SC486 datasheet(11/26 Pages) SEMTECH

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 2006 Semtech Corp.

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SC486

POWER MANAGEMENT

EN/PSV: Enable, PSAVE and Soft Discharge (Cont.)

If the EN/PSV pin is pulled low, all three outputs will be

shut down and discharged using switches with a nominal

resistance of 22 Ohms, regardless of the state of the

VTTEN pin. This will ensure that the outputs will be in a

defined state next time they are enabled and also

ensure, since this is a soft discharge, that there are no

dangerous negative voltage excursions to be concerned

about. In order for the soft discharge circuitry to

function correctly, the chip supply must be present.

VTTEN

The VTTEN pin is used to enable the VTT regulator only.

Pulling it high enables the regulator as long as VDDQ/

REF are present. Pulling VTTEN low while EN/PSV is

floating or high will turn off the VTT regulator and leave it

in a high-impedance state for S3 mode (VDDQ and REF

present, VTT high-Z).

VDDQ Output Voltage Selection and Output Sense

The output voltage is set by the feedback resistors R5 &

R9 of Figure 2 below. The internal reference is 1.5V, so

the voltage at the feedback pin will match the 1.5V

reference. Therefore the output can be set to a

minimum of 1.5V. The equation for setting the output

voltage is:

VOUT

•











 +

VDDQS is used to sense the output voltages for the on-

time one-shot, t

ON, and also to generate REF, which is 1/

2 of VDDQ. An RC filter consisting of 10

Ω and 1µF from

VDDQ to VSSA is required (R4 and C2 in Figure 2) to filter

switching frequency ripple.

VDDQ Current Limit Circuit

Current limiting of the SC486 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 SC486

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 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

overleaf.

Figure 2

C10

VDDQ

C12

1uF

C11

20uF

VTT

1uF

10R

5VRUN

5VSUS

no-pop

R7 10R

REF

10R

1uF

R4 10R

1uF

VDDQ

PGOOD

VBAT

1nF

no-pop

R8 0R

PGND1

REF

EN/PSV

TON

VDDQS

VCCA

PGD

VSSA

PGND2

VDDP

ILIM

BST

VTTEN

VTT

PGND2

VTTIN

VTT

VTTS

COMP

SC486

VDDQ

R10

C13

1uF

0.1uF

R3 470k

5VSUS

10uF

VBAT

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