NB679A

–28V VIN, FIXED 5.1V-8A BUCK CONVERTER WITH LDO AND LP# VOUT SCALING

NB679A Rev. 1.03

www.MonolithicPower.com

15

7/27/2017

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2017 MPS. All Rights Reserved.

If an application requires a higher under-voltage

lockout (UVLO), use EN to adjust the input

voltage UVLO using two external resistors (see

Figure 5). Note that there is a

600kΩ internal pull

low resistor on the EN Pin. The Calculation of the

two resistors need to consider this resistor.

It is recommended to use the enable resistors to

set the Vin falling threshold above 5.5V. The

rising threshold should be set to provide enough

hysteresis to allow for any input supply variations.

Figure 5: Adjustable UVLO

To avoid too much sink current on EN when

Rdown is not applied, the EN resistor (Rup) is

usually in the range of 1M-2M

Ω. A typical pull-up

resistor is 2M

Ω.

Thermal Shutdown

Thermal shutdown is employed in the NB679A.

The junction temperature of the IC is monitored

internally. If the junction temperature exceeds the

threshold value (140ºC, typically), the converter

shuts off. This is a non-latch protection. There is

about

25ºC

hysteresis.

Once

the

junction

temperature drops to about 115ºC, it initiates a

SS.

Output Discharge

NB679A discharges the output when EN is low,

or the controller is turned off by the protection

functions UVP, OCP, OCP, OVP, UVLO, and

thermal shutdown. The part discharges outputs

using an internal 6

Ω MOSFET from Vout Pin, so

it is suggest that the Vout trace need to be over

20mil.