VCC does not have current limit protection, so selecting a higher value capacitance can stress the internal VCC

regulator and can damage the device. A lower capacitance than required is not sufficient to drive the internal

gate of the power MOSFET. An internal diode connects from the VCC regulator to the BST pin to replenish the

charge in the high-side gate drive bootstrap capacitor when the SW voltage is low.

8.3.3 Regulation Comparator

The feedback voltage at FB is compared to an internal 1.2-V reference. The LM5012 voltage regulation loop

regulates the output voltage by maintaining the FB voltage equal to the internal reference voltage, VREF. A

resistor divider programs the ratio from output voltage VOUT to FB.

For a target VOUT setpoint, use Equation 2 to calculate RFB2 based on the selected RFB1.

FB2

FB1

OUT

1.2 V

R

R

V

1.2 V

˜

(2)

TI recommends selecting RFB1 in the range of 100 kΩ to 1 MΩ for most applications. A larger RFB1 consumes

less DC current, which is mandatory if light-load efficiency is critical. RFB1 larger than 1 MΩ is not recommended

as the feedback path becomes more susceptible to noise. Ensure to route the feedback trace away from the

noisy area of the PCB, minimize the feedback node size, and keep the feedback resistors close to the FB pin.

8.3.4 Internal Soft Start

The LM5012 employs an internal soft-start control ramp that allows the output voltage to gradually reach

a steady-state operating point, thereby reducing start-up stresses and current surges. The soft-start feature

produces a controlled, monotonic output voltage start-up. The soft-start time is internally set to 3 ms.

8.3.5 On-Time Generator

The on time of the LM5012 high-side FET is determined by the RRON resistor and is inversely proportional to the

input voltage, VIN. The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. Use

Equation 3 to calculate the on time.

RON

ON

IN

R

k

t

V

V

V

2.5

:

˜

(3)

Use Equation 4 to determine the RRON resistor to set a specific switching frequency in CCM.

OUT

RON

SW

V

(V) 2500

R

(k )

F

(kHz)

˜

:

(4)

Select RRON for a minimum on time (at maximum VIN) greater than 50 ns for proper operation. In addition to this

minimum on time, the maximum frequency for this device is limited to 1 MHz.

8.3.6 Current Limit

The LM5012 manages overcurrent conditions with cycle-by-cycle current limiting of the peak inductor current.

The current sensed in the high-side MOSFET is compared every switching cycle to the current limit threshold

(3.2 A). There is a 100-ns leading-edge blanking time following the high-side MOSFET turn-on transition to

eliminate false tripping off the current limit comparator. To protect the converter from potential current runaway

conditions, the LM5012 includes a tOFF timer that is proportional to VIN and VOUT that is enabled if a 3.2-A

peak current limit is detected. As shown in Figure 8-1, if the peak current in the high-side MOSFET exceeds

3.2 A (typical), the present cycle is immediately terminated regardless of the programmed on time (tON), the

high-side MOSFET is turned off and the tOFF timer is activated. This action allows the peak inductor current to

fall from 3.2-A peak to an acceptable value to ensure no excessive current in the power stage. This method

folds back the switching frequency to prevent overheating and limits the average output current to less than 3.2

A to ensure proper short-circuit and heavy-load protection of the LM5012. This innovative current limit scheme

www.ti.com

LM5012

SNVSCA9 – OCTOBER 2022

Copyright © 2022 Texas Instruments Incorporated

Submit Document Feedback

13

Product Folder Links: LM5012