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© 2003 Semtech Corp.

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POWER MANAGEMENT

SC2616

maximum voltage applied to these pins do not exceed

the chipsets specifications. A separate lower pullup supply

may be necessary to avoid damage to the chipset.

“Back Feeding” the Input Supply

When in S3 state, VDDQ is supplied by the linear regulator

and current can flow back from the VDDQ supply through

the body diode of the Top switching MOSFET to the 5V

supply of the Silver Box, which is off during the S3 state.

This in turn shorts out the VDDQ supply and is not

acceptable.

An addittional MOSFET should be addded to avoid the

reverse current flow. The MOSFETs should have the drains

to each other(common- Drain).

During S0 to S3 transition, As soon as SLP_S3 signal

goes low, BG signal stops chopping. This can prevent the

inductor to build up its current in the reverse direction.

To avoid the MOSFET damaged by overshoot, the input

bulk capacitor must be added to the node of common -

Drain.

Current Limit

Current limit is implemented by sensing the VDDQ voltage.

If it falls to 75% off its nominal voltage, as sensed by the

FB pin, the TG and BG pins are latched off and the

switcher and the linear converters are shut down. To

recover from the current limit condition, either the power

rails, 5VCC or 12VCC have to be recycled, or the SS/EN

pin must be pulled low and released to restart switcher

operation.

Thermal Shutdown

There are three independent Thermal Shutdown

protection circuits in the SC2616: the VDDQ linear

regulator, the VTT source regulator, and the VTT sink

regulator. If any of the three regulators’ temperature

rises above the threshold, that regulator will turn off

independently, until the temperature falls below the

thermal shutdown limit.

OUTPUT INDUCTOR - A good starting point for output

filter component selection is to choose an inductor value

Applications Information (Cont.)

that will give an inductor ripple current of approximately

20% of max. output current.

Inductor ripple current is given by:-

OSC

IN

O

O

RIPPLE

L

f

L

V

V

V

I

⋅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⋅

=

1

So choose inductor value from:-

OSC

O

IN

O

O

f

I

V

V

V

L

⋅

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⋅

⋅

=

1

5

OUTPUT CAPACITOR(S) - The output capacitors should

be selected to meet output ripple and transient response

criteria. Output ripple voltage is caused by the inductor

ripple current flowing in the output capacitor’s ESR (There

is also a component due to the inductor ripple current

charging and discharging the output capacitor itself, but

this component is usually small and can often be ignored).

Given a maximum output voltage ripple requirement, ESR

is given by:-

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⋅

⋅

⋅

<

IN

O

O

RIPPLE

OSC

ESR

V

V

V

V

f

L

R

1

Output voltage transient excursions are a function of load

current transient levels, input and output voltages and

inductor and capacitor values.

Capacitance and R

sient condition can be calculated from

release)

(load

transients

positive

for

V

V

and

n)

applicatio

(load

transients

negative

for

V

V

V

where

V

V

I

L

C

I

V

R

O

A

O

IN

A

A

T

T

T

T

ESR

=

−

=

⋅

⋅

⋅

>

<

2

2

values for positive and negative transients must be cal-

culated seperately and the worst case value chosen. For

Capacitor values, the calculated value should be doubled

to allow for duty cycle limitation and voltage drop issues.