LM5017 datasheet(11/33 Pages) TI1

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VIN

UVLO

VIN

RUV1

CIN

RUV2

LM5017

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SNVS783I – JANUARY 2012 – REVISED OCTOBER 2015

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Feature Description (continued)

7.3.8 Synchronous Rectifier

The LM5017 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 with light loads which would otherwise result in discontinuous operation.

7.3.9 Undervoltage Detector

The LM5017 device contains a dual level undervoltage lockout (UVLO) circuit. A summary of threshold voltages

and operational states is provided in Device Functional Modes . When the UVLO pin voltage is below 0.66 V, the

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

1.225 V, the regulator 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 connected 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 LM5017 device 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 LM5017 in the event of a higher than

normal junction temperature. When activated, typically at 165°C, the regulator 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 falls below 145°C (typical hysteresis = 20°C), the VCC

regulator is enabled, and normal operation is resumed.

7.3.11 Ripple Configuration

LM5017 uses Constant-On-Time (COT) control 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 larger than 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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