SC1544 datasheet(13/15 Pages) SEMTECH

13 / 15 page

 2002 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC1544

PRELIMINARY

Applications Information (Cont.)

Linear Regulator Stability

When extremely low ESR capacitors (Oscons, Polymers,

high value ceramics) are used on outputs requiring fast

transients (3.3V/2.5V memory), the linear regulators may

exhibit some overshoot and/or transient instability.

External compensation may be necessary (5VSB to 3.3V

Dual Memory shown):

3.3V DUAL MEM

5VSB

SC1544-3.3

3.3VDM

AGP

TYPEDET

PWR_OK

/S3

/S5

USB

P CI

-CAP

+CAP

5VSB

5VD

G2/3

GND

3.3VD

1.8V

IRLR3103

22nF

Another possibility is to use dual capacitor types for the

load capacitor - a ceramic capacitor will provide high

frequency decoupling for the load while an electrolytic

capacitor (high ESR) will tame the Q to prevent instability:

Electrolytic - bulk

Ceramic - high frequency decoupling

3.3V DUAL MEM

Adjusting the Output Voltage of the Linear Regulators

It is possible to adjust the output voltage of the linear

regulators (1.8V, 2.5V Dual Memory and AGP) by

applying an external resistor divider to the sense pin (see

below). Since the sense pins sink a nominal 100µA (1.8V

LDO, 150µA for the 1.5V AGP LDO, 325µA for the 3.3V

AGP LDO and 350µA for the 2.5V Dual Memory LDO),

the resistor values should be selected to allow 100x that

current to flow through the divider. This will ensure that

variations in the sense current will have negligible affect

on the output voltage regulation. Thus a target value for

R2 (maximum) can be calculated:

Ω

•

≤

100

SENSE

)

FIXED

(

OUT

.or the 2.5V Dual Memory linear regulator:

Ω

≤

•

≤

100

350

100uF/6.3V

SC1544-2.5

2.5VDM

AGP

TYPEDET

PWR_OK

/S3

/S5

USB

PCI

-CAP

+CAP

5VSB

5VD

G2/3

GND

3.3VD

1.8V

2.5V < VOUT (AJUSTED) < 3.3V

I SENSE

VOUT (FIXED)

GND

The output voltage can only be adjusted upwards from

the fixed output voltage, and can be calculated using the

following equation:

Volts

SENSE

)

FIXED

(

OUT

)

ADJUSTED

(

OUT

•











 +

•

Therefore to set the 2.5V Dual Memory linear regulator

to 2.6V, for example, R1 = 2.8Ω and R2 = 69.8Ω. The

maximum voltage to which an output can be set using

this method is limited by the input voltage to the .ET(s)

and the R

DS(ON) of the .ET(s).

Please note that this technique cannot be used for the

3.3V Dual Memory, 3.3V Dual and 5V Dual outputs since

the output is switched via a pass device (i.e. not

regulated) when not in S0.

.ault Protection Hints

Loss of AC Power: if it is possible during brownouts or

momentary loss of AC power to the computer silver box

that PWR_OK can assert low while S3 and S5 remain

high then the over current protection may trigger. This is

because the state machine will ignore this illegal

transition and monitor all outputs as if they are still in

S0, despite the fact that the inputs are going away. If

5VSB decays slowly, S3 and S5 remain high, and one

output drops below the OCP threshold for long enough

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