Datasheet LTM4648 (Analog Devices) - 10

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APPLICATIONS INFORMATION
The typical LTM4648 application circuit is shown in error due to this current, an additional VOUT_LCL pin can Figure  18. External component selection is primarily be tied to VOUT, and an additional RFB resistor can be used determined by the maximum load current and output to lower the total Thevenin equivalent resistance seen by voltage. Refer to Table 3 for specific external capacitor this current. requirements for particular applications.
Input Capacitors VIN to VOUT Step-Down Ratios
The LTM4648 module should be connected to a low AC There are restrictions in the VIN to VOUT step-down ratio impedance DC source. Additional input capacitors are that can be achieved for a given input voltage. The VIN to needed for the RMS input ripple current rating. The ICIN(RMS) VOUT minimum dropout is a function of load current and equation which follows can be used to calculate the input at very low input voltage and high duty cycle applications capacitor requirement. Typically 22µF X7R ceramics are a output power may be limited as the internal top power good choice with RMS ripple current ratings of ~2A each. MOSFET is not rated for 10A operation at higher ambient A 47µF to 100µF surface mount aluminum electrolytic bulk temperatures. At very low duty cycles the minimum 90ns capacitor can be used for more input bulk capacitance. on-time must be maintained. See the Frequency Adjust- This bulk input capacitor is only needed if the input source ment section and temperature derating curves. impedance is compromised by long inductive leads, traces or not enough source capacitance. If low impedance power
Output Voltage Programming
planes are used, then this bulk capacitor is not needed. The PWM controller has an internal 0.6V ±1% reference For a buck converter, the switching duty cycle can be voltage. As shown in the Block Diagram, a 10k 0.5% estimated as: internal feedback resistor connects the VOUT_LCL and V V FB pins together. When the remote sense amplifier is D = OUT used, then DIFFOUT is connected to the VOUT_LCL pin. VIN If the remote sense amplifier is not used, then VOUT_LCL connects to V Without considering the inductor ripple current, for each OUT. The output voltage will default to 0.6V with no feedback resistor. Adding a resistor R output, the RMS current of the input capacitor can be FB from VFB to ground programs the output voltage: estimated as: 10k IOUT(MAX) V + RFB I • D • (1− D) OUT = 0.6V • CIN(RMS) = η% RFB In the previous equation,
Table 1. V
η% is the estimated efficiency of
FB Resistor Table vs Various Output Voltages
the power module. The bulk capacitor can be a switcher-
VOUT(V) 0.6 1.0 1.2 1.5 1.8 2.5 3.3 5.0
rated electrolytic aluminum capacitor or a Polymer RFB(k) OPEN 15 10 6.65 4.99 3.09 2.21 1.37 capacitor. For parallel operation of N LTM4648, the following equa- tion can be used to solve for R
Output Capacitors
FB: 10k The LTM4648 is designed for low output voltage ripple noise. The bulk output capacitors defined as COUT are R N FB = V chosen with low enough effective series resistance (ESR) OUT – 1 to meet the output voltage ripple and transient require- 0.6 ments. COUT can be a low ESR tantalum capacitor, low In parallel operation the V ESR Polymer capacitor or ceramic capacitors. The typical FB pins have an IFB current of 20nA maximum each channel. To reduce output voltage output capacitance range is from 200µF to 470µF. Additional Rev A 10 For more information www.analog.com Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Design Resources Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Design Resources Related Parts