LTM4659EVPBF datasheet(9/26 Pages) AD

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LTM4659

Rev. 0

For more information www.analog.com

See LTM4659 Typical Application circuit. External compo-

nentselectionisprimarilydeterminedbytheinputvoltage,

the output voltage and the maximum load current. See

Table 8 for specific external capacitor requirements for a

particular application.

VIN to VOUT Step-Down Ratios

The minimum VOUT step-down ratio that can be achieved

for a given input voltage is limited by the minimum on-

time of the regulator.

Theminimumon-timelimitimposesaminimumdutycycle

of the converter which can be calculated using Equation 1.

DMIN = TON(MIN) • fSW

(1)

where TON(MIN) is the minimum on-time, 45ns typical

for the LTM4659. In the rare cases where the minimum

duty cycle is surpassed, the output will overvoltage and

a slower switching frequency is needed to accommodate

the high VIN/VOUT ratio.

The LTM4659 is capable of a maximum duty cycle of

100%; therefore, the VIN-to-VOUT dropout is limited by

the RDS(ON) of the top switch, the inductor DCR, and the

load current.

Output Voltage Programming and Output

Voltage Sensing

The PWM controller has an internal 0.5V reference volt-

age. The resistor divider from the VOUT remote sensing

point to the FB pin and from the FB pin to the AGND pin

programs the output voltage (Equation 2). See the Block

Diagram for more information.

VOUT = 0.5V •

RA +RB

(2)

In high current operation, a ground offset may be present

between the LTM4659 local ground and ground at the

load. To overcome this offset, AGND should have a Kelvin

connection to the load ground, and the lowest potential

node of the resistor divider should be connected to AGND.

The internal error amplifier senses the difference between

this feedback voltage and a 0.5V AGND referenced volt-

age. This scheme overcomes any ground offsets between

APPLICATIONS INFORMATION

local ground and remote output ground, resulting in a

more accurate output voltage. The LTM4659 allows for

remote output ground deviations as much as ±100mV

with respect to the local ground.

Input Decoupling Capacitors

The LTM4659 module should be connected to a low

AC-impedance DC source. All of the VIN pins must be

connected together with short, wide traces and bypassed

to PGND with low ESR capacitors located as close as

possible to the pins. For the regulator, a one-piece 22µF

input ceramic capacitor is recommended for RMS ripple

current decoupling. A bulk input capacitor is only needed

when the input source impedance is compromised by

long inductive leads, traces or not enough source capaci-

tance. The bulk capacitor can be an electrolytic aluminum

capacitor and polymer capacitor.

Without considering the inductor current ripple, the

RMS current of the input capacitor can be estimated

using Equation 3.

ICIN(RMS) =

IOUT(MAX)

η%

• D •(1

−D)

(3)

where η% is the estimated efficiency of the power module.

Output Decoupling Capacitors

Withanoptimizedhigh-frequency,high-bandwidthdesign,

onlytwopiecesof47μFlowESRoutputceramiccapacitors

are required for LTM4659 to achieve low output voltage

ripple and very good transient response. Additional output

filtering may be required by the system designer, if further

reduction of output ripples or dynamic transient spikes is

required. Table 8 shows a matrix of different output volt-

ages and output capacitors to minimize the voltage droop

and overshoot during a 2.5A (25%) load-step transient.

The multiphase operation will reduce effective output

ripple as a function of the number of phases. Analog De-

vices Application Note 77 discusses this noise reduction

versus output ripple current cancellation, but the output

capacitancewillbemoreafunctionofstabilityandtransient

response.TheAnalogDevicesLTpowerCAD®designtoolis

available to download online for output ripple, stability and

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