LTM4600EVPBF datasheet(10/24 Pages) LINER

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LTM4600

4600fa

APPLICATIO S I FOR ATIO

the module input pins in the PCB layout to minimize the

trace inductance and high frequency AC noise.

Output Capacitors

The LTM4600 is designed for low output voltage ripple.

The bulk output capacitors COUT is chosen with low enough

effective series resistance (ESR) to meet the output voltage

ripple and transient requirements. COUT can be low ESR

tantalum capacitor, low ESR polymer capacitor or ceramic

capacitor (X5R or X7R). The typical capacitance is 200µF

if all ceramic output capacitors are used. The internally

optimized loop compensation provides sufﬁcient stability

margin for all ceramic capacitors applications. Additional

output ﬁltering may be required by the system designer,

if further reduction of output ripple or dynamic transient

spike is required. Refer to Table 2 for an output capaci-

tance matrix for each output voltage Droop, peak to peak

deviation and recovery time during a 5A/µs transient with

a speciﬁc output capacitance.

Fault Conditions: Current Limit and Over current

Foldback

The LTM4600 has a current mode controller, which inher-

ently limits the cycle-by-cycle inductor current not only in

steady state operation, but also in transient.

To further limit current in the event of an over load condi-

tion, the LTM4600 provides foldback current limiting. If the

output voltage falls by more than 50%, then the maximum

output current is progressively lowered to about one sixth

of its full current limit value.

Soft-Start and Latchoff with the RUN/SS pin

The RUN/SS pin provides a means to shut down the

LTM4600 as well as a timer for soft-start and over-cur-

rent latchoff. Pulling the RUN/SS pin below 0.8V puts

the LTM4600 into a low quiescent current shutdown (IQ

≤ 75µA). Releasing the pin allows an internal 1.2µA cur-

rent source to charge up the timing capacitor CSS. Inside

LTM4600, there is an internal 1000pF capacitor from

RUN/SS pin to ground. If RUN/SS pin has an external

capacitor CSS_EXT to ground, the delay before starting

is about:

CpF

DELAY

SS EXT

1000

•(

)

When the voltage on RUN/SS pin reaches 1.5V, the

LTM4600 internal switches are operating with a clamping

of the maximum output inductor current limited by the

RUN/SS pin total soft-start capacitance. As the RUN/SS pin

voltage rises to 3V, the soft-start clamping of the inductor

current is released.

VIN to VOUT Stepdown Ratios

There are restrictions in the maximum VIN to VOUT step

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

These contraints are shown in the Typical Performance

Characteristics curves labeled “VIN to VOUT Stepdown

Ratio”. Note that additional thermal de-rating may apply.

See the Thermal Considerations and Output Current De-

Rating sections of this data sheet.

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