LM5176PWPR datasheet(16/45 Pages) TI2

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between the shutdown mode and the standby mode. When EN/UVLO is greater than the operating threshold,

VEN(OP), and VCC is above the undervoltage threshold, VUV(VCC), the controller starts operation. A hysteresis

current ΔIHYS(OP) is sourced out of the EN/UVLO pin when the EN/UVLO input exceeds the operating threshold

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

voltage.

The VIN UVLO threshold is typically set by a resistor divider from VIN to AGND (Figure 7-2). The turnon threshold

VIN(UV) is calculated using Equation 1 where RUV2 is the upper resistor and RUV1 is the lower resistor in the

EN/UVLO resistor divider:

UV2

IN(UV)

EN(OP)

UV2

EN(STBY)

UV1

(1)

The hysteresis between the UVLO turnon threshold and turnoff threshold is set by the upper resistor in the

EN/UVLO resistor divider and is given by:

HYS(UV)

HYS(OP)

UV2

(2)

UV2

UV1

EN/UVLO

LM5176

Figure 7-2. UVLO Threshold Programming

7.3.4 Soft Start

The LM5176 soft-start time is programmed using a soft-start capacitor from the SS pin to AGND. When the

converter is enabled, an internal current source (ISS) charges the soft-start capacitor. When the SS pin voltage

is below the feedback reference voltage, VREF, the soft-start pin controls the regulated FB voltage. Once SS

exceeds VREF, the soft-start interval is complete and the error amplifier is referenced to VREF. The soft-start time

is given by Equation 3:

REF

(3)

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

below the operating 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.

7.3.5 Overcurrent Protection

The LM5176 provides cycle-by-cycle current limit to protect against overcurrent and short circuit conditions. In

buck operation, the sensed valley voltage across the CSG and CS pins is limited to VCS(BUCK). The high-side

buck switch skips a cycle if the sensed voltage does not fall below this threshold during the buck switch off time.

In boost operation, the maximum peak voltage across CS and CSG is limited to VCS(BOOST). If the peak current

in the low-side boost switch causes the voltage across CS and CSG to exceed this threshold voltage, the boost

switch is turned off for the remainder of the clock cycle.

LM5176

SNVSAI1D – JUNE 2017 – REVISED AUGUST 2021

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