LTM4650 datasheet(16/38 Pages) AD

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LTM4652

Rev. 0

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APPLICATIONS INFORMATION

In Equation 4, η% is the estimated efficiency of the power

module. The bulk capacitor can be a switcher-rated elec-

trolytic aluminum capacitor, Polymer capacitor.

Output Capacitors

The LTM4652 is designed for low output voltage rip-

ple noise and good transient response. The bulk output

capacitors defined as COUT are chosen with low enough

effective series resistance (ESR) to meet the output

voltage ripple and transient requirements. COUT can be a

low ESR tantalum capacitor, the low ESR polymer capac-

itor or ceramic capacitor. The typical output capacitance

range for each output is from 300µF to 800µF per output

channel. Additional output filtering may be required by

the system designer, if further reduction of output ripples

or dynamic transient spikes is required. Table 6 shows a

matrix of different output voltages and output capacitors

to minimize the voltage droop and overshoot during a

25% load step. In multi LTM4652 paralleling applications,

Table 6 RC compensation value is still valid in terms of

having one set of RC filters on each of the paralleling

modules while connecting all the COMP, FB and VOUT

pins together. See Figure 34 and Multiphase Operation

section. The table optimizes total equivalent ESR and

total bulk capacitance to optimize the transient perfor-

mance. Stability criteria are considered in the Table 6

matrix, and the Analog Devices LTpowerCAD Design Tool

provides 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. The Analog

Devices µModule Power Design Tool can calculate the

output ripple reduction as the number of implemented

phases increases by N times. A small value 10Ω to 50Ω

resistor can be placed in series from VOUT to the VOUTS pin

to allow for a bode plot analyzer to inject a signal into the

control loop and validate the regulator stability. The same

resistor could be placed in series from VOUT to DIFFP and

a bode plot analyzer could inject a signal into the control

loop and validate the regulator stability.

Pulse-Skipping Mode Operation

In applications where low output ripple and high efficiency

at intermediate currents are desired, pulse-skipping

mode should be used. Pulse-skipping operation allows

the LTM4652 to skip cycles at low output loads, thus

increasing efficiency by reducing switching loss. Tying

the MODE_PLLIN pin to INTVCC enables pulse-skipping

operation. At light loads the internal current compara-

tor may remain tripped for several cycles and force the

top MOSFET to stay off for several cycles, thus skipping

cycles. The inductor current does not reverse in this mode.

Applications with transient and/or sustained negative (sink-

ing) output current should operate the LTM4652 in forced

continuous mode, since there is no way for the control

loop to command reverse (negative) inductor current in

pulse-skipping operation except by deliberately allowing

the output voltage to rise high enough (~10% above nom-

inal regulation) to activate an output overvoltage response.

Forced Continuous Operation

In applications where fixed frequency operation is more

critical than low current efficiency, and where the low-

est output ripple is desired, forced continuous operation

should be used. Forced continuous operation can be

enabled by tying the MODE_PLLIN pin to SGND. In this

mode, inductor current is allowed to reverse during low

output loads, the COMP voltage is in control of the current

comparator threshold throughout, and the top MOSFET

always turns on with each oscillator pulse. During start-up,

forced continuous mode is disabled and inductor current

is prevented from reversing until the LTM4652’s output

voltage is in regulation. Either regulator can be configured

for forced continuous mode.

Multiphase Operation

For output loads that demand more than ±25A of current,

two outputs in LTM4652 or even multiple LTM4652s can

be paralleled to run out of phase to provide more output

current without increasing input and output voltage ripples.

The MODE_PLLIN pin allows the LTM4652 to synchronize

to an external clock (between 300kHz* and 780kHz) and

the internal phase-locked-loop allows the LTM4652 to lock

*Note that synchronization below 400kHz is not tested in ATE.

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