LTM4686BEVPBF datasheet(48/130 Pages) AD

48 / 130 page

LTM4686B

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

VIN TO VOUT STEP-DOWN RATIOS

There are restrictions in the maximum VIN and VOUT step-

down ratio that can be achieved for a given input voltage.

Each output of the LTM4686B is capable of 95% duty

cycle at 500kHz, but the VIN to VOUT minimum dropout

is still a function of its load current and will limit output

current capability related to high duty cycle on the topside

switch. Minimum on-time tON(MIN) is another consider-

ation in operating at a specified duty cycle while operating

at a certain frequency due to the fact that tON(MIN) < D/fSW,

where D is duty cycle and fSW is the switching frequency.

tON(MIN) is specified in the electrical parameters as 45ns.

See Note 6 in the Electrical Characteristics section for

output current guideline.

INPUT CAPACITORS

The LTM4686B module should be connected to a low

AC-impedance DC source. For the regulator input four

22µF input ceramic capacitors are used to handle the 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 esti-

mated with Equation 2.

Dn =

VOUT

VIN

(2)

Without considering the inductor current ripple, for each

output, the RMS current of the input capacitor can be

estimated with Equation 3.

ICINn(RMS) =

IOUTn(MAX)

η%

• Dn • 1−Dn

(

)

(3)

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

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

electrolytic aluminum capacitor, or a Polymer capacitor.

OUTPUT CAPACITORS

The LTM4686B 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

ceramic capacitor. The typical output capacitance range

for each output is from 400µF to 700µF. Additional output

filtering may be required by the system designer, if further

reduction of output ripple or dynamic transient spikes

is required. Table 20 shows a matrix of different output

voltages and output capacitors to minimize the voltage

droop and overshoot during a 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 20 matrix, and the Analog

Devices, Inc. µModule Power Design Tool will be provided

for stability analysis. Multiphase operation reduces effec-

tive output ripple as a function of the number of phases.

Analog Devices 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, Inc. µModule Power Design Tool can

calculate the output ripple reduction as the number of

implemented phases increases by N times. A small value

10Ω resistor can be placed in series from VOUTn to the

VOSNS0+ or VOSNS1 pin to allow for a bode plot analyzer

to inject a signal into the control loop and validate the

regulator stability.

LIGHT LOAD CURRENT OPERATION

TheLTM4686Bhastwomodesofoperation:highefficiency,

discontinuousconductionmodeorforcedcontinuouscon-

duction mode. The mode of operation is configured by bit

0 of the MFR_PWM_MODEn command (discontinuous

conduction is always the start-up mode, forced continu-

ous is the default running mode).

LTM4686BEVPBF 数据表(PDF) 48 Page - Analog Devices

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