Datasheet LTM4619 (Analog Devices) - 9

LTM4619
applicaTions inForMaTion
The typical LTM4619 application circuit is shown in Without considering the inductor ripple current, for each Figure 19. External component selection is primarily deter- output, the RMS current of the input capacitor can be mined by the maximum load current and output voltage. estimated as:
Output Voltage Programming
I I OUT(MAX) • D•(1−D) CIN(RMS) = η The PWM controller has an internal 0.8V reference voltage. As shown in the block diagram, a 60.4k internal feedback In the above equation, η is the estimated efficiency of the resistor RFB connects VOUT to VFB pin. The output voltage power module. The bulk capacitor can be a switcher-rated will default to 0.8V with no feedback resistor. Adding a aluminum electrolytic capacitor or a polymer capacitor. One resistor RSET from VFB pin to SGND programs the output 10µF ceramic input capacitor is typically rated for 2A of voltage: RMS ripple current, so the RMS input current at the worst 60.4k +R case for each output at 4A maximum current is about 2A. V SET If a low inductance plane is used to power the device, then OUT = 0.8V • RSET two 10µF ceramic capacitors are enough for both outputs or equivalently at 4A load and no external input bulk capacitor is required. 60.4k R
Output Capacitors
SET =  VOUT –1 The LTM4619 is designed for low output voltage ripple  0.8V  noise. The bulk output capacitors defined as COUT are chosen with low enough effective series resistance (ESR)
Table 1. RSET Resistor Table vs Various Output Voltages
to meet the output voltage ripple and transient require-
VOUT (V)
0.8 1.2 1.5 1.8 2.5 3.3 5 ments. COUT can be a low ESR tantalum capacitor, a low
RSET (k
Ω
)
Open 121 68.1 48.7 28.0 19.1 11.5 ESR polymer capacitor or ceramic capacitor. The typical
Input Capacitors
output capacitance range for each output is from 47µF to 220µF. Additional output filtering may be required by The LTM4619 module should be connected to a low AC- the system designer. If further reduction of output ripple impedance DC source. Two 1.5µF input ceramic capacitors or dynamic transient spikes is required, LTpowerCAD is are included inside the module. Additional input capacitors available for stability analysis. Multiphase operation will are needed if a large load is required up to the 4A level. reduce effective output ripple as a function of the num- A 47µF to 100µF surface mount aluminum electrolytic ber of phases. Application Note 77 discusses this noise capacitor can be used for more input bulk capacitance. reduction versus output ripple current cancellation, but This bulk capacitor is only needed if the input source im- the output capacitance should be considered carefully as pedance is compromised by long inductive leads, traces a function of stability and transient response. LTpowerCAD or not enough source capacitance. calculates the output ripple reduction as the number of For a buck converter, the switching duty-cycle can be implemented phases increased by N times. estimated as: V D= OUT VIN 4619fc For more information www.linear.com/LTM4619 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Simplified Block Diagram Decoupling Requirements Operation Applications Information Typical Applications Package Description Revision History Package Photograph Related Parts Design Resources