LTM4645 datasheet(16/34 Pages) LINER

16 / 34 page

LTM4646

4646f

For more information www.linear.com/LTM4646

APPLICATIONS INFORMATION

Frequency Selection and Phase-Locked Loop(MODE_

PLLIN and fSET Pins)

The LTM4646 device is operated over a range of frequen-

cies to improve power conversion efficiency . It is recom-

mended to operate the lower output voltages or lower

duty cycle conversions at lower frequencies to improve

efficiency by lowering power MOSFET switching losses.

Higher output voltages or higher duty cycle conversions

can be operated at higher frequencies to limit inductor

ripple current. The efficiency graphs will show an operat-

ing frequency chosen for that condition An internal oscil-

lator (clock generator) provides phase interleaved internal

clock signals for individual channels to lock up to. The

switching frequency and phase of each switching channel

is independently controlled by adjusting the top MOSFET

turn-on time (on-time) through the one-shot timer. This

is achieved by sensing the phase relationship between

a top MOSFET turn-on signal and its internal reference

clock through a phase detector, and the time interval of the

one-shot timer is adjusted on a cycle-by-cycle basis, so

that the rising edge of the top MOSFET turn-on is always

trying to synchronize to the internal reference clock signal

for the respective channel.

The frequency of the internal oscillator can be pro-

grammed from 250kHz to 1.3MHz by connecting a resis-

tor, RT , from the FREQ pin to signal ground (SGND). The

FREQ pin is regulated to 1.2V internally. The value of this

resistor can be chosen according to the formula:

RT(kΩ) =

41550

f(kHz)

– 2.2

For applications with stringent frequency or interfer-

ence requirements, an external clock source connected

to the MODE_PLLIN pin can be used to synchronize the

internal clock signals through a clock phase-locked loop

(Clock PLL). The LTM4646 operates in forced continuous

mode of operation when it is synchronized to the exter-

nal clock. The external clock frequency has to be within

±30% of the internal oscillator frequency for successful

synchronization.

The clock input levels should be no less than 2V for “high”

and no greater than 0.5V for “low”. The MODE_PLLIN pin

has an internal 600k pull-down resistor.

During dynamic transient conditions either in the line volt-

age or load current (e.g., load step or release), the top

switch will turn on more or less frequently in response

to achieve faster transient response. This is the benefit of

the LTM4646’s controlled on-time, valley current mode

architecture. However, this process may understandably

lose phase and even frequency lock momentarily. For

relatively slow changes, phase and frequency lock can

still be maintained. For large load current steps with fast

slew rates, phase lock will be lost until the system returns

back to a steady-state condition . It may take up to several

hundred microseconds to fully resume the phase lock, but

the frequency lock generally recovers quickly, long before

phase lock does.

Minimum On-Time and Minimum Off-Time

Minimum on-time tON is the smallest time duration that

the LTM4646 is capable of turning on the top MOSFET on

either channel. It is determined by internal timing delays,

and the gate charge required to turn on the top MOSFET.

The LTM4646 has a minimum on time of ~30ns, and far

lower than what would be a concern based on the maxi-

mum operating frequency of 1.3MHz .

The below equation can be checked against the VIN , VOUT,

and (FREQ) frequency of operation to insure the minimum

on time tON(MIN) is above 30ns.

VOUT

VIN • FREQ

tON(MIN)

If the duty cycle falls below what can be accommodated

by the minimum on-time, the controller will begin to skip

cycles. The output voltage will continue to be regulated,

but the output ripple will increase. The on-time can be

increased by lowering the switching frequency. The

below equation can be checked against the VIN , VOUT,

and (FREQ) frequency of operation to insure the minimum

off time tOFF(MIN) is above 90ns.

VOUT(MAX)

VIN(MAX)

DMAX

DMAX 1 – FREQ • tOFF(MIN)

LTM4645 Datasheet(PDF) 16 Page - Linear Technology

LTM4645 Datasheet(HTML) 16 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

Datasheet Download

Link URL