NB679A

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

NB679A Rev. 1.03

www.MonolithicPower.com

13

7/27/2017

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© 2017 MPS. All Rights Reserved.

Figure 3: DCM Operation

As the output current increases from the light-

load condition, the time period within which the

current modulator regulates becomes shorter.

The HS-FET is turned on more frequently. Hence,

the switching frequency increases accordingly.

The output current reaches the critical level when

the current modulator time is zero. The critical

level of the output current is determined using

Equation (1):

IN

OUT

OUT

OUT

SW

IN

(V

V

) V

I

2 L F

V





 



(1)

The device enters PWM mode once the output

current exceeds the critical level. After that, the

switching frequency stays fairly constant over the

output current range.

DC Auto-Tune Loop

NB679A applies the DC auto-tune loop to

balance the DC error between VFB and VREF by

adjusting the comparator input REF to make VFB

always follow VREF. This loop is quite small, so it

improves the load and line regulation without

affecting

the

transient

performance.

The

relationship between VFB, VREF, and REF is

shown in Figure 4.

VFB

REF

VREF

DC

Error

Figure 4: DC Auto-Tune Loop Operation

Large Duty Operation

The NB679A supports larger duty operation

(5.6Vin to 5.1Vout) by its internal Ton extension

function. When FB is lower than VREF and

Vin - Vo < 2V, the Ton can be extended,

extending the duty cycle. If FB > REF or Ton

meets its limitation, Ton will stop extending

Light-Load Ultrasonic Mode (USM)

Ultrasonic mode (USM) is designed to keep the

switching frequency above an audible frequency

area during light-load or no-load conditions.

Once the part detects both the HS-FET and the

LS-FET are off (for about 32µs), it reduces the

Ton so as to keep Vout under regulation with

optimal efficiency. If the load continues to reduce,

the part discharges the Vout to make sure the FB

is smaller than 102% of the internal reference.

The HS-FET will turn on again once the internal

FB reaches VREF and then stops switching.

USM is selected by the EN voltage level. When

EN is in the range of 1.4V to 1.8V, it enters USM.

If EN is in the range of 2.6V to

3.6V, then it

operates in normal mode.

Configuring the EN Control

The NB679A has one EN pin to control the on/off

of the internal regulators and LDO. See Table 2

for the NB679A EN logics

Table 2: EN Control

State

EN

VCC

VOUT

5V LDO

S0

1

ON

ON

ON

S3/S5

0

OFF

OFF

OFF

For automatic start-up, EN can be pulled up to

the input voltage through a resistive voltage

divider.

See the “UVLO Protection” section for

additional details.

Configuring the LP# Control

The NB679A implements a voltage scaling

function on low-power mode by controlling LP#

(see Table 3).

Table 3: LP# Control

State

LP#

VOUT(V)

S0

1

5.1V

S3/S5

0

4.85V

Soft Start (SS)

The

NB679A

employs

a

soft-start

(SS)

mechanism to ensure smooth output during

power-up. When EN goes high, the internal

reference voltage ramps up gradually; hence, the

output voltage ramps up smoothly as well. Once

the reference voltage reaches the target value,

the soft start finishes, and the part

enters

steady-state operation.