LTC7151S datasheet(17/28 Pages) AD

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LTC3311S

Rev. A

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APPLICATIONS INFORMATION

LTC3311S when the output of the other regulator reaches

a predetermined level.

Output Voltage Tracking and Soft-Start

The LTC3311S allows the user to program its output volt-

age ramp rate by means of the SSTT pin.

An internal 10μA pulls up the SSTT pin. Putting an external

capacitor on SSTT enables soft-starting the output to pre-

vent current surge on the input supply and output voltage

overshoot. During the soft-start ramp, the output volt-

age will proportionally track the SSTT pin voltage. When

the soft-start is complete, the pin will servo to a voltage

proportional to the LTC3311S junction temperature. See

Figure 3 showing the SSTT pin operating range.

The soft-start time is calculated as follows:

tSS = CSS •

500mV

10µA

(10)

For output tracking applications, SSTT can be externally

driven by another voltage source. From 0V to 0.5V, the

SSTT voltage will override the internal 0.5V reference

input to the error amplifier, thus regulating the FB pin

voltage to that of SSTT pin. When SSTT is above 0.5V,

tracking is disabled and the feedback voltage will regulate

to the internal reference voltage

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

to discharge the external soft-start capacitor in the case

of fault conditions. The ramp will restart when the fault is

cleared. Fault conditions that clear the soft-start capacitor

are the EN/UV pin transitioning low, VIN voltage falling too

low or thermal shutdown.

Temperature Monitor

Once the soft-start cycle has completed and the output

power good flag thrown, the SSTT pin reports the die

junction temperature. The LTC3311S regulates the SSTT

pin to a voltage proportional to the junction temperature.

While reporting the temperature, the SSTT voltage is not

valid below 1V. The junction temperature is calculated

with the following formula:

TJ (°C)=

VSSTT

4mV

– 273

The following procedure is used for a more accurate mea-

surement of the junction temperature:

1. Measure the ambient temperature TA.

2. Measure the SSTT voltage while in pulse skip mode

with the VOUT pulled up slightly higher than the

regulated VOUT.

3. Calculate the slope of the temperature sensing circuit

as follows:

Slope (mV /

°C)=

VSSTT

TA +273

4. Calculate the junction temperature with the new cali-

brated slope.

When the output voltage goes out of regulation and the

power good pin is pulled low, the soft-start pin no longer

reports the temperature.

Figure 3. Soft-Start and Temperature Monitor Operation

DIE TEMP

(°C)

(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

3311S F03

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

Output Power Good

When the LTC3311S’s output voltage is within the

–2/+10% window of the nominal regulation voltage the

output is considered good and the open-drain PGOOD pin

goes high impedance and is typically pulled high with an

external resistor. Otherwise, the internal pull-down device

will pull the PGOOD pin low. To prevent glitching both the

upper and lower thresholds, include 1% of hysteresis as

well as a built in time delay, typically 125µs. The PGOOD

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