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LM5175

Copyright © 2016, Texas Instruments Incorporated

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LM5175

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SNVSA37A – OCTOBER 2015 – REVISED MAY 2016

Product Folder Links: LM5175

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Feature Description (continued)

8.3.3 Enable/UVLO

The LM5175 has a dual function enable and undervoltage lockout (UVLO) circuit. The EN/UVLO pin has three

distinct voltage ranges: shutdown, standby, and operating (see Shutdown, Standby, and Operating Modes).

When the EN/UVLO pin is below the standby threshold (0.7 V typical), the converter is held in a low power

shutdown mode. When EN/UVLO voltage is greater than the standby threshold but less than the 1.23 V

operating threshold, the internal bias rails and the VCC regulator are enabled but the soft-start (SS) pin is held

low and the PWM controller is disabled. A 1.5 µA pull-up current is sourced out of the EN/UVLO pin in standby

mode to provide hysteresis between the shutdown mode and the standby mode. When EN/UVLO is greater than

the 1.23 V operating threshold, the controller commences operation if VCC is above VCC UV threshold (3.3 V). A

hysteresis current of 3.5 µA is sourced into the EN/UVLO pin when the EN/UVLO input exceeds the 1.23 V

operation threshold to provide hysteresis that prevents on/off chattering in the presence of noise with a slowly

changing input voltage.

(1)

The hysteresis between the UVLO turn-on threshold and turn-off threshold is set by the upper resistor in the

EN/UVLO resistor divider and is given by:

(2)

Figure 21. UVLO Threshold Programming

8.3.4 Soft-Start

The LM5175 soft-start time is programmed using a soft-start capacitor from the SS pin to AGND. When the

converter is enabled, an internal 5-µA current source charges the soft-start capacitor. When the SS pin voltage is

is given by Equation 3:

(3)

The soft-start capacitor is internally discharged when the converter is disabled because of EN/UVLO falling

below the operation threshold or VCC falling below the VCC UV threshold. The soft-start pin is also discharged

when the converter is in hiccup mode current limiting or in thermal shutdown. When average input or output

current limiting is active, the soft-start capacitor is discharged by the constant current loop transconductance

(gm) amplifier to limit either input or output current.