13

LM2840-Q1, LM2841-Q1, LM2842-Q1

www.ti.com

SNVSBE5 – APRIL 2019

Product Folder Links: LM2840-Q1 LM2841-Q1 LM2842-Q1

Submit Documentation Feedback

Copyright © 2019, Texas Instruments Incorporated

8.2.2.5 Output Capacitor

The selection of COUT is driven by the maximum allowable output voltage ripple. The output ripple in the constant

frequency, PWM mode is approximated by Equation 6.

VRIPPLE = IRIPPLE (ESR + (1 / (8fSWCOUT)))

(6)

The ESR term usually plays the dominant role in determining the voltage ripple. Low-ESR ceramic capacitors are

recommended. Capacitors in the range of 22 µF to 100 µF are a good starting point with an ESR of 0.1

Ω or

less.

Table 3. Recommended Input and Output Capacitors

MANUFACTURER

CAPACITOR

CONTACT INFORMATION

Vishay Sprague

293D, 592D, and 595D series tantalum

www.vishay.com

407-324-4140

Taiyo Yuden

High capacitance MLCC ceramic

www.t-yuden.com

408-573-4150

Cornell Dubilier

ESRD seriec Polymer Aluminum Electrolytic

SPV and AFK series V-chip series

www.cde.com

MuRata

High capacitance MLCC ceramic

www.murata.com

8.2.2.6 Bootstrap Capacitor

A 0.15-µF ceramic capacitor or larger is recommended for the bootstrap capacitor CBOOT). For applications where

the input voltage is less than twice the output voltage a larger capacitor is recommended, generally 0.15 µF to 1

µF to ensure plenty of gate drive for the internal switches and a consistently low RDSON.

8.2.2.7 Soft-Start Components

The devices have circuitry that is used in conjunction with the SHDN pin to limit the inrush current on start-up of

the DC/DC switching regulator. The SHDN pin in conjunction with a RC filter is used to tailor the soft start for a

specific application. When a voltage applied to the SHDN pin is between 0 V and up to 2.3 V it causes the cycle-

by-cycle current limit in the power stage to be modulated for minimum current limit at 0 V up to the rated current

limit at 2.3 V. Thus controlling the output rise time and inrush current at start-up. The resistor value must be

selected so the current injected into the SHDN pin is greater then the leakage current of the SHDN pin (1.5 µA)

when the voltage at SHDN is equal or greater then 2.3 V.

8.2.2.8 Shutdown Operation

The SHDN pin of the LM284x-Q1 is designed so that it may be controlled using 2.3 V or higher logic signals. If

the shutdown function is not to be used the SHDN pin may be tied to VIN. This input must not be allowed to float

The maximum voltage to the SHDN pin should not exceed 42 V. If the use of a higher voltage is desired due to

system or other constraints it may be used; however, a 100 k

Ω or larger resistor is recommended between the

applied voltage and the SHDN pin to protect the device.

8.2.2.9 Schottky Diode

The breakdown voltage rating of the diode (D1) is preferred to be 25% higher than the maximum input voltage.

The current rating for the diode must be equal to the maximum output current for best reliability in most

applications. In cases where the duty cycle is greater than 50%, the average diode current is lower. In this case it

is possible to use a diode with a lower average current rating, approximately (1 – D)IOUT; however, the peak

current rating should be higher than the maximum load current. A 0.5-A to 1-A rated diode is a good starting

point.