LTM4630IV View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTM4630IV

Dual 18A or Single 36A DC/DC µModule Regulator

Linear Technology 

LTM4630

APPLICATIONS INFORMATION

Input Capacitors

The LTM4630 module should be connected to a low ac-

impedance DC source. For the regulator input four 22µF

input ceramic capacitors are used 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 capacitance. 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)

=

IO U T(M AX )

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 LTM4630 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 200µF to 470µF. 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 4.5A/µs 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

µModule Power Design Tool will be provided for stability

analysis. 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 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 reduc-

tion 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 LTM4630 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 LTM4630 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.

4630fa

12

For more information www.linear.com/LTM4630

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