SC470 datasheet(9/31 Pages) SEMTECH

9 / 31 page

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SC470

POWER MANAGEMENT

Application Information (Cont.)

Output Voltage Selection

The output voltage is set by the feedback resistors R3 &

R7 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:

Figure 2: Setting The Output Voltage

Current Limit Circuit

Current limiting of the SC470 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

efficient and less expensive. In both cases, the R

ILIM

resistor 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 SC470 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 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 Under-voltage 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

Figure 3: Valley Current Limiting

The equation for the current limit threshold is as follows:

10e

SENSE

ILIM

LIMIT

•

Where (referring to Figure 8 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

developed across the R

SENSE resistor reaches the voltage

20k0

EN/PSV

TON

VOUT

VCCA

PGD

VSSA

PGND

VDDP

ILIM

BST

SC470

VOUT

0402

56p

0402

OUT

•

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