SC4626 datasheet(16/21 Pages) SEMTECH

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SC4626

Over-Voltage Protection

In the event of a 5% over-voltage on the output, the

PWM drive is disabled with LX pin floating.

Soft-Start

The soft-start mode is activated after VIN reaches its

UVLO and EN signal is set high to enable the part. An over

temperature shutdown event will also activate the soft

start sequence. Soft-start mode controls the maximum

current during startup thus limiting in-rush current. The

PMOS current limit is stepped through four soft start

levels of approximately 20%, 25%, 40%, & 00%. Each

step is maintained for 20μs following internal reference

start up of 20μs giving the total nominal startup period

of 00μs. During startup, the chip operates in controlling

the inductor current swings between 0A and current

limit. If V

OUT reaches 90% of the target within the first 2

current levels, the chip continues in hysteretic mode till

the end of the soft-start time period before switching to

PWM mode. If V

OUT does not reach 90% by the end of the

second current limit level, soft start will continue to level 3

or level 4 till the output voltage reaches 96% and will then

transition into PWM mode. After the full soft start time

period, the SC4626 will switch into PWM mode operation

regardless of the V

OUT level.

The SC4626 is capable of starting up into a pre-biased

output. When the output is precharged by another supply

rail, the SC4626 will not discharge the output during the

soft start interval.

Shut Down

When the EN pin voltage goes low, the SC4626 will run in

shutdown mode, drawing less than μA from the input

power supply. The internal switches and bandgap voltage

will be immediately turned off.

Inductor Selection

The SC4626 converter has internal loop compensation.

The compensation is designed to work with a output filter

corner frequency is less than 00kHz over any operating

condition, tolerance and bias effect. The corner frequency

of output filter can be defined by the equation

OUT

Values outside this range may lead to instability,

malfunction, or out-of-specification performance.

When choosing an inductor, it is important to consider

the change in inductance with DC bias current. The

inductor saturation current is specified as the current at

which the inductance drops a specific percentage from

the nominal value. This is approximately 30%. Except for

short-circuit or other fault conditions, the peak current

must always be less than the saturation current specified

by the manufacturer. The peak current is the maximum

load current plus one half of the inductor ripple current at

the maximum input voltage. Load and/or line transients

can cause the peak current to exceed his level for short

durations. Maintaining the peak current below the

inductor saturation specification keeps the inductor ripple

current and the output voltage ripple at acceptable levels.

Manufacturers often provide graphs of actual inductance

and saturation characteristics versus applied inductor

current. The saturation characteristics of the inductor can

varysignificantlywithcoretemperature.Coreandambient

temperatures should be considered when examining the

core saturation characteristics.

When the inductance has been determined, the DC

resistance (DCR) must be examined. The efficiency that

can be achieved is dependent on the DCR of the inductor.

The lower values give higher efficiency. The RMS DC

current rating of the inductor is associated with losses in

the copper windings and the resulting temperature rise of

the inductor. This is usually specified as the current which

produces a 40˚C temperature rise. Most copper windings

are rated to accommodate this temperature rise above

maximum ambient.

Magnetic fields associated with the output inductor can

interfere with nearby circuitry. This can be minimized by

the use of low noise shielded inductors which use the

minimum gap possible to limit the distance that magnetic

fields can radiate from the inductor. However shielded

inductors typically have a higher DCR and are thus less

efficient than a similar sized non-shielded inductor.

Applications Information (continued)

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