Datasheet LTM4618 (Analog Devices) - 10

LTM4618
applicaTions inForMaTion
The typical LTM4618 application circuit is shown in the 6A level. A 47µF to 100µF surface mount aluminum Figure 21. External component selection is primarily electrolytic bulk capacitor can be used for more input bulk determined by the maximum load current and output capacitance. This bulk input capacitor is only needed if voltage. the input source impedance is compromised by long in- ductive leads, traces or not enough source capacitance.
VIN to VOUT Step-Down Ratios
If low impedance power planes are used, then this 47µF There are restrictions in the maximum V capacitor is not needed. IN to VOUT step- down ratio that can be achieved for a given input voltage. For a buck converter, the switching duty-cycle can be One of the restrictions is the minimum on-time tON(MIN), estimated as: which is the smallest time duration that the LTM4618 can operate. Make sure that the operating on-time is larger than D = VOUT the minimum on-time as shown in the equation below. See VIN the Thermal Considerations and Output Current Derating Without considering the inductor current ripple, the RMS sections in this data sheet for the current restrictions. current of the input capacitor can be estimated as: tON(MIN) is approximately 90ns, guardband to 110ns. IOUT(MAX) t ICIN(RMS) = • D • (1– D) ON(MIN) < VOUT η VIN • ƒ In the above equation, η is the estimated efficiency of
Output Voltage Programming
the power module. One 10µF ceramic input capacitor is The PWM controller has an internal 0.8V reference voltage. typically rated for 2A of RMS ripple current, so the RMS As shown in the Block Diagram, a 60.4k internal feedback input current at the worst case 6A maximum current is resistor connects V about 3A. If a low inductance plane is used to power the OUT to the VFB pin. Adding a resistor RFB from the V device, then two 10µF ceramic capacitors are enough for FB pin to SGND programs the output voltage: the output at 6A load and no external input bulk capacitor V is required. The input RMS ripple current can be cancelled OUT = 0.8V • 60.4k + RFB RFB by paralleling multiple LTM4618 power modules out of phase, allowing the use of fewer input capacitors. Ap-
Table 1. VFB Resistor Table vs Various Output Voltages
plication Note 77 explains the details.
VOUT (V)
0.8 1 1.2 1.5 1.8 2.5 3.3 5
RFB (k
Ω
)
Open 243 121 68.1 48.7 28.0 19.1 11.5
Output Capacitors Input Capacitors
The LTM4618 is designed for low output voltage ripple noise. The bulk output capacitors defined as COUT are The LTM4618 module should be connected to a low AC- chosen with low enough effective series resistance (ESR) to impedance DC source. One 1.5µF input ceramic capacitor meet the output voltage ripple and transient requirements. is included inside the module. Additional input capacitors COUT can be a low ESR tantalum capacitor, a low ESR are only needed if a large load step is required up to polymer capacitor or ceramic capacitor. The typical output 4618fb 10 For more information www.linear.com/LTM4618 Document Outline Features Description Applications 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 Photograph Package Description Revision History Typical Application Related Parts