SC251 datasheet(10/19 Pages) SEMTECH

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SC251

POWER MANAGEMENT

Applications Information (Cont.)

Inductor Selection

The SC251 is designed for use with a 4.7μH inductor. The

magnitude of the inductor current ripple is dependent on

the inductor value and can be determined by the following

equation:

OSC

OUT

⎟

⎠

⎞

⎜

⎝

⎛ −

The inductor should have a low DC Resistance (DCR) to

minimize the conduction losses and maximize efﬁciency.

As a minimum requirement, the DC current rating of the

inductor should be equal to the maximum load current

plus half of the inductor current ripple as shown by the

following equation:

)

MAX

(

OUT

LPK

Final inductor selection will depend on various design

considerations such as efﬁciency, EMI, size and cost.

Table 1 lists the manufacturers of practical inductor

options.

IN Selection

The source input current to a buck converter is non-

continuous. To prevent large input voltage ripple a low

ESR ceramic capacitor is required. A minimum value of

10μF should be used for sufﬁcient input voltage ﬁltering

and a 22μF should be used for improved input voltage

ﬁltering.

OUT Selection

The internal compensation is designed to work with a

certain output ﬁlter corner frequency deﬁned by the

equation:

OUT

This single pole ﬁlter is designed to operate with an output

capacitor value of 4.7μF.

Output voltage ripple is a combination of the voltage

ripple from the inductor current charging and discharging

the output capacitor and the voltage created from the

inductor current ripple through the output capacitor ESR.

Selecting an output capacitor with a low ESR reduces the

bias supply for power ampliﬁers. This output provides a

regulated 2.85V with output current capability up to 10mA.

The 2.85V output is guaranteed for input supply voltages

in excess of 2.95V. When input voltages below 2.95V are

used, V

REF is equal to VIN - VREF DO. This reference supply

is controlled by the same enable pin as the switching

regulator.

Protection Features

The SC251 provides the following protection features:

Thermal shutdown

Current limit

Under voltage lockout

Thermal Shutdown

The device has a thermal shutdown feature to protect

the device if the junction temperature exceeds 160°C. In

thermal shutdown the on-chip power devices are disabled,

effectively tri-stating the LX output. Switching will resume

when the temperature drops by 15°C.

Short Circuit Protection

The PMOS and NMOS power devices of the buck switcher

stage are protected by current limit functions. In the event

of a short to ground on the output, the LX pin will switch

with minimum duty cycle. The duty cycle is short enough to

allow the inductor to discharge during each cycle, thereby

preventing the inductor current from “staircasing”.

The pass-through PMOS is protected by a current limit

function. When the part is enabled in pass-through, the

output capacitor charges up with a large surge current.

In order to support this surge current and to protect

against short circuits, an internal timer is used. A short

circuit condition must exist for more than 128 clock

cycles before the pass-through device is disabled. After

an additional 2048 clock cycles, the pass-through device

will turn back on. This cycle will continue until the short

circuit is removed. This method allows the part to manage

thermal dissipation and recover when the fault condition

is removed.

Under Voltage Lockout

The part will turn itself off if the input supply voltage falls

below 2.4V typical. The device is allowed to turn on again

when the input supply voltage increases above the lockout

voltage. Hysteresis is included to prevent chattering.

•

Applications Information (Cont.)

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