LM5117, LM5117-Q1

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SNVS698F – APRIL 2011 – REVISED AUGUST 2015

Feature Description (continued)

7.3.11 Maximum Duty Cycle

When operating with a high PWM duty cycle, the high-side NMOS device is forced off each cycle for 320ns to

forced off-time limits the maximum duty cycle of the controller. When designing a regulator with high switching

frequency and high duty cycle requirements, a check should be made of the required maximum duty cycle

against the graph shown in Figure 28. The actual maximum duty cycle varies with the switching frequency as

follows:

Figure 28. Maximum Duty Cycle vs Switching Frequency

7.3.12 Thermal Protection

Internal thermal shutdown circuitry is provided to protect the controller in the event the maximum junction

temperature is exceeded. When activated, typically at 165°C, the controller is forced into a low power shutdown

mode, disabling the output drivers and the VCC regulator. This feature is designed to prevent overheating and

destroying the device.

7.4 Device Functional Modes

7.4.1 Diode Emulation

A fully synchronous buck regulator implemented with a freewheeling NMOS rather than a diode has the

capability to sink current from the output in certain conditions such as light load, over-voltage or pre-bias startup.

The LM5117 provides a diode emulation feature that can be enabled to prevent reverse current flow in the low-

side NMOS device. When configured for diode emulation, the low-side NMOS driver is disabled when SW pin

voltage is greater than -5mV during the off-time of the high-side NMOS driver, preventing reverse current flow.

A benefit of the diode emulation is lower power loss at no load or light load conditions. The negative effect of

diode emulation is degraded light load transient response.

The diode emulation feature is configured with the DEMB pin. To enable diode emulation, connect the DEMB pin

to GND or leave the pin floating. If continuous conduction operation is desired, the DEMB pin should be tied to a

voltage greater than 3 V and may be connected to VCC. The LM5117 forces the regulator to operate in diode

emulation mode when SS pin voltage is less than the internal 0.8-V reference, allowing for startup into a pre-

biased load with the continuous conduction configuration.

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