LTM4645 datasheet(14/34 Pages) LINER

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LTM4646

4646f

For more information www.linear.com/LTM4646

APPLICATIONS INFORMATION

ESR tantalum capacitor, low ESR polymer capacitor or

ceramic capacitor. The typical output capacitance range

for each output is from 200F to 470F. Additional output

filtering may be required by the system designer, if further

reduction of output ripples or dynamic transient spikes

is required. Table 5 shows a matrix of different output

voltages and output capacitors to minimize the voltage

droop and overshoot during a 7.5A/s transient. The table

optimizes total equivalent ESR and total bulk capacitance

to optimize the transient performance. Stability criteria are

considered in the Table 5 matrix, and LTpowerCAD will be

provided for stability analysis. Multiphase operation will

reduce effective output ripple as a function of the num-

ber of phases. Application Note 77 discusses this noise

reduction versus output ripple current cancellation, but

the output capacitance should be considered carefully as a

function of stability and transient response. LTpowerCAD

can calculate the output ripple reduction as the number of

implemented phases increases by N times.

Continuous and Discontinuous Mode Operation

If the MODE_PLLIN pin is tied to INTVCC or an external

clock is applied to MODE_PLLIN, the LTM4646 will be

forced to operate in continuous mode. With load current

less than one-half of the full load peak-to-peak ripple, the

inductor current valley can drop to zero or become nega-

tive. This allows constant-frequency operation but at the

cost of low efficiency at light loads.

If the MODE_PLLIN pin is left open or connected to sig-

nal ground, the channel will transition into discontinu-

ous mode operation, where a current reversal compara-

tor shuts off the bottom MOSFET as the inductor current

approaches zero, thus preventing negative inductor cur-

rent and improving light-load efficiency. In this mode,

both switches can remain off for extended periods of time.

As the output capacitor discharges by load current and the

output voltage droops lower, EA will eventually move the

ITH voltage above the zero current level (0.8V) to initiate

another switching cycle.

Multiphase Operation

For output loads that demand more than 10A of current,

two outputs in LTM4646 or even multiple LTM4646s can

be paralleled to run out of phase to provide more output

current without increasing input and output voltage ripple.

The MODE_PLLIN pin allows the LTM4646 to synchronize

to an external clock (between 250kHz and 1.3MHz) and

the internal phase-locked loop allows the LTM4646 to lock

onto an incoming clock phase as well. The CLKOUT signal

can be connected to the MODE_PLLIN pin of the following

stage to line up both the frequency and the phase of the

entire system. Tying the PHASMD pin to INTVCC, SGND,

or floating the pin will select VOUT2 and CLKOUT phases

relative to VOUT1.

Up to of 12 phases can be cascaded to run simultaneously

with respect to each other by programming the PHASMD

pin of each LTM4646 channel to different levels. Figure 3

shows a 2-phase design, 4-phase design and a 6-phase

design example for clock phasing with the PHASMD table.

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. The LTM4646

device is an inherently current mode controlled device, so

parallel modules will have very good current sharing. This

will balance the thermals on the design. Figure 19 shows

an example of parallel operation and pin connection.

Input RMS Ripple Current Cancellation

Application Note 77 provides a detailed explanation of

multiphase operation. The input RMS ripple current can-

cellation mathematical derivations are presented, and

a graph is displayed representing the RMS ripple cur-

rent reduction as a function of the number of interleaved

phases. Figure4 shows this graph.

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