LTC3313JVPBF datasheet(18/28 Pages) AD

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LTC3313

Rev. 0

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APPLICATIONS INFORMATION

conditions. The VIN(EN) threshold prevents the regulator

from operating at source voltages where problems may

occur. This threshold can be adjusted by setting the values

R1 and R2 such that they satisfy the following equation:

VIN EN

( ) =

⎛

⎝

⎜

⎞

⎠

⎟ • 400mV

(9)

where the LTC3313 will remain off until VIN is above

VIN(EN). Due to the comparator’s hysteresis, switching

will not stop until the input falls slightly below VIN(EN).

Alternatively, a resistor divider from an output of

another regulator to the enable pin of the LTC3313 pro-

vides event-based power-up sequencing, enabling the

LTC3313 when the output of the other regulator reaches

a predetermined level.

Output Voltage Tracking and Soft-Start

The LTC3313 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 voltage

will proportionally track the SSTT pin voltage. When the

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

tional to the LTC3313 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 the 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 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 LTC3313 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 Equation 11.

TJ (°C)=

VSSTT

4mV

– 273

(11)

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-skipping

mode with the VOUT pulled up slightly higher than the

regulated VOUT.

3. Calculate the slope of the temperature sensing circuit

with Equation 12.

Slope (mV /

°C)=

VSSTT

TA +273

(12)

4. Calculate the junction temperature with the new cali-

brated slope.

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