LTM4602 datasheet(10/24 Pages) LINER

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LTM4602

4602fa

Output Capacitors

The LTM4602 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 3A/μs transient with

a speciﬁc output capacitance.

Fault Conditions: Current Limit and Overcurrent

Foldback

The LTM4602 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 LTM4602 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

LTM4602 as well as a timer for soft-start and overcurrent

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

LTM4602 into a low quiescent current shutdown (IQ ≤

100μA). Releasing the pin allows an internal 1.2μA cur-

rent source to charge up the timing capacitor CSS. Inside

LTM4602, 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:

tDELAY =

1.5V

1.2μA

•(CSS_EXT +1000pF)

When the voltage on RUN/SS pin reaches 1.5V, the LTM4602

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 Step-Down Ratios

There are restrictions in the maximum VIN to VOUT step

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

These constraints are shown in the Typical Performance

Characteristics curves labeled “VIN to VOUT Step-Down

Ratio”. Note that additional thermal derating may apply. See

the Thermal Considerations and Output Current Derating

sections of this data sheet.

APPLICATIONS INFORMATION

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