L6739 datasheet(13/30 Pages) STMICROELECTRONICS

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L6739

Output voltage setting and protections

The L6739 device is capable of precisely regulating an output voltage as low as 0.8 V. In

fact, the device comes with a fixed 0.8 V internal reference that guarantees the output

regulated voltage to be within ± 0.5% tolerance over line and temperature variations

(excluding output resistor divider tolerance, when present).

Output voltage higher than 0.8 V can be easily achieved by adding a resistor ROS between

the FB pin and ground. Referring to Figure 1 on page 4, the steady state DC output voltage

is:

Equation 2

where VREF is 0.8 V.

The L6739 monitors the voltage at the VSEN pin and compares it to the internal reference

voltage in order to provide undervoltage and overvoltage protections, as well as PGOOD

signal. According to the level of VSEN, different actions are performed from the controller:



PGOOD

If the voltage monitored through VSEN exits from the PGOOD window limits, the device

de-asserts the PGOOD signal. PGOOD is asserted at the end of the soft-start phase

with 3x clock cycles delay.



Undervoltage protection (UV)

If the voltage at the VSEN pin drops below the UV threshold, the device turns off both

HS and LS MOSFETs, latching the condition. Cycle VCC or EN to recover.

UV is also active during SS acting as VIN detection protection. See the description

below.



Overvoltage protection (OV)

If the voltage at the VSEN pin rises over the OV threshold, overvoltage protection turns

off the HS MOSFET and turns on the LS MOSFET. The LS MOSFET is turned off as

soon as VSEN goes below Vref/2. The condition is latched, cycle VCC/EN to recover.

Note that, even if the device is latched, the device still controls the LS MOSFET and

can switch it on whenever VSEN rises above the OV threshold.



PreOVP protection

Monitors VSEN when IC is disabled. If VSEN surpasses the OV threshold, IC turns on

the low-side MOSFET to protect the load. On the EN rising edge, the protection is

disabled and the IC implements the SS procedure. PreOVP is disabled when EN is

high but the OV protection becomes operative.



VIN detection

UV protection active during SS allows the IC to detect whether input voltage VIN is

present. If UV is triggered during the soft-start, it resets the SS procedure: the controller

re-implements the initial delay and re-ramps up the reference with the same SS timings

described in Section 5. The UV protection is then preventing that IC starts-up when VIN

is not present.

VOUT

VREF 1

RFB

ROS

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