idle duration exceeds 15 μs, the converter transitions into an ultra-low IQ mode, consuming only 10-μA quiescent

current from the input.

6.3.2 Internal VCC Regulator and Bootstrap Capacitor

The LM5164 contains an internal linear regulator that is powered from VIN with a nominal output of 5 V,

eliminating the need for an external capacitor to stabilize the linear regulator. The internal VCC regulator

supplies current to internal circuit blocks including the synchronous FET driver and logic circuits. The input

pin (VIN) can be connected directly to line voltages up to 100 V. As the power MOSFET has a low total gate

charge, use a low bootstrap capacitor value to reduce the stress on the internal regulator. It is required to select

a high-quality 2.2-nF 50-V X7R ceramic bootstrap capacitor as specified in the Absolute Maximum Ratings

section. Selecting a higher value capacitance stresses the internal VCC regulator and damages 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.

6.3.3 Regulation Comparator

The feedback voltage at FB is compared to an internal 1.2-V reference. The LM5164 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 10 to calculate RFB2 based on the selected RFB1.

FB2

FB1

OUT

1.2 V

R

R

V

1.2 V

˜

(10)

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. It is important to route the feedback trace away from

the noisy area of the PCB and keep the feedback resistors close to the FB pin.

6.3.4 Internal Soft Start

The LM5164 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.

6.3.5 On-Time Generator

The on-time of the LM5164 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 11 to calculate the on-time.

RON

ON

IN

R

k

t

V

V

V

2.5

:

˜

(11)

Use Equation 12 to determine the RRON resistor to set a specific switching frequency in CCM.

OUT

RON

SW

V

(V) 2500

R

(k )

F

(kHz)

˜

:

(12)

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.

LM5164

SNVSAU4C – SEPTEMBER 2018 – REVISED APRIL 2024

www.ti.com

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