LTM8049 datasheet(8/20 Pages) LINER

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LTM4661

4661f

For more information www.linear.com/LTM4661

The typical LTM4661 application circuit is shown in

Figure 17. External component selection is primarily de-

termined by the input voltage, the output voltage and the

maximum load current.

Minimum Input Voltage

The LTM4661 is designed to allow start-up from input

voltages as low as 1.8V. When VOUT exceeds 2.5V, the

LTM4661 continues to regulate its output, even when VIN

falls as low as 0.7V. This feature extends operating times

by maximizing the amount of energy that can be extracted

from the input source. The limiting factors for the applica-

tion become the availability of the power source to supply

sufficient power to the output at the low input voltage,

and the maximum duty cycle, which is clamped at 94%.

In a step-up boost converter, the duty cycle can be cal-

culated at:

D = 1–

VIN • η

VOUT

where ηistheconverterefficiency.85%isagoodestimate

to start with.

Note that at low input voltages, voltage drops due to series

resistance become critical and greatly limit the power

delivery capability of the converter.

Output Current Capability

The LTM4661 is designed to provide up to 4A switch cur-

rent. Due to the nature of the boost converter, the actually

output current capability depends highly on the input/

output voltage ratio. The peak inductor current, same as

switch current, in a boost converter can be calculated as:

ISW =

VIN • D

2 • fS • L

IOUT

1– D

where D is the duty cycle showing above and fS = 1MHz

and L = 2.2µH/2 = 1.1µH.

APPLICATIONS INFORMATION

Based on common input and output values, Table 1 lists

differentoutputcurrentcapabilityoftheLTM4661module.

Table 1. Output Current Capability vs Input Voltage

VIN (V)

3.3

VOUT (V)

Output Current (A)

1.9

0.7

0.5

1.7

0.7

Efficiency (%)

Peak Switch Current (A)

3.8

3.9

3.94 4.1

4.1

Output Voltage Programming

The PWM controller has an internal 1.2V reference volt-

age. As shown in the Block Diagram, a 100k 0.5% internal

feedback resistor connects VOUT and FB pins together.

Adding a resistor RFB from FB pin to GND, programs the

output voltage:

RFB =

1.2V

VOUT – 1.2V

• 100k

Table 2. VFB Resistor Value vs Various Output Voltages

VOUT (V)

1.2

2.5

3.3

RFB (k)

OPEN

93.1

57.6

31.6

17.8

11.0

8.66

For parallel operation of N-piece of LTM4661 modules, the

following equation can be used to solve for RFB:

RFB =

1.2V

VOUT – 1.2V

•

100k

Multiphase Operation

The LTM4661 uses a unique dual-phase single-output

architecture, rather than the conventional single phase of

otherboostconverters.Byinterleavingtwophasesequally

spaced 180° apart, both input and output current ripple

get significantly reduced as well as the amount of input

and output decoupling capacitor required.

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