Datasheet LTM4637 (Analog Devices) - 10

LTM4637
applicaTions inForMaTion
The typical LTM4637 application circuit is shown in
Input Capacitors
Figure 22. External component selection is primarily determined by the maximum load current and output The LTM4637 module should be connected to a low AC- voltage. Refer to Table 5 for specific external capacitor impedance DC source. Additional input capacitors are requirements for particular applications. needed for the RMS input ripple current rating. The ICIN(RMS) equation which follows can be used to calculate the input
V
capacitor requirement. Typically 22µF X7R ceramics are a
IN to VOUT Step-Down Ratios
good choice with RMS ripple current ratings of ~ 2A each. There are restrictions in the VIN to VOUT step-down ratio A 47µF to 100µF surface mount aluminum electrolytic bulk that can be achieved for a given input voltage. The duty capacitor can be used for more input bulk capacitance. cycle is 94% typical at 500kHz operation. The VIN to VOUT This bulk input capacitor is only needed if the input source minimum dropout is a function of load current and operation impedance is compromised by long inductive leads, traces at very low input voltage and high duty cycle applications. or not enough source capacitance. If low impedance power At very low duty cycles the minimum 100ns on-time must planes are used, then this bulk capacitor is not needed. be maintained. See the Frequency Adjustment section and temperature derating curves. For a buck converter, the switching duty cycle can be estimated as:
Output Voltage Programming
VOUT The PWM controller has an internal 0.6V ±1% reference D= VIN voltage. As shown in the Block Diagram, a 60.4k internal feedback resistor connects the V Without considering the inductor ripple current, for each OUT_LCL and VFB pins together. When the remote sense amplifier is used, then output the RMS current of the input capacitor can be DIFF_OUT is connected to the V estimated as: OUT_LCL pin. If the remote sense amplifier is not used, then VOUT_LCL connects to IOUT(MAX) V I • D•(1–D) OUT. The output voltage will default to 0.6V with no feed- CIN(RMS)= η% back resistor. Adding a resistor R FB from VFB to ground programs the output voltage: where η% is the estimated efficiency of the power mod- ule. The bulk capacitor can be a switcher-rated aluminum 60.4k +R V FB electrolytic capacitor or a Polymer capacitor. OUT = 0.6V • RFB
Output Capacitors Table 1. VFB Resistor Table vs Various Output Voltages
The LTM4637 is designed for low output voltage ripple
VOUT (V)
0.6 1.0 1.2 1.5 1.8 2.5 3.3 5.0 noise. The bulk output capacitors defined as COUT are
RFB (k)
Open 90.9 60.4 40.2 30.1 19.1 13.3 8.25 chosen with low enough effective series resistance (ESR) to meet the output voltage ripple and transient require- For parallel operation of N LTM4637s, the following ments. COUT can be a low ESR tantalum capacitor, low equation can be used to solve for R ESR Polymer capacitor or ceramic capacitors. The typical FB: output capacitance range is from 200µF to 800µF. Additional 60.4k /N R output filtering may be required by the system designer FB= VOUT –1 if further reduction of output ripple or dynamic transient 0.6V spikes is required. Table 5 shows a matrix of different output Tie the V voltages and output capacitors to minimize the voltage FB pins together for each parallel output. The COMP pins must be tied together also. droop and overshoot during a 10A/µs transient. The table optimizes total equivalent ESR and total bulk capacitance 4637fc 10 For more information www.linear.com/LTM4637 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 Typical Applications Package Description Package PHOTO Typical Application Related Parts Design Resources