LTM4658EVPBF datasheet(9/26 Pages) AD

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LTM4658

Rev. 0

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ThetypicalLTM4658applicationcircuitisshownonpage 1.

External component selection is primarily determined by

the input voltage, the output voltage and the maximum

loadcurrent.RefertoTable 8forspecificexternalcapacitor

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 minimum on-time

of the regulator.

Theminimumon-timelimitimposesaminimumdutycycle

of the converter which can be calculated with Equation 1.

DMIN = TON(MIN) • fSW

(1)

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

for the LTM4658. In the rare cases where the minimum

duty cycle is surpassed, output will overvoltage and a

slower switching frequency is needed to accommodate

the high VIN/VOUT ratio.

The LTM4658 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.

It should be noted that the practical transconductance ampli-

fier has a limited output voltage range, to stay in regulation,

COMP voltage in the compensation loop must reflect the peak

current value. If the output is set to below 0.7V and the switch-

ing frequency is 2MHz or above, due to the small ripple cur-

rent, the output voltage may not be regulated at no load since

the transconductance amplifier is running to its low output

voltage limit. To avoid this issue, reduce to a lower frequency

or change the operation mode to pulse-skipping mode.

Output Voltage Programming and Output Voltage

Sensing

The PWM controller has an internal 0.5V reference volt-

age. Resistor divider from VOUT remote sensing point to

FB pin and from FB pin to AGND pin programs the output

voltage (Equation 2). Refer to the Block Diagram.

VOUT = 0.5V •

RA +RB

(2)

APPLICATIONS INFORMATION

In high current operation, a ground offset may be present

between the LTM4658 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

local ground and remote output ground, resulting in a

more accurate output voltage. The LTM4658 allows for

remote output ground deviations as much as ±100mV

with respect to the local ground.

Input Decoupling Capacitors

The LTM4658 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 pos-

sible to the pins. For the regulator, one-piece 22µF input

ceramic capacitor is recommended for RMS ripple current

decoupling. Bulk input capacitor is only needed when the

input source impedance is compromised by long induc-

tive leads, traces or not enough source capacitance. 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

with Equation 3.

ICIN(RMS) =

IOUT(MAX)

η%

• D •(1

−D)

(3)

where η%istheestimatedefficiencyofthepowermodule.

Output Decoupling Capacitors

With an optimized high frequency, high bandwidth

design, only two pieces of 47μF low ESR output ce-

ramic capacitor is required for LTM4658 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 voltages and output

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