LM5019 datasheet(11/30 Pages) TI

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VIN

UVLO

VIN

RUV1

CIN

RUV2

LM5019

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SNVS788F – JANUARY 2012 – REVISED DECEMBER 2014

Feature Description (continued)

7.3.8 Synchronous Rectifier

The LM5019 provides an internal synchronous N-Channel MOSFET rectifier. This MOSFET provides a path for

the inductor current to flow when the high-side MOSFET is turned off.

The synchronous rectifier has no diode emulation mode, and is designed to keep the regulator in continuous

conduction mode even during light loads which would otherwise result in discontinuous operation.

7.3.9 Undervoltage Detector

The LM5019 contains a dual level Undervoltage Lockout (UVLO) circuit. When the UVLO pin voltage is below

0.66 V, the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than 0.66 V but

less than 1.225 V, the controller is in standby mode. In standby mode the VCC bias regulator is active while the

regulator output is disabled. When the VCC pin exceeds the VCC undervoltage threshold and the UVLO pin

voltage is greater than 1.225 V, normal operation begins. An external set-point voltage divider from VIN to GND

can be used to set the minimum operating voltage of the regulator.

UVLO hysteresis is accomplished with an internal 20-

μA current source that is switched on or off into the

impedance of the set-point divider. When the UVLO threshold is exceeded, the current source is activated to

quickly raise the voltage at the UVLO pin. The hysteresis is equal to the value of this current times the resistance

RUV2.

If the UVLO pin is wired directly to the VIN pin, the regulator will begin operation once the VCC undervoltage is

satisfied.

Figure 11. UVLO Resistor Setting

7.3.10 Thermal Protection

The LM5019 should be operated so the junction temperature does not exceed 150°C during normal operation.

An internal Thermal Shutdown circuit is provided to protect the LM5019 in the event of a higher than normal

junction temperature. When activated, typically at 165°C, the controller is forced into a low power reset state,

disabling the buck switch and the VCC regulator. This feature prevents catastrophic failures from accidental

device overheating. When the junction temperature reduces below 145°C (typical hysteresis = 20°C), the VCC

regulator is enabled, and normal operation is resumed.

7.3.11 Ripple Configuration

LM5019 uses Constant-On-Time (COT) control scheme, in which the on-time is terminated by an on-timer, and

the off-time is terminated by the feedback voltage (VFB) falling below the reference voltage (VREF). Therefore, for

stable operation, the feedback voltage must decrease monotonically, in phase with the inductor current during

the off-time. Furthermore, this change in feedback voltage (VFB) during off-time must be large enough to

suppress any noise component present at the feedback node.

Table 1 shows three different methods for generating appropriate voltage ripple at the feedback node. Type 1

and Type 2 ripple circuits couple the ripple at the output of the converter to the feedback node (FB). The output

voltage ripple has two components:

1. Capacitive ripple caused by the inductor current ripple charging/discharging the output capacitor.

2. Resistive ripple caused by the inductor current ripple flowing through the ESR of the output capacitor.

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