Datasheet LTM4622A (Analog Devices) - 10

link to page 20 LTM4622A
APPLICATIONS INFORMATION Output Voltage Programming Output Decoupling Capacitors
The PWM controller has an internal 0.6V reference voltage. With an optimized high frequency, high bandwidth design, As shown in the Block Diagram, a 60.4k 0.5% internal only single piece of 47µF low ESR output ceramic capaci- feedback resistor connects VOUT and FB pins together. tor is required for each LTM4622A output to achieve low Adding a resistor RFB from FB pin to GND programs the output voltage ripple and very good transient response. output voltage: Additional output filtering may be required by the system designer, if further reduction of output ripples or dynamic 0.6V R • 60.4k transient spikes is required. Table 7 shows a matrix of dif- FB = VOUT – 0.6V ferent output voltages and output capacitors to minimize the voltage droop and overshoot during a 1A (50%) load step
Table 1. VFB Resistor Table vs Various Output Voltages
transient. Multiphase operation will reduce effective output
(1% Resistor)
ripple as a function of the number of phases. Application
VOUT (V) 1.5 1.8 2.5 3.3 5.0 8.0 10.0 12.0
Note 77 discusses this noise reduction versus output
RFB (k)
40.2 30.1 19.1 13.3 8.25 4.87 3.83 3.16 ripple current cancellation, but the output capacitance will be more a function of stability and transient response. Pease note that for output above 3.3V, a higher operating The Linear Technology LTpowerCAD® Design Tool is avail- frequency is required to optimize inductor current ripple. able to download online for output ripple, stability and See Operating Frequency section. transient response analysis and calculating the output For parallel operation of N-channels LTM4622A, the fol- ripple reduction as the number of phases implemented lowing equation can be used to solve for RFB: increases by N times. 0.6V 60.4k RFB = •
Burst Mode Operation
VOUT – 0.6V N In applications where high efficiency at intermediate current are more important than output voltage ripple, Burst Mode
Input Decoupling Capacitors
operation could be used by connecting SYNC/MODE pin The LTM4622A module should be connected to a low AC- to INTVCC to improve light load efficiency. In Burst Mode impedance DC source. For each regulator channel, one piece operation, a current reversal comparator (IREV) detects 4.7µF input ceramic capacitor is required for RMS ripple the negative inductor current and shuts off the bottom current decoupling. Bulk input capacitor is only needed power MOSFET, resulting in discontinuous operation and when the input source impedance is compromised by long increased efficiency. Both power MOSFETs will remain inductive leads, traces or not enough source capacitance. off and the output capacitor will supply the load current The bulk capacitor can be an electrolytic aluminum capaci- until the COMP voltage rises above the zero current level tor and polymer capacitor. to initiate another cycle. Without considering the inductor current ripple, for each
Force Continuous Current Mode (CCM) Operation
output, the RMS current of the input capacitor can be estimated as: In applications where fixed frequency operation is more critical than low current efficiency, and where the lowest I I OUT(MAX) • D •(1–D) output ripple is desired, forced continuous operation should CIN(RMS) = η% be used. Forced continuous operation can be enabled by tying the SYNC/MODE pin to GND. In this mode, induc- where is the estimated efficiency of the power module. tor current is allowed to reverse during low output loads, the COMP voltage is in control of the current comparator threshold throughout, and the top MOSFET always turns Rev B 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 Package Photos Design Resources Related Parts