L =

(VIN - VOUT)VOUT

VIN x IRIPPLE x fSW

12

LM2840-Q1, LM2841-Q1, LM2842-Q1

SNVSBE5 – APRIL 2019

www.ti.com

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

Submit Documentation Feedback

Copyright © 2019, Texas Instruments Incorporated

•

Run electrical simulations to see important waveforms and circuit performance

•

Run thermal simulations to understand board thermal performance

•

Export customized schematic and layout into popular CAD formats

•

Print PDF reports for the design, and share the design with colleagues

Get more information about WEBENCH tools at www.ti.com/WEBENCH.

This section presents guidelines for selecting external components.

8.2.2.2 Setting the Output Voltage

The output voltage is set using the feedback pin and a resistor divider connected to the output as shown in

Typical Application Circuit. The feedback pin voltage 0.765 V, so the ratio of the feedback resistors sets the

output voltage according to Equation 3:

VOUT = 0.765 V (1 + (R1 / R2))

(3)

Typically R2 is given as 100 Ω to 10 kΩ for a starting value. To solve for R1 given R2 and VOUT, use Equation 4:

R1 = R2 ((VOUT / 0.765 V) – 1)

(4)

8.2.2.3 Inductor Selection

The most critical parameters for the inductor are the inductance, peak current, and the DC resistance. The

inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages.

(5)

A higher value of ripple current reduces inductance, but increases the conductance loss, core loss, and current

stress for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage

ripple requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Because

the ripple current increases with the input voltage, the maximum input voltage is always used to determine the

inductance. The DC resistance of the inductor is a key parameter for the efficiency. Lower DC resistance is

available with a bigger winding area. A good tradeoff between the efficiency and the core size is letting the

inductor copper loss equal 2% of the output power. See Selecting Inductors for Buck Converters for more

information on selecting inductors. A good starting point for most applications is a 10 µH to 22 µH with 1.1 A or

greater current rating for the LM2842-Q1 or a 0.7 A or greater current rating for the LM284x-Q1. Using such a

rating enables the device to current limit without saturating the inductor. This is preferable to the device going

into thermal shutdown mode and the possibility of damaging the inductor if the output is shorted to ground or

other long-term overload.

Table 2. Recommended Inductors

MANUFACTURER

INDUCTOR

CONTACT INFORMATION

Coilcraft

LPS4018, DO1608C, DO3308, and LPO2506 series

www.coilcraft.com

800-3222645

MuRata

LQH55D and LQH66S series

www.murata.com

Coiltronics

MP2 and MP2A series

www.cooperbussman.com

8.2.2.4 Input Capacitor

A low ESR ceramic capacitor (CIN) is needed between the VIN pin and GND pin. This capacitor prevents large

voltage transients from appearing at the input. Use a 2.2-µF to 10-µF value with X5R or X7R dielectric.

Depending on construction, a ceramic capacitor’s value can decrease up to 50% of its nominal value when rated

voltage is applied. Consult with the capacitor manufacturer's data sheet for information on capacitor derating over

voltage and temperature.