LTM4659EVPBF datasheet(14/26 Pages) AD

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LTM4659

Rev. 0

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components of the typical application circuits are a good

starting point for component values. The output voltage

settling behavior is related to the stability of the closed-

loop system. For a detailed explanation of optimizing the

compensation components, including a review of control

loop theory, refer to the Analog Devices Application Note

76.

RUN Threshold Programming

The LTM4659 has a precision threshold RUN pin to en-

able or disable switching. Pulling the RUN pin to ground

forces the LTM4659 into its shutdown state, turning off

both power MOSFETs and most of its internal control

circuitry. Bringing the RUN pin above 0.4V will turn on

the entire chip.

The rising threshold of the RUN comparator is 400mV

with 60mV of hysteresis. Connect the RUN pin to VIN if

the shutdown feature is not used. Adding a resistor divider

from VIN to RUN programs the LTM4659 to regulate the

output only when VIN is above a desired voltage (see the

Block Diagram). Typically, this threshold, VIN(RUN),isused

in situations where the input supply is current limited or

has a relatively high source resistance. A switching regula-

tor draws constant power from the source, so the source

current increases as the source voltage drops. This looks

like a negative resistance load to the source and can cause

the source to current limit or latch low under low source

voltage conditions. The VIN(RUN) threshold prevents the

regulator from operating at source voltages where prob-

lems may occur. This threshold can be adjusted by setting

the values R1 and R2 such that they satisfy Equation 8.

VIN(RUN) =

⎛

⎝

⎜

⎞

⎠

⎟• 400mV

(8)

where the LTM4659 will remain off until VIN is above

VIN(RUN). Due to the comparator’s hysteresis, switching

will not stop until the input falls slightly below VIN(RUN).

Alternatively, a resistor divider from an output of another

regulator to the enable RUN pin of the LTM4659 provides

event-based power up sequencing, enabling the LTM4659

when the output of the other regulator reaches a prede-

termined level.

Output Overvoltage Protection

During an output overvoltage event, when the FB pin volt-

age is greater than 110% of nominal, the LTM4659 top

power switch will be turned off. If the output remains out

of regulation for more than 100μs, the PGOOD pin will be

pulledlow.Anoutputovervoltageeventshouldnothappen

under normal operating conditions.

Output Short-Circuit Protection and Recovery

The peak inductor current at which the current compara-

tor shuts off the top power switch is controlled by the

voltage on the COMP pin. If the output current increases,

the error amplifier raises the COMP pin voltage until the

average inductor current matches the new load current. In

normal operation, the LTM4659 clamps at the maximum

COMP pin voltage.

When the output is shorted to ground, the inductor current

decays very slowly during the switch-off time because of

the low voltage across the inductor. To keep the current

in control, a secondary limit is also imposed on the valley

inductorcurrent.Iftheinductorcurrentmeasuredthrough

the bottom power switch increases beyond IVALLEY(MAX),

the top power switch will be held off and switching cycles

will be skipped until the inductor current is reduced.

Recovery from a short circuit can be abrupt, and because

the output is shorted and below regulation, the regulator

is requesting the maximum current to charge the output.

When the short circuit condition is removed, the inductor

current could cause an extreme voltage overshoot in the

output. The LTM4659 addresses this potential issue by

regulatingtheSSTTvoltagejustabovetheFBvoltagewhen

the output is out of regulation. Therefore, a recovery from

an output short circuit goes through a soft-start cycle. The

output ramp is controlled, and the overshoot is minimized.

Thermal Considerations and Output Current Derating

The thermal resistances reported in the Pin Configuration

section are consistent with those parameters defined by

JESD51-9. They are intended for use with finite element

analysis (FEA) software modeling tools that leverage the

outcome of thermal modeling, simulation, and correlation

APPLICATIONS INFORMATION

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