LM5045 datasheet(14/46 Pages) TI

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LM5045

SNVS699H – FEBRUARY 2011 – REVISED JANUARY 2015

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7.3 Feature Description

7.3.1 High-Voltage Start-Up Regulator

The LM5045 contains an internal high-voltage start-up regulator that allows the input pin (VIN) to be connected

directly to the supply voltage over a wide range from 14 V to 100 V. The input can withstand transients up to

105 V. When the UVLO pin potential is greater than 0.4 V, the VCC regulator is enabled to charge an external

capacitor connected to the VCC pin. The VCC regulator provides power to the voltage reference (REF) and the

gate drivers (HO1/HO2 and LO1/LO2). When the voltage on the VCC pin exceeds its undervoltage (UV)

threshold, the internal voltage reference (REF) reaches its regulation set point of 5 V and the UVLO voltage is

greater than 1.25 V, the soft-start capacitor is released and normal operation begins. The regulator output at

VCC is internally current limited. The value of the VCC capacitor depends on the total system design, and its

start-up characteristics. The recommended range of values for the VCC capacitor is 0.47

μF to 10 µF.

The internal power dissipation of the LM5045 can be reduced by powering VCC from an external supply. The

output voltage of the VCC regulator is initially regulated to 9.5 V. After the synchronous rectifiers are engaged

(which is approximately when the output voltage in within regulation), the VCC voltage is reduced to 7.7 V. In

typical applications, an auxiliary transformer winding is connected through a diode to the VCC pin. This winding

must raise the VCC voltage above 8V to shut off the internal start-up regulator. Powering VCC from an auxiliary

winding improves efficiency while reducing the power dissipation of the controller. The VCC UV circuit will still

function in this mode, requiring that VCC never falls below its UV threshold during the start-up sequence. The

VCC regulator series pass transistor includes a diode between VCC and VIN that should not be forward biased

in normal operation. Therefore, the auxiliary VCC voltage should never exceed the VIN voltage.

An external DC bias voltage can be used instead of the internal regulator by connecting the external bias voltage

to both the VCC and the VIN pins. This implementation is shown in the Application and Implementation section.

The external bias must be greater than 10 V and less than the VCC maximum voltage rating of 14 V.

7.3.2 Line Undervoltage Detector

The LM5045 contains a dual level undervoltage lockout (UVLO) circuit. When the UVLO pin voltage is below 0.4

V, the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than 0.4 V but less

than 1.25 V, the controller is in standby mode. In standby mode the VCC and REF bias regulators are active

while the controller outputs are disabled. When the VCC and REF outputs exceed their respective under-voltage

thresholds and the UVLO pin voltage is greater than 1.25 V, the soft-start capacitor is released and the normal

operation begins. An external set-point voltage divider from VIN to GND can be used to set the minimum

operating voltage of the converter. The divider must be designed such that the voltage at the UVLO pin will be

greater than 1.25 V when VIN enters the desired operating range. UVLO hysteresis is accomplished with an

internal 20

μA current sink that is switched on or off into the impedance of the set-point divider. When the UVLO

threshold is exceeded, the current sink is deactivated to quickly raise the voltage at the UVLO pin. When the

UVLO pin voltage falls below the 1.25 V threshold, the current sink is enabled causing the voltage at the UVLO

pin to quickly fall. The hysteresis of the 0.4 V shutdown comparator is internally fixed at 50 mV.

The UVLO pin can also be used to implement various remote enable / disable functions. Turning off the

converter by forcing the UVLO pin to standby condition (0.4 V < UVLO < 1.25 V) provides a controlled soft-stop.

Refer to the Soft-Stop section for more details.

7.3.3 Overvoltage Protection

An external voltage divider can be used to set either an overvoltage or an overtemperature protection. During an

OVP condition, the SS and SSSR capacitors are discharged and all the outputs are disabled. The divider must

be designed such that the voltage at the OVP pin is greater than 1.25 V when overvoltage/temperature condition

exists. Hysteresis is accomplished with an internal 20

μA current source. When the OVP pin voltage exceeds

1.25 V, the 20

μA current source is activated to quickly raise the voltage at the OVP pin. When the OVP pin

voltage falls below the 1.25 V threshold, the current source is deactivated causing the voltage at the OVP to

quickly fall. Refer to the Application and Implementation section for more details.

7.3.4 Reference

The REF pin is the output of a 5-V linear regulator that can be used to bias an optocoupler transistor and

external housekeeping circuits. The regulator output is internally current limited to 15 mA. The REF pin must be

locally decoupled with a ceramic capacitor, the recommended range of values are from 0.1

μF to 10 μF

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