minimize the voltage 1.67e11 droop and overshoot during a 3A load-step transient. f(Hz) = Additional output filtering may be required by the sys- 274k ||RFSET (Ω) tem designer if further reduction of output ripple or The programmable operating frequency range is up dynamic transient spikes is required. The Analog Devices to 3MHz. LTpowerCAD™ design tool is available to download online for output ripple, stability and transient response analysis
Frequency Synchronization and Clock In
for further optimization. The power module has a phase-locked loop comprised of
Discontinuous Current Mode (DCM)
an internal voltage controlled oscillator and a phase detec- tor. This al ows the internal top MOSFET turn-on to be In applications where low output ripple and high efficiency locked to the rising edge of the external clock. The exter- at intermediate current are desired, discontinuous current nal clock frequency range must be within ±30% around mode (DCM) should be used by connecting the MODE/ the resistor set operating frequency. A pulse detection CLKIN pin to GND. At light loads the internal current com- circuit is used to detect a clock on the CLKIN pin to turn parator may remain tripped for several cycles and force the on the phase-locked loop. The pulse width of the clock has top MOSFET to stay off for several cycles, thus skipping to be at least 100ns. The clock high level must be above cycles. The inductor current does not reverse in this mode. 1V and clock low level below 0.3V. During the start-up of
Forced Continuous Current Mode (CCM)
the regulator, the phase-locked loop function is disabled. In applications where fixed frequency operation is more
Multiphase Operation
critical than low current efficiency, and where the low- For output loads that demand more than 12A of current, est output ripple is desired, forced continuous opera- multiple LTM4626s can be paralleled to run out of phase tion should be used. Forced continuous operation can to provide more output current without increasing input be enabled by tying the MODE/CLKIN pin to INTVCC. In and output voltage ripples. this mode, inductor current is allowed to reverse dur- ing low output loads, the COMPa voltage is in control of The CLKOUT signal can be connected to the MODE/CLKIN the current comparator threshold throughout, and the pin of the following LTM4626 stage to line up both the top MOSFET always turns on with each oscillator pulse. frequency and the phase of the entire system. Tying the During start-up, forced continuous mode is disabled and PHMODE pin to INTVCC, GND or FLOAT generates a phase inductor current is prevented from reversing until the difference (between CLKIN and CLKOUT) of 180°, 120°, or LTM4626’s output voltage is in regulation. 90° respectively, which corresponds to 2-phase, 3-phase or 4-phase operation. A total of 6 phases can be cascaded
Operating Frequency
to run simultaneously out of phase with respect to each The operating frequency of the LTM4626 is optimized other by programming the PHMODE pin of each LTM4626 to achieve the compact package size and the minimum to different levels. Figure 3 shows a 4-phase design and output ripple voltage while still keeping high efficiency. a 6-phase design example for clock phasing. The default operating frequency is 600kHz. In most appli-
Table 2. PHMODE Pin Status and Corresponding Phase
cations, no additional frequency adjustment is required.
Relationship (Relative to CLKIN)
If an operating frequency other than 600kHz is required by
PHMODE INTVCC GND FLOAT
the application, the operating frequency can be increased CLKOUT 180° 120° 90° Rev B 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Pin Configuration Typical Performance Characteristics Pin Functions Block Diagram Decoupling Requirements Operation Applications Information Package Description Revision History Package Photo Design Resources Related Parts