LTC3312SAAVPBF datasheet(21/28 Pages) AD

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LTC3312SA

Rev. 0

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SSTT to AGND to set the soft-start time, calculated using

Equation 11.

tSS = CSS • 500mV/4µA

(11)

From 0V to 0.5V, the SSTT voltage will override the inter-

nal 0.5V reference input to the error amplifier, thus regu-

lating the FB pin voltage to that of the SSTT pin. When

SSTT is above 0.5V, tracking is disabled, and the feedback

voltage will regulate to the internal reference voltage.

An alternative is to connect an SSTT pin to AVIN, which

selects an internally-set 1ms default soft-start time for

that buck regulator.

An active pull-down circuit is connected to either the SSTT

pin, or to the internal soft-start node if SSTT is connected

high. This is used to discharge it in the case of fault con-

ditions. The soft-start ramp will restart when the fault is

cleared. Fault conditions that clear the soft-start voltage

ramp are the EN pin transitioning low, VIN voltage falling

too low, or thermal shutdown.

Figure 2 (above) and Figure 3 (below) show examples for

some of the ways to control power-up sequencing using

the EN, SSTT and PGOOD pins.

LTC3312SA

PGOOD1

500k

PGOOD

EN1

VIN

TIME

ENABLE

VOUT1

VOUT2

EN2

PGOOD2

3312SA F03a

(a) Sequential Voltage Power-Up

LTC3312SA

SSTT1

EN1

EN2

TIME

ENABLE

VOUT1

VOUT2

SSTT2

•

3312SA F03b

(b) Tracking Voltage Power-Up

Figure 3. Power-Up Sequencing

Temperature Monitor

After the soft-start cycle has completed and the output

power good flag is high, the SSTT pin voltages are set to a

PTAT (proportional to absolute temperature) voltage. The

following procedure can be used to accurately measure

the die junction temperature:

1. Measure the ambient temperature (TA), and one of the

SSTT pin voltages (VSSTT), when both buck regulators

are unloaded. Calculate an accurate slope for the tem-

perature sensing circuit using Equation 12.

Slope (V /°K)=

VSSTT

TA +273

(12)

2. The calibrated slope can then be used to accurately

determine the die temperature for any load or ambient

conditions using Equation 13.

TJDIE(°C)=

VSSTT

Slope

−273

(13)

When an output voltage goes out of regulation or is

disabled, and the PGOOD pin is pulled low, the corre-

sponding SSTT pin no longer reports the temperature.

DIE TEMP

(°C)

VFB

(V)

150

0.1

125

0.6

0.3

100

0.5

0.4

0.2

0.5

1.5

0.6

1.6

0.3

1.3

3312SA F04

1.7

0.4

1.4

0.2

1.2

0.1

SSTT (V)

TEMP

MONITOR

~4mV/°C

SSTT PIN VOLTAGE

OPERATING RANGE

SOFT-START

AND TRACKING

Figure 4. Soft-Start and Temperature Monitor Operation

PCB Layout Considerations

The LTC3312SA is a high performance IC designed for

high efficiency and fast transient response. For optimal

results carefully consider the layout of the PCB board

APPLICATIONS INFORMATION

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