Datasheet LTM4632 (Analog Devices) - 10

link to page 8 LTM4632
APPLICATIONS INFORMATION Channel 1 Output Voltage Programming (Configured
Without considering the inductor current ripple, for each
as VDDQ)
output, the RMS current of the input capacitor can be The PWM controller for the V estimated as: OUT1 has an internal 0.6V reference voltage. As shown in the Block Diagram, a 60.4k IOUT(MAX) internal feedback resistor connects V I • D • (1– D) OUT1 and FB1 pins CIN(RMS) = η% together. Adding a resistor RFB from FB1 pin to GND pro- grams the output voltage: where η% is the estimated efficiency of the power module. RFB = 0.6V •60.4k
Output Decoupling Capacitors
VOUT – 0.6V With an optimized high frequency, high bandwidth design,
Table 1. V
only single piece of 22µF low ESR output ceramic capaci-
FB Resistor Table (1%) vs Various Output Voltages V
tor is required for each LTM4632 output to achieve low
OUT(V) 0.6 1.0 1.2 1.3 1.5 1.8 2.5
R output voltage ripple and very good transient response. FB(k) OPEN 90.9 60.4 52.3 40.2 30.1 19.1 Additional output filtering may be required by the sys-
Channel 2 Output Voltage Programming (Configured
tem designer, if further reduction of output ripples or
as VTT)
dynamic transient spikes is required. Table 5 shows a matrix of different output voltages and output capacitors The PWM controller for the VOUT2 uses VTTR voltage as to minimize the voltage droop and overshoot during a a reference voltage. VOUT2 is directly connected to the 0.75A (25%) load step transient. Multiphase operation negative side of the error compiler to internally program will reduce effective output ripple as a function of the VOUT2 to equal to VTTR voltage, which equals to one half number of phases. Application Note 77 discusses this of VDDQIN voltage. noise reduction versus output ripple current cancel a- VOUT2 = VTTR = VDDQIN/2 tion, but the output capacitance will be more a function In a complete DDR memory power application which of stability and transient response. The Linear Technology require both VDDQ supply and VTT terminal outputs, LTpowerCAD Design Tool is available to download online configure LTM4632 Channel 1 as VDDQ output by add- for output ripple, stability and transient response analysis ing a feed-back resistor from FB1 pin to GND. Feed V and calculating the output ripple reduction as the number OUT1 (VDDQ output) voltage to V of phases implemented increases by N times. DDQIN pin to program Channel 2 as VTT output which equals half of the Channel 1 (VDDQ
Burst Mode Operation
output) voltage. In applications where high efficiency at intermediate
Input Decoupling Capacitors
current are more important than output voltage ripple, The LTM4632 module should be connected to a low burst mode operation could be used on Channel 1 by AC-impedance DC source. For each regulator channel, connecting SYNC/MODE pin to INTVCC to improve light one piece 4.7µF input ceramic capacitor is required for load efficiency. In Burst Mode operation, a current rever- RMS ripple current decoupling. Bulk input capacitor is sal comparator (IREV) detects the negative inductor cur- only needed when the input source impedance is com- rent and shuts off the bottom power MOSFET, resulting promised by long inductive leads, traces or not enough in discontinuous operation and increased efficiency. Both source capacitance. The bulk capacitor can be an electro- power MOSFETs will remain off and the output capacitor lytic aluminum capacitor and polymer capacitor. will supply the load current until the COMP voltage rises above the zero current level to initiate another cycle. 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