LTM4650-1 datasheet(11/36 Pages) LINER

11 / 36 page

LTM4636

4636f

For more information www.linear.com/LTM4636

applicaTions inForMaTion

The typical LTM4636 application circuit is shown in

Figure 20. External component selection is primarily

determined by the maximum load current and output

voltage. Refer to Table 5 for specific external capacitor

requirements for particular applications.

VIN to VOUT Step-Down Ratios

TherearerestrictionsintheVINtoVOUTstep-downratiothat

can be achieved for a given input voltage. The maximum

duty cycle is 94% typical at 500kHz operation. The VIN to

VOUT minimum dropout is a function of load current and

operation at very low input voltage and high duty cycle

applications. At very low duty cycles the minimum 100ns

on-time must be maintained. See the Frequency Adjust-

ment section and temperature derating curves.

Output Voltage Programming

The PWM controller has an internal 0.6V ±1% reference

voltage. As shown in the Block Diagram, a 4.99k internal

feedback resistor connects the VOUTS1+ and VFB pins to-

gether. When the remote sensing is used, then VOUTS1+

and VOUTS1– are connected to the remote VOUT and GND

points. If no remote sense the VOUTS1+ connects to VOUT.

The output voltage will default to 0.6V with no feedback

resistor. Adding a resistor RFB from VFB to ground pro-

grams the output voltage:

VOUT = 0.6V •

4.99k

+RFB

RFB

Table 1. VFB Resistor Table vs Various Output Voltages

VOUT (V)

0.6

1.0

1.2

1.5

1.8

2.5

3.3

RFB (k)

Open

7.5

4.99

3.32

2.49

1.58

1.1

For parallel operation of N LTM4636s, the following

equation can be used to solve for RFB:

RFB=

4.99k /N

VOUT

0.6V

–1

Or use VOUTS1 on one channel and connect all feedback

pins together utilizing a single feedback resistor.

TietheVFBpinstogetherforeachparalleloutput.TheCOMP

pins must be tied together also. See Typical Application

section examples.

Input Capacitors

The LTM4636 module should be connected to a low AC-

impedance DC source. Additional input capacitors are

neededfortheRMSinputripplecurrentrating.TheICIN(RMS)

equation which follows can be used to calculate the input

capacitor requirement. Typically 22µF X7R ceramics are a

good choice with RMS ripple current ratings of ~4A each.

A47µFto100µFsurfacemountaluminumelectrolyticbulk

capacitor can be used for more input bulk capacitance.

This bulk input capacitor is only needed if the input source

impedanceiscompromisedbylonginductiveleads,traces

ornotenoughsourcecapacitance.Iflowimpedancepower

planes are used, then this bulk capacitor is not needed.

For a buck converter, the switching duty cycle can be

estimated as:

VOUT

VIN

Without considering the inductor ripple current, for each

output the RMS current of the input capacitor can be

estimated as:

ICIN(RMS)=

IOUT(MAX)

η%

• D•(1–D)

where

η% is the estimated efficiency of the power mod-

ule. The bulk capacitor can be a switcher-rated aluminum

electrolytic capacitor or a Polymer capacitor.

LTM4650-1 データシート(PDF) 11 Page - Linear Technology

LTM4650-1 データシート(HTML) 11 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

データシートダウンロード