NB681- 28V, 6 A, HIGH-CURRENT SYNCHRONOUS BUCK CONVERTER WITH 2-BIT VID

NB681 Rev. 1.04

www.MonolithicPower.com

16

12/28/2017

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To protect the part from damage, there is an

absolute 3.6 V OVP on VOUT. Once Vout

reaches this value, it latches off. The LS-FET

behaves the same as at 130 percent OVP. This

OVP is active even in LP# mode.

When the feedback voltage becomes lower than

75 percent of Vref, the UVP-1 comparator output

goes high, and the part latches if the FB voltage

stays in this range for about 64 µs (latching the

HS-FET off and LS-FET on). The LS-FET

remains on until the inductor current hits zero.

During this period, the valley current limit helps

control the inductor current.

When the feedback voltage drops below 50

percent of the Vref, the UVP-2 comparator output

goes high, and the part latches off directly after

the comparator and logic delay (latching the HS-

FET off and the LS-FET on). The LS-FET

remains on until the inductor current hits zero.

Fault latching can be reset by EN going low or

the power cycling of VIN or VCC.

UVLO Protection

The NB681 has two kinds of under-voltage lock-

out protection: a 3 V VCC UVLO and a 4.2 V Vin

UVLO. The part starts up only when both the

VCC and Vin exceed their own UVLO. The part

shuts down when either the VCC voltage is lower

than the UVLO falling threshold voltage (2.8 V,

typically), or the VIN is lower than the 3.9 V Vin

falling threshold. Both UVLO protections are non-

latch off.

Thermal Shutdown

Thermal shutdown is employed in the NB681.

The junction temperature of the IC is monitored

internally. If the junction temperature exceeds the

threshold value (145ºC, typically), the converter

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

about 25ºC hysteresis. Once the junction

temperature drops to about 120ºC, it initiates a

SS.

Output Discharge

NB681 discharges the output when EN is low, or

the controller is turned off by the protection

functions UVP, OVP, UVLO, and thermal

shutdown. The part discharges outputs using an

internal MOSFET.

Remote Sense

For NB681 (where the remote sense is required),

AGND acts as Remote Sen-, which is connected

to the remote ground of the output capacitors.

VOUT acts as Remote Sen+. The VCC capacitor

connected between 3V3 and AGND is still

required and should be placed very close to the

IC. For additional details please see Figure 9 for

the SCH with remote sense connection or refer to

AN086 for remote sense details.