SC1486AEVB datasheet(10/30 Pages) SEMTECH

10 / 30 page

 2006 Semtech Corp.

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SC1486A

POWER MANAGEMENT

Application Information (Cont.)

Figure 3: Valley Current Limiting

The equation for the current limit threshold is as follows:

10e

SENSE

ILIM

LIMIT

•

Where (referring to Figure 2) R

ILIM is R5 and RSENSE is the

DS(ON) of Q3.

For resistor sensing, a sense resistor is placed between

the source of Q3 and PGND. The current through 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 the voltage

drop across R

ILIM, a positive over-current exists and the

high side MOSFET will not be allowed to turn on. When

using an external sense resistor R

SENSE is the resistance

of the sense resistor.

The current limit circuitry also protects against negative

over-current (i.e. when the current is flowing from the

load to PGND through the inductor and bottom MOSFET).

In this case, when the bottom MOSFET is turned on, the

phase node, LX, will be higher than PGND initially. The

SC1486A monitors the voltage at LX, and if it is greater

than a set threshold voltage of 140mV (nom.) the

bottom MOSFET is turned off. The device then waits for

approximately 2µs and then DL goes high for 300ns (typ.)

once more to sense the current. This repeats until

either the over-current condition goes away or the part

OUT1 Output Voltage Selection

The output voltage is set by the feedback resistors R10

& R13 of Figure 2 below. The internal reference is 1.5V,

so the voltage at the feedback pin is multiplied by three

to match the 1.5V reference. Therefore the

output can be set to a minimum of 0.5V. The equation

for setting the output voltage is:

VOUT

•











 +

5VSUS

0402

2n2/50V

IRF7811AV

SOD

0402

7343

0603

0402

323

R5 13k3

0u1/25V

0603

R13

17k4

C10 0u1

0402

R10

45k3

1210

C14

330u/25m

C20

1.8V

0402

2u4

5VSUS

C17

27p

PGOOD1

VCCA1

0402

R1 10R

VCCA1

0603

VCCA1

0402

10u/25V

SC1486A

VDDP1

PGND1

TON1

DH1

BST1

LX1

ILIM1

EN/PSV1

PGD1

VOUT1

FB1

VCCA1

VSSA1

VDDP2

PGND2

TON2

DH2

BST2

LX2

ILIM2

PGD2

FB2

REFIN

REFOUT

VCCA2

VSSA2

DL1

DL2

10u/25V

C21

TON1

0402

7343

C13

330u/25m

PGD1

R14 470k

FDS6676S

VSSA1

0402

1.8V, 10A

PWR_SRC

1210

C12

0u1

Figure 2: Setting VDDQ Output Voltage

Current Limit Circuit

Current limiting of the SC1486A can be accomplished in

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

MOSFETs can be used as the current sensing element

or sense resistors in series with the low-side sources

can be used if greater accuracy is desired. R

DS(ON)

sensing is more efficient and less expensive. In both

cases, the R

ILIM resistors between the ILIM pin and LX pin

set the over current threshold. This resistor R

ILIM is

connected to a 10µA current source within the SC1486A

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 RILIM resisor,

positive current limit will activate. The high side MOSFET

will not be turned on until the voltage drop across the

sense element (resistor or MOSFET) falls below the

voltage across the R

ILIM resistor. In an extreme over-

current situation, the top MOSFET will never turn back

on and eventually the part will latch off due to output

undervoltage (see Output Undervoltage Protection).

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. The

equations for setting the valley current and calculating

the average current through the inductor are shown

below:

ILIMIT

ILOAD

IPEAK

TIME

Valley Current-Limit Threshold Point

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