LTM4650 datasheet(14/36 Pages) AD

14 / 36 page

LTM4650-2

Rev. 0

For more information www.analog.com

A multiphase power supply significantly reduces the

amount of ripple current in both the input and output

capacitors. The RMS input ripple current is reduced by,

and the effective ripple frequency is multiplied by the

number of phases used (assuming that the input volt-

age is greater than the number of phases used times

the output voltage). The output ripple amplitude is also

reduced by the number of phases used when all of the

outputs are tied together to achieve a single high-output

current design.

In multi LTM4650-2s parallel applications, CTH and RTH

values in Table 4 are still valid to achieve a ±3% tran-

sient response in a 25% load step. Connect one set of RC

(RTH and CTH) networks to the COMP pin of each paral-

leling module. Then, connect the COMP pins, FB pins,

TRACK/SS pin and VOUT pins from different modules

together. See Figure 29 for an example of parallel opera-

tion. LTpowerCAD power design tool can also be used to

optimize loop compensation and transient performance if

only one set of RC (RTH and CTH) network is to be added

to the common COMP pins.

The LTM4650-2 device is an inherently current mode-

controlled device so that the parallel modules will have

very good current sharing. This will balance the thermals

on the design. Figure 29 shows an example of parallel

operation and pin connection.

Input RMS Ripple Current Cancellation

Application Note 77 provides a detailed explanation of mul-

tiphase operation. The input RMS ripple current cancella-

tion mathematical derivations are presented, and a graph

is displayed representing the RMS ripple current reduction

as a function of the number of interleaved phases. Figure 6

shows this graph.

APPLICATIONS INFORMATION

Frequency Selection and Phase-Lock Loop

(MODE_PLLIN and fSET Pins)

The LTM4650-2 device is operated over a range of fre-

quencies to improve power conversion efficiency. It is

recommended to operate the module at 400kHz for output

voltage below 1.0V, 500kHz for output voltage between

1.0V to 1.5V and 600kHz for output voltage above 1.5V

for the best efficiency and inductor current ripple.

The LTM4650-2 switching frequency can be set with an

external resistor from the fSET pin to SGND. An accurate

10µA current source into the resistor will set a voltage

that programs the frequency, or a DC voltage can be

applied. Figure 7 shows a graph of frequency setting

versus programming voltage. An external clock can be

applied to the MODE_PLLIN pin from 0V to INTVCC over

a frequency range of 400kHz to 750kHz. The clock input

high threshold is 1.6V, and the clock input low threshold

is 1V. The LTM4650-2 has the PLL loop filter components

on board. The frequency setting resistor should always

be present to set the initial switching frequency before

locking to an external clock. Both regulators will operate

in continuous mode while being externally clock.

The output of the PLL phase detector has a pair of com-

plementary current sources that charge and discharge the

internal filter network. When the external clock is applied,

the fSET frequency resistor is disconnected with an inter-

nal switch, and the current sources control the frequency

adjustment to lock to the incoming external clock. When

no external clock is applied, the internal switch is on,

thus connecting the external fSET frequency set resistor

for free-run operation.

LTM4650 データシート(PDF) 14 Page - Analog Devices

LTM4650 データシート(HTML) 14 Page - Analog Devices

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