LM5145 datasheet(17/60 Pages) TI2

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VOUT

HO-SW

Low Dropout Mode

PWM Mode

Approximately 99%

Duty Cycle

VIN

one

tOFF skip

two

tOFF skip

three

tOFF skip

up to 15

tOFF skip

up to 15

tOFF skip

Figure 6-2. Dropout Mode Operation

6.3.2 High-Voltage Bias Supply Regulator (VCC)

The LM25139 contains an internal high-voltage VCC bias regulator that provides the bias supply for the PWM

controller and the gate drivers for the external MOSFETs. The controller is rated to an input voltage up to 42V.

However, when the input voltage is below the VCC setpoint level, the VCC voltage tracks VIN minus a small

voltage drop.

The VCC regulator output current limit is 210mA (typical). At power up, the controller sources current into the

capacitor connected at the VCC pin. When the VCC voltage exceeds 3.8V (typical) and the EN pin is above 1V,

the soft-start sequence begins. The output remains active unless the VCC voltage falls below the VCC UVLO

falling threshold of 3.5V (typical) or EN is switched to a low state. Connect a ceramic capacitor from VCC to

PGND. The recommended range of the VCC capacitor is from 2.2µF to 10µF.

6.3.3 Precision Enable (EN)

The EN pin can be connected to a voltage as high as 42V. The LM25139 has a precision enable function. When

the EN voltage is greater than 1V, switching is enabled. If the EN voltage is below 0.5V, the LM25139 is in

shutdown with an IQ of 2.3μA (typical) current consumption from VIN. When the EN voltage is between 0.5V and

1V, the LM25139 is in standby mode, the VCC regulator is active, and the controller is not switching. When the

controller is in standby mode, the input quiescent current is 30μA (typical). Enable the LM25139 with a voltage

greater than 1V. However, many applications benefit from using a resistor divider RUV1 and RUV2, as shown in

Figure 6-3, to establish a precision input UVLO. TI recommends to avoid leaving the EN pin floating.

Use Equation 2 and Equation 3 to calculate the UVLO resistors given the required input turn-on and turn-off

voltages.

RUV1= VINON −VINOFF

IHYS

(2)

RUV2= RUV1×VEN

VINON − VEN

(3)

www.ti.com

LM25139

SLVSJ80 – OCTOBER 2025

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