Datasheet LTM4623 (Analog Devices) - 10

LTM4623 APPLICATIONS INFORMATION The typical LTM4623 application circuit is shown in For parallel operation of N-channels LTM4623, tie all the Figure  24. External component selection is primarily FB pins together and use the following equation to solve determined by the input voltage, the output voltage and for RFB: the maximum load current. Refer to Table 7 for specific external capacitor requirements for a particular application. 0.6V 60.4k RFB = • V N
V
OUT – 0.6V
IN to VOUT Step-Down Ratios
There are restrictions in the maximum VIN and VOUT step-
Input Decoupling Capacitors
down ratios that can be achieved for a given input voltage The LTM4623 module should be connected to a low AC due to the minimum off-time and minimum on-time limits impedance DC source. For the regulator, a 10µF input of the regulator. The minimum off-time limit imposes a ceramic capacitor is required for RMS ripple current de- maximum duty cycle which can be calculated as: coupling. Bulk input capacitance is only needed when the DMAX = 1 – (tOFF(MIN) • fSW) input source impedance is compromised by long inductive where t leads, traces or not enough source capacitance. The bulk OFF(MIN) is the minimum off-time, typically 70ns for LTM4623, and f capacitor can be an aluminum electrolytic capacitor or SW (Hz) is the switching frequency. Conversely the minimum on-time limit imposes a minimum polymer capacitor. duty cycle of the converter which can be calculated as: Without considering the inductor ripple current, the RMS D current of the input capacitor can be estimated as: MIN = tON(MIN) • fSW where t I ON(MIN) is the minimum on-time, typically 40ns I OUT(MAX) • D •(1 for LTM4623. In the rare cases where the minimum duty CIN(RMS) = − D) η% cycle is surpassed, the output voltage will still remain in regulation, but the switching frequency will decrease where η% is the estimated efficiency of the power module. from its programmed value. Note that additional thermal derating may be applied. See the Thermal Considerations
Output Decoupling Capacitors
and Output Current Derating section in this data sheet. With an optimized high frequency, high bandwidth design, only a single low ESR output ceramic capacitor is required
Output Voltage Programming
for the LTM4623 to achieve low output ripple voltage and The PWM controller has an internal 0.6V reference voltage. very good transient response. Additional output filtering As shown in the Block Diagram, a 60.4k internal feedback may be required by the system designer if further reduction resistor connects the VOUT and FB pins together. Adding a of output ripple or dynamic transient spikes is required. resistor, RFB, from FB pin to SGND programs the output Table 7 shows a matrix of different output voltages and voltage: output capacitors to minimize the voltage droop and overshoot during a 1A load-step transient. The Linear 0.6V R Technology LTpowerCAD™ design tool is available to FB = • 60.4k VOUT − 0.6V download online for output ripple, stability and transient response analysis for further optimization.
Table 1. RFB Resistor Table vs Various Output Voltages V Discontinuous Current Mode (DCM) OUT (V) 0.6 1.0 1.2 1.5 1.8 2.5 3.3 5.0
RFB (kΩ) OPEN 90.9 60.4 40.2 30.1 19.1 13.3 8.25 In applications where low output ripple and high efficiency Pease note that for 3.3V and 5V output, a higher operating frequency at intermediate current are desired, discontinuous current (2MHz) is required to optimize inductor current ripple. See Operating Frequency section. mode (DCM) should be used by connecting the MODE pin Rev D 10 For more information www.analog.com 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 Package Description Revision History Package Photos Design Resources Related Parts