LTM4644 datasheet(13/34 Pages) LINER

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LTM4650

4650fb

For more information www.linear.com/LTM4650

APPLICATIONS INFORMATION

Input Capacitors

The LTM4650 module should be connected to a low AC-

impedance DC source. For the regulator input two 22µF

input ceramic capacitors are required for each channel

for RMS ripple current. A 47µF to 100µF surface mount

aluminum electrolytic bulk capacitor can be used for more

input bulk capacitance. This bulk input capacitor is only

needed if the input source impedance is compromised by

long inductive leads, traces or not enough source capaci-

tance. If low impedance power planes are used, then this

bulk capacitor is not needed.

For a buck converter, the switching duty cycle can be

estimated as:

D =

VOUT

VIN

Without considering the inductor current ripple, for each

output, the RMS current of the input capacitor can be

estimated as:

ICIN(RMS) =

IOUT(MAX)

η%

• D • 1−D

(

)

In the above equation, η% is the estimated efficiency of

the power module. The bulk capacitor can be a switcher-

rated electrolytic aluminum capacitor, Polymer capacitor.

Output Capacitors

The LTM4650 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%) load step

transient.ThetableoptimizestotalequivalentESRandtotal

bulk capacitance to optimize the transient performance.

Stability criteria are considered in the Table 4 matrix, and

the Linear Technology LTpowerCAD Design Tool 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. The Linear

Technology 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 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 is capable of Burst Mode operation on each

regulator in which the power MOSFETs operate intermit-

tently based on load demand, thus saving quiescent cur-

rent. 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 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.

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