LM5177DCPR.A datasheet(22/60 Pages) TI2

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VIN

BIAS

Sup ply Vo ltag e S electio n

to Internal

Regula tors

Figure 7-7. VMAX Supply Scenario 2

VIN

BIAS

V(AUX)

Sup ply Vo ltag e S electio n

to Internal

Regula tors

Figure 7-8. VMAX Supply Scenario 3

7.3.5 Enable and Undervoltage Lockout

The LM51770 has a dual function enable and undervoltage lockout (UVLO) pin. The internal device logic and

reference system powers up once the pin voltage is above the VT+(EN) threshold. Once this condition is met, the

device is in standby mode. If the EN/UVLO pin voltage is below the VT-(EN) threshold, the device is in shutdown

mode to save quiescent current. Find the device operation modes description in Section 7.4.

The UVLO function of the device can detect an low input voltage condition for the power stage to avoid a

brownout condition. The detection threshold as well as the required hysteresis are adjustable with an external

voltage divider on the EN/UVLO pin.

If the EN/UVLO pin voltage is above the VT+(EN) threshold, the internal current source for the UVLO hysteresis

is active. If the EN/UVLO pin voltage is above the VT+(UVLO) threshold, the internal current source for the UVLO

hysteresis is off.

The UVLO features an internal delay time (td(UVLO)) for the shutdown to avoid any undesired converter shutdown

due to input noise on the UVLO detection pin. The voltage on the EN/UVLO pin must be below the VT-(UVLO)

threshold for the delay time, td(UVLO). Once these conditions are met, the device logic immediately stops the

converter operation.

The UVLO threshold is typically set by a resistor divider from VIN to AGND. The effective turn-on threshold is

calculated using Equation 1. The hysteresis between the UVLO turn-on threshold and turn-off threshold is set by

the upper resistor and the internal hysteresis current.

VVIN,IT+,UVLO =VIT+ UVLO × 1+RUVLO,top

RUVLO,bot + RUVLO,top ×I(UVLO,ℎyst)

(1)

where

• R(UVLO,top) is the upper resistor.

• R(UVLO,bot) is the lower resistor in the divider.

7.3.6 Oscillator Frequency Selection

The has a low tolerance internal trimmed oscillator. With the RT pin left open, the oscillator frequency is 75 kHz.

With the RT pin grounded, the switching frequency is at the maximum of 2.5 MHz. The oscillator frequency can

be programmed up or down by connecting a resistor from the RT pin to ground. To calculate the RT resistor for a

specific oscillator frequency, use Equation 2.

LM5177

SNVSBU4F – JUNE 2022 – REVISED JANUARY 2025

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