Datasheet LTM4614 (Analog Devices) - 8

LTM4614
applicaTions inForMaTion Dual Switching Regulator
For a buck converter, the switching duty cycle can be A typical LTM4614 application circuit is shown in Figure 12. estimated as: External component selection is primarily determined by VOUT the maximum load current and output voltage. Refer to D= V Table 4 for specific external capacitor requirements for a IN particular application. Without considering the inductor current ripple, the RMS current of the input capacitor can be estimated as:
VIN to VOUT Step-Down Ratios
I There are restrictions in the maximum V OUT(MAX) IN and VOUT step- ICIN(RMS) = • D • (1–D) down ratio than can be achieved for a given input voltage η% on the two switching regulators. The LTM4614 is 100% duty cycle capable, but the V In the above equation, η% is the estimated efficiency of IN to VOUT minimum dropout will be a function the load current. A typical 0.5V minimum the power module. The bulk capacitor can be a switcher- is sufficient. See Typical Performance Characteristics. rated aluminum electrolytic OS-CON or polymer capacitor. If a low inductance plane is used to power the device,
Output Voltage Programming
then no input capacitance is required. The internal 4.7µF ceramics on each channel input are typically rated for 1A Each regulator channel has an internal 0.8V reference of RMS ripple current up to 85°C operation. The worst- voltage. As shown in the Block Diagram, a 4.99k internal case ripple current for the 4A maximum current is 2A or feedback resistor connects the VOUT and FB pins together. less. An additional 10µF or 22µF local ceramic capacitor The output voltage will default to 0.8V with no externally can be used to supplement the internal capacitor with an applied feedback resistor. Adding a resistor RFB from the additional 1A to 2A ripple current rating. See Figure 11 FB pin to GND programs the output voltage: for recommended PCB layout. 4.99k +R V FB OUT = 0.8V •
Output Capacitors
RFB The LTM4614 switchers are designed for low output volt-
Table 1. FB Resistor Table vs Various Output Voltages
age ripple on each channel. The bulk output capacitors
V
are chosen with low enough effective series resistance
OUT
0.8V 1.0V 1.2V 1.5V 1.8V 2.5V 3.3V
R
(ESR) to meet the output voltage ripple and transient
FB
Open 20k 10k 5.76k 3.92k 2.37k 1.62k requirements. The output capacitors can be low ESR tan-
Input Capacitors
talum capacitors, low ESR polymer capacitors or ceramic The LTM4614 module should be connected to a low AC capacitors. The typical output capacitance range is 66µF impedance DC source. One 4.7µF ceramic capacitor is to 100µF. Additional output filtering may be required by included inside the module for each regulator channel. the system designer if further reduction of output ripple Additional input capacitors are needed if a large load step or dynamic transient spikes is required. Table 4 shows a is required up to the full 4A level and for RMS ripple cur- matrix of different output voltages and output capacitors rent requirements. A 47µF bulk capacitor can be used for to minimize the voltage droop and overshoot during a 2A/ more input bulk capacitance. This 47µF capacitor is only µs transient. The table optimizes total equivalent ESR and needed if the input source impedance is compromised by total bulk capacitance to maximize transient performance. long inductive leads or traces. See Figure 11 for recommended PCB layout. 4614fb 8 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 Package Description Revision History Package Photograph Related Parts