SC1485ITSTRT datasheet(9/27 Pages) SEMTECH

9 / 27 page

 2005 Semtech Corp.

www.semtech.com

SC1485

POWER MANAGEMENT

Output Voltage Selection

The output voltage (OUT2 shown) is set by the feedback

resistors R9 & 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:

OUT

•











 +

0402

C15

56p

0402

VOUT2

VSSA2

20k0

R13

20k0

VOUT

EN/PSV2

TON2

VOUT2

VCCA2

FB2

PGD2

VSSA2

PGND2

DL2

VDDP2

ILIM2

LX2

DH2

BST2

SC1485 OUT2

Figure 2: Setting The Output Voltage

Current Limit Circuit

Current limiting of the SC1485 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 sets the over

current threshold. This resistor R

ILIM is connected to a

10µA current source within the SC1485 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 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-to-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 16) R

ILIM is R10 and

SENSE is the RDS(ON) of Q4.

For resistor sensing, a sense resistor is placed between

the source of Q4 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.

SC1485ITSTRT 数据表(PDF) 9 Page - Semtech Corporation

SC1485ITSTRT 数据表(HTML) 9 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

数据表下载