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LTM4650-1

46501fc

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applicaTions inForMaTion

Output Capacitors

The LTM4650-1 is designed for low output voltage ripple

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 volt-

age ripple and transient requirements. COUT can be a low

ESR tantalum capacitor, the low ESR polymer capacitor

or ceramic capacitor. The typical output capacitance

range for each output is from 400µF to 600µF. Additional

output filtering may be required by the system designer,

if further reduction of output ripples or dynamic transient

spikes is required. Table 4 shows a matrix of different

output voltages and output capacitors to minimize the

voltage droop and overshoot during a 12.5A (25%) and

25A (50%) load step transient. The table optimizes total

equivalent ESR and total bulk capacitance to optimize the

transient performance. Stability criteria are considered in

the Table 4 matrix, and the Linear Technology LTpower-

CAD Design Tool will be provided for stability analysis.

In multi LTM4650-1 paralleling applications, Table 4 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 29 and Multiphase Operation section. Multiphase

operationwillreduceeffectiveoutputrippleasafunctionof

the number of phases. Application Note 77 discusses this

noise reduction versus output ripple current cancellation,

but the output capacitance should be considered carefully

asafunctionofstabilityandtransientresponse. TheLinear

Technology LTpowerCAD 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 place 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 place in series from VOUT to DIFFP and

a bode plot analyzer could inject a signal into the control

loop and validate the regulator stability.

Burst Mode Operation

The LTM4650-1 is capable of Burst Mode operation on

each regulator in which the power MOSFETs operate in-

termittently based on load demand, thus saving quiescent

current. For applications where maximizing the efficiency

at very light loads is a high priority, Burst Mode operation

should be applied. Burst Mode operation is enabled with

the MODE_PLLIN pin floating. During this operation, the

peak current of the inductor is set to approximately one

third of the maximum peak current value in normal opera-

tion even though the voltage at the COMP pin indicates

a lower value. The voltage at the COMP pin drops when

the inductor’s average current is greater than the load

requirement. As the COMP voltage drops below 0.5V, the

BURST comparator trips, causing the internal sleep line

to go high and turn off both power MOSFETs.

In sleep mode, the internal circuitry is partially turned off,

reducing the quiescent current to about 450µA for each

output. The load current is now being supplied from the

output capacitors. When the output voltage drops, caus-

ing COMP to rise above 0.5V, the internal sleep line goes

low, and the LTM4650-1 resumes normal operation. The

next oscillator cycle will turn on the top power MOSFET

and the switching cycle repeats. Either regulator can be

configured for Burst Mode operation.

Pulse-Skipping Mode Operation

In applications where low output ripple and high effi-

ciencyatintermediatecurrentsaredesired,pulse-skipping

mode should be used. Pulse-skipping operation allows

the LTM4650-1 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 comparator

may remain tripped for several cycles and force the top

MOSFETtostayoffforseveralcycles,thusskippingcycles.

The inductor current does not reverse in this mode. This

mode will maintain higher effective frequencies thus lower

output ripple and lower noise than Burst Mode operation.

Eitherregulatorcanbeconfiguredforpulse-skippingmode.

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