Datasheet LTM4622 (Analog Devices) - 10

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APPLICATIONS INFORMATION Output Voltage Programming
is required for each LTM4622 output to achieve low output voltage ripple and very good transient response. Additional The PWM controller has an internal 0.6V reference volt- output filtering may be required by the system designer, if age. As shown in the Block Diagram, a 60.4k 0.5% internal further reduction of output ripples or dynamic transient feedback resistor connects VOUT and FB pins together. spikes is required. Table 6 shows a matrix of different out- Adding a resistor RFB from FB pin to GND programs the put voltages and output capacitors to minimize the volt- output voltage: age droop and overshoot during a 1.25A (50%) load step 0.6V transient. Multiphase operation will reduce effective output RFB = • 60.4k VOUT – 0.6V ripple as a function of the number of phases. Application Note 77 discusses this noise reduction versus output rip-
Table 1. VFB Resistor Table vs Various Output Voltages
ple current cancellation, but the output capacitance will be
VOUT (V) 0.6 1.0 1.2 1.5 1.8 2.5 3.3 5.0
more a function of stability and transient response. The
RFB (k)
OPEN 90.9 60.4 40.2 30.1 19.1 13.3 8.25 Analog Devices, Inc. LTpowerCAD® Design Tool is available to download online for output ripple, stability and transient Pease note that for 2.5 to 5V output, a higher operating response analysis and calculating the output ripple reduc- frequency is required to optimize inductor current ripple. tion as the number of phases implemented increases by See Operating Frequency section. N times. For parallel operation of N-channels LTM4622, the follow-
Burst Mode Operation
ing equation can be used to solve for RFB: In applications where high efficiency at intermediate cur- 0.6V 60.4k RFB = • rent are more important than output voltage ripple, Burst VOUT – 0.6V N Mode operation could be used by connecting SYNC/ MODE pin to INTVCC to improve light load efficiency. In
Input Decoupling Capacitors
Burst Mode operation, a current reversal comparator (IREV) detects the negative inductor current and shuts off The LTM4622 module should be connected to a low the bottom power MOSFET, resulting in discontinuous AC-impedance DC source. For each regulator channel, operation and increased efficiency. Both power MOSFETs one piece 4.7µF input ceramic capacitor is required for will remain off and the output capacitor will supply the RMS ripple current decoupling. Bulk input capacitor is load current until the COMP voltage rises above the zero only needed when the input source impedance is com- current level to initiate another cycle. promised by long inductive leads, traces or not enough source capacitance. The bulk capacitor can be an electro-
Force Continuous Current Mode (CCM) Operation
lytic aluminum capacitor and polymer capacitor. In applications where fixed frequency operation is more Without considering the inductor current ripple, for each critical than low current efficiency, and where the low- output, the RMS current of the input capacitor can be est output ripple is desired, forced continuous opera- estimated as: tion should be used. Forced continuous operation can IOUT(MAX) be enabled by tying the SYNC/MODE pin to GND. In this ICIN(RMS) = • D • (1–D) η% mode, inductor current is allowed to reverse during low output loads, the COMP voltage is in control of the current where is the estimated efficiency of the power module. comparator threshold throughout, and the top MOSFET always turns on with each oscillator pulse. During start-
Output Decoupling Capacitors
up, forced continuous mode is disabled and inductor current is prevented from reversing until the LTM4622’s With an optimized high frequency, high bandwidth design, output voltage is in regulation. only single piece of 22µF low ESR output ceramic capacitor Rev. H 10 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Decoupling Requirements Operation Applications Information Safety Considerations Layout Checklist/Example Package Description Revision History Package Photo Design Resources Related Parts