SC488 datasheet(10/24 Pages) SEMTECH

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POWER MANAGEMENT

SC488

Referencing Figure 2, the equation for setting the output

voltage is:

⋅

1.5

Vout

⎛⎜

⎝

⎞⎟

⎠

Current Limit Circuit

Current limiting of the SC488 can be accomplished in two

ways. The on-state resistance of the low-side MOSFETs

can be used as the current sensing element, or a sense

resistor in the low-side source can be used if greater ac-

curacy is desired. RDSON sensing is more efﬁcient and

less expensive. In both cases, the R

ILIM resistor between

the ILIM pin and LX sets the over-current threshold. This

resistor R

ILIM is connected to a 10μA current source within

the SC488 which is turned on when the low-side MOSFET

turns on. When the voltage drop across the sense resistor

or low-side MOSFET equals the voltage across the R

ILIMresis-

tor, current limit will activate. The high-side MOSFET will

not be allowed to turn on until the voltage drop across the

sense element (resistor or MOSFET) falls below the voltage

across the R

ILIM resistor.

The current sensing circuit actually regulates the inductor

valley current, see Figure 3. This means that if the current

limit is set to 10A, the peak current through the inductor

would be 10A plus the peak ripple current, and the average

current through the inductor would be 10A plus 1/2 the

peak-to-peak ripple current.

I LIMIT

I LOAD

IPEAK

TIME

Valley Current - Limit Threshold Point

Figure 3

Application Information

The schematic of RDS

ON sensing circuit is shown in Figure

4 with R

ILIM = R1 and RDSON of Q2.

Vout

+5V

+VIN

PGND

VDDP

ILIM

BST

SC488

Figure 4

Similarly, for resistor sensing, the current through the lower

MOSFET and the source sense resistor develops a voltage

that opposes the voltage developed across R

ILIM. When the

voltage developed across the R

SENSE resistor reaches voltage

drop across R

ILIM, an over-current exists and the high-side

MOSFET will not be allowed to turn on. The over-current

equation when using an external sense resistor is:

SENSE

ILIM

Valley

Schematic of resistor sensing circuit is shown in Figure 5

with R

ILIM = R1 and RSENSE = R4.

Vout

+5V

PGND

VDDP

ILIM

BST

+VIN

SC488

Figure 5

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