Datasheet LTM4647 (Analog Devices) - 10

LTM4647
APPLICATIONS INFORMATION
The typical LTM4647 application circuit is shown in In multiphase single output application. Only one set of Figure 23 and Figure 24. External component selection differential sensing amplifier and one set of feedback resis- is primarily determined by the maximum load current tor are required while connecting VOUT, VFB and COMP of and output voltage. Refer to Table 5 for specific external different channels together. See Figure 25 for paralleling capacitor requirements for particular applications. application.
VIN to VOUT Step-Down Ratios and Minimum On-Time Input Capacitors
There are restrictions in the VIN to VOUT step-down ratio The LTM4647 module should be connected to a low that can be achieved for a given input, output voltage AC-impedance DC source. Additional input capacitors and frequency. The minimum on-time, tON(MIN), limits are needed for the RMS input ripple current rating. The the smallest time duration that the module is capable of ICIN(RMS) equation which follows can be used to calculate turning on the top MOSFET. It is determined by internal the input capacitor requirement. Typically 22µF ceramics timing delays, and the gate charge required turning on are a good choice with RMS ripple current ratings of ~2A the top MOSFET. At very low duty cycles, the minimum each. A 47µF to 100µF surface mount aluminum electrolytic 90ns on-time must be maintained and satisfy the equation: bulk capacitor can be used for more input bulk capacitance. V This bulk input capacitor is only needed if the input source t OUT ON = > 90ns impedance is compromised by long inductive leads, traces VIN •FREQ or not enough source capacitance. If low impedance power If the duty cycle falls below what can be accommodated planes are used, then this bulk capacitor is not needed. by the minimum on-time, the controller will begin to skip For a buck converter, the switching duty cycle can be cycles. The output voltage will continue to be regulated, estimated as: but the output ripple voltage of inductor ripple and current V will increase. The minimum on-time can be increased by D= OUT lowering the switching frequency. VIN
Output Voltage Programming
Without considering the inductor ripple current, for each output, the RMS current of the input capacitor can be The PWM controller has an internal 0.6V reference voltage. estimated as: As shown in the Block Diagram, a 60.4k, 0.5% accuracy internal feedback resistor connects from the V + I SNS pin I OUT(MAX) • D•(1–D) to the V CIN(RMS) = FB pin. η% The output voltage will default to 0.6V with no feedback In the previous equation, η% is the estimated efficiency of resistor. Adding a resistor R – FB from VFB to VSNS programs the power module. The bulk capacitor can be a switcher- the output voltage: rated electrolytic aluminum capacitor or a Polymer 60.4k capacitor. V +RFB OUT = 0.6V • RFB
Output Capacitors Table 1. VFB Resistor Table vs Various Output Voltages
The LTM4647 is designed for low output voltage ripple
VOUT (V)
0.6 0.8 1.0 1.2 1.5 1.8 noise. The bulk output capacitors defined as COUT are
R
chosen with low enough effective series resistance (ESR)
FB (kΩ)
OPEN 182 90.9 60.4 40.2 30.1
Frequency (kHz)
400 400 500 500 600 700 to meet the output voltage ripple and transient require-
R
ments. C
FREQ (kΩ)
37.4 37.4 43.2 43.2 47.5 53.6 OUT can be a low ESR tantalum capacitor, low ESR Polymer capacitor or ceramic capacitors. Please note small 4647fb 10 For more information www.linear.com/LTM4647 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Decoupling Requirements Operation Applications Information Typical Applications