NB679, 28 V VIN, FIXED 5V-8A BUCK CONVERTER WITH LDO

NB679 Rev. 1.03

www.MonolithicPower.com

12

7/27/2017

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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 with

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

NB679 applies a 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. The small DC loop improves the load

and line regulation without affecting 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 NB679 supports larger duty operation (5.5 V-

5 V) by its internal Ton extension function. When

the parts detects its FB is lower than Vref and

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

extending the duty. If FB > REF or Ton reaches

its limitation, Ton will stop extending.

Light load Ultrasonic Mode

Ultrasonic mode (USM) keeps 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 30 µs), it shrinks 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 FB is

smaller than 102 percent

of the internal

reference. The HS-FET turns 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 1.4 V to 1.8 V, it enters

USM. If EN is in the range of 2.6 V to 3.6 V, it

operates in normal mode.

Configuring the EN Control

The NB679 has two enable pins to control the

on/off of the internal regulators and LDO.

For NB679, the buck regulator and LDO are

controlled by EN and ENLDO, respectively. For

example, when the application is only used with

the buck regulator, the buck regulator is realized

by pulling ENLDO low, allowing the buck to be

controlled by EN.

See Table 2 for the NB679 EN logics.

Table 2

—ENLDO/EN control

State

ENLDO

EN

VCC

VOUT

5V LDO

S0

1

1

ON

ON

ON

S3

1

0

ON

OFF

ON

S5

0

0

OFF

OFF

OFF

Others

0

1

ON

ON

OFF

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

the input voltage through a resistive voltage

divider. Refer to

the “UVLO Protection” section

for more details.

Soft Start (SS)

The NB679 employs a soft-start (SS) mechanism

to ensure smooth output during power-up. When

EN becomes high, the internal reference voltage

ramps up gradually; this causes the output

voltage to ramp up smoothly as well. Once the

reference voltage reaches the target value, the

soft start finishes, and the part enters steady-

state operation.

If the output is pre-biased to a certain voltage

during start-up, the IC disables the switching of

both the high-side and low-side switches until the

voltage on the internal reference exceeds the

sensed output voltage at the internal FB node.