LTM4680 datasheet(12/36 Pages) AD

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LTM4650-2

Rev. 0

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APPLICATIONS INFORMATION

Output Capacitors

The LTM4650-2 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 volt-

age ripple and transient requirements. COUT can be a low

ESR tantalum capacitor, a low ESR polymer capacitor or

a ceramic capacitor. The typical output capacitance range

for each output is from 400µF to 600µF. Additional out-

put 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. Table 4 optimizes total

equivalent ESR and total bulk capacitance to optimize the

transient performance. Stability criteria are considered in

the Table 4 matrix, and the Analog Devices LTpowerCAD®

design tool will be provided for stability analysis. In multi

LTM4650-2 paralleling applications, Table 4 RC compen-

sation 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 the Multiphase Operation section. The multiphase

operation will reduce effective output ripple as a function

of the number of phases. Application Note 77 discusses

this noise reduction versus output ripple current cancel-

lation, but the output capacitance should be considered

carefully as a function of stability and transient response.

The Analog Devices 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 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.

Burst Mode Operation

The LTM4650-2 is capable of Burst Mode operation on

each regulator in which the power MOSFETs operate inter-

mittently based on load demand, thus saving quiescent

current. For applications where maximizing the efficiency

at very light loads is a high priority, Burst Mode opera-

tion 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

operation, even though the voltage at the COMP pin indi-

cates 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,

causing COMP to rise above 0.5V, the internal sleep line

goes low, and the LTM4650-2 resumes normal opera-

tion. 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 efficiency

at intermediate currents are desired, pulse-skipping

mode should be used. Pulse-skipping operation allows

the LTM4650-2 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. This mode will maintain higher effective frequen-

cies thus lower output ripple and lower noise than Burst

Mode operation. Either regulator can be configured for

pulse-skipping mode.

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