LM5145 datasheet(16/70 Pages) TI

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8.3 Feature Description

8.3.1 Input Voltage Range (VIN)

The LM5148 operational input voltage range is from 3.5 V to 80 V. The device is intended for step-down

conversions from 12-V, 24-V, and 48-V supply rails. The application circuit in Figure 9-4 shows all the necessary

components to implement an LM5148 based wide-VIN single-output step-down regulator using a single supply.

The LM5148 uses an internal LDO to provide a 5-V VCC bias rail for the gate drive and control circuits

(assuming the input voltage is higher than 5 V with additional voltage margin necessary for the subregulator

dropout specification).

In high input voltage applications, take extra care to ensure that the VIN and SW pins do not exceed their

absolute maximum voltage rating of 85 V during line or load transient events. Voltage excursions that exceed the

applicable voltage specifications can damage the device.

Care must be taken in applications where there are fast input transients that cause the voltage at VIN to

suddenly drop more than 2 V below the VOUT setpoint. The LM5148 has an internal ESD diode from the VOUT

to the VIN pins that can conduct under such conditions causing the output to discharge. To prevent damage to

the internal ESD diode under the said conditions, TI recommends adding a Schottky diode in series with the VIN

pin of the LM5148 to prevent reverse current flow from VOUT to VIN.

As VIN approaches VOUT, the LM5148 skips tOFF cycles to allow the controller to extend its duty cycle up to

approximately 99%. Refer to Dropout Mode.

Use Equation 1 to calculate when the LM5148 enters dropout mode.

VIN=VOUT× tPtP − tOFF

(1)

• tP is the oscillator period

• tOFF is the minimum off time, typical 90 ns

VOUT

HO-SW

Low Dropout Mode

PWM Mode

~99% Duty Cycle

VIN

one

tOFF skip

two

tOFF skip

three

tOFF skip

up to 15

tOFF skip

up to 15

tOFF skip

Figure 8-1. Dropout Mode Operation

8.3.2 High-Voltage Bias Supply Regulator (VCC, VCCX, VDDA)

The LM5148 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 input voltage pin (VIN) can be connected directly

to an input voltage source up to 80 V. 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 115 mA (minimum). At power up, the controller sources current into the

capacitor connected at the VCC pin. When the VCC voltage exceeds 3.3 V and the EN pin is connected to a

voltage greater than 1 V, the soft-start sequence begins. The output remains active unless the VCC voltage falls

LM5148

SNVSC01 – FEBRUARY 2023

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