LTM4644MPY datasheet(10/36 Pages) LINER

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LTM4644/LTM4644-1

4644fe

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APPLICATIONS INFORMATION

The typical LTM4644 application circuit is shown in

Figure 33. External component selection is primarily

determined by the input voltage, the output voltage and

the maximum load current. Refer to Table 7 for specific

externalcapacitorrequirementsforaparticularapplication.

VIN to VOUT Step-Down Ratios

There are restrictions in the maximum VIN and VOUT step-

down ratio that can be achieved for a given input voltage

due to the minimum off-time and minimum on-time limits

of each regulator. The minimum off-time limit imposes a

maximum duty cycle which can be calculated as:

DMAX = 1 – tOFF(MIN) • fSW

where tOFF(MIN) is the minimum off-time, 70ns typical for

LTM4644, and fSW is the switching frequency. Conversely

the minimum on-time limit imposes a minimum duty cycle

of the converter which can be calculated as:

DMIN = tON(MIN) • fSW

where tON(MIN) is the minimum on-time, 40ns typical for

LTM4644. In the rare cases where the minimum duty

cycle is surpassed, the output voltage will still remain

in regulation, but the switching frequency will decrease

from its programmed value. Note that additional thermal

derating may be applied. See the Thermal Considerations

and Output Current Derating section in this data sheet.

Output Voltage Programming (LTM4644)

ThePWMcontrollerhasaninternal0.6Vreferencevoltage.

As shown in the Block Diagram, a 60.4k internal feedback

resistor connects each regulator channel from VOUT pin

to FB pin. Adding a resistor RFB(BOT) from FB pin to GND

programs the output voltage:

RFB(BOT) =

60.4k

VOUT

0.6

−1

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

5.0

RFB(BOT) (k) Open 90.9 60.4 40.2 30.1 19.1 13.3 8.25

For parallel operation of N channels, use the following

equation can be used to solve for RFB(BOT). Tie the VOUT

and the FB and COMP pins together for each paralleled

output with a single resistor to GND as determined by:

RFB(BOT) =

60.4k

⎛

⎝

⎜

⎞

⎠

⎟

VOUT

0.6

−1

⎛

⎝

⎜

⎞

⎠

⎟

OUTPUT VOLTAGE PROGRAMMING (LTM4644-1)

ThePWMcontrollerhasaninternal0.6Vreferencevoltage.

Adding two resistors RFB(TOP) from VOUT to FB pin and

RFB(BOT) from FB pin to GND programs the output voltage:

RFB(BOT) =

RFB(TOP)

VOUT

0.6

–1

For parallel operation of N Channels, only one set of

RFB(TOP) and RFB(BOT) is needed while tying the VOUT, FB

and COMP pins from different channels together. See

Figure 1 for example.

Figure 1. LTM4644-1 Feedback Resistor

for Paralleling Application

4644 F01

LTM4644-1

COMP3

RFB(TOP)

RFB(BOT)

VOUT1

FB1

COMP1

VOUT2

FB2

COMP2

VOUT3

FB3

LTM4644

LTM4644-1

Top Feedback Resistor

from VOUT-to-VFB

(one resistor per channel)

Integrated

60.4k 0.5%

Resistor

External (to be added on

PCB)

Application

General

Applications

To Interface with

PMBus power system

management supervisory

ICs such as the LTC2975

LTM4644MPY Datasheet(PDF) 10 Page - Linear Technology

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