SC411 datasheet(10/27 Pages) SEMTECH

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SC411

POWER MANAGEMENT

Application Information (Cont.)

Output Voltage Selection

The output voltage is set by the feedback resistors R3 &

R5 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:

OUT = ( 1 + ――― ) • 0.5

Figure 2: Setting The Output Voltage

Current Limit Circuit

Current limiting of the SC411 can be accomplished in two

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

can be used as the current sensing element or sense

resistors in series with the low-side source can be used

if greater accuracy is desired. R

DS(ON) sensing is more ef-

ﬁcient and less expensive. In both cases, the R

ILIM resis-

tor between the ILIM pin and LX pin sets the over current

threshold. This resistor RILIM is connected to a 10μA cur-

rent source within the SC411 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 resistor, 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 resis-

tor. In an extreme over-current situation, the top MOSFET

will never turn back on and eventually the part will latch

off due to output under-voltage (see Output Under-voltage

Protection).

The current sensing circuit actually regulates the induc-

tor 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:

Figure 3: Valley Current Limiting

The equation for the current limit threshold is as follows:

LIMIT

= 10μA × R

ILIM

/ R

SENSE

(Amps)

Where (referring to Figure 4 on Page 17) R

ILIM is R4 and

SENSE is the RDS(ON) of Q2.

For resistor sensing, a sense resistor is placed between

the source of Q2 and PGND. The current through the

source sense resistor develops a voltage that opposes the

voltage developed across R

ILIM.

When the voltage devel-

oped 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.

VOUT

VCCA

PGD

VDDP

ILIM

SC411

56p

0402

20k0

14k3

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