NB679

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

NB679 Rev. 1.03

www.MonolithicPower.com

11

7/27/2017

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OPERATION

PWM Operation

The NB679 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter with

a fixed 5 V output. Constant-on-time (COT)

control provides fast transient response and

eases loop stabilization. At the beginning of each

cycle, the high-side MOSFET (HS-FET) is turned

on when the feedback voltage (VFB) is below the

reference

voltage

(VREF),

which

indicates

insufficient output voltage. The on period is

determined by the output voltage and the input

voltage to make the switching frequency fairly

constant over the input voltage range.

After the on period elapses, the HS-FET is turned

off or enters an off state. It is turned on again

when VFB drops below VREF. By repeating

operation this way, the converter regulates the

output voltage. The integrated low-side MOSFET

(LS-FET) is turned on when the HS-FET is in its

off state to minimize the conduction loss. There is

a dead short between the input and GND if both

the HS-FET and the LS-FET are turned on at the

same time (shoot-through). In order to avoid

shoot-through, a dead time (DT) is generated

internally between the HS-FET off and the LS-

FET on period or the LS-FET off and the HS-FET

on period.

Internal compensation is applied for COT control

for

stable

operation

even

when

ceramic

capacitors are used as output capacitors. This

internal

compensation

improves

the

jitter

performance without affecting the line or load

regulation.

CCM Operation

Figure 2

—CCM operation

Continuous conduction mode (CCM) occurs

when the output current is high, and the inductor

current is always above zero amps (see Figure 2).

When VFB is below VREF, the HS-FET is turned on

for a fixed interval. When the HS-FET is turned

off, the LS-FET is turned on until the next period.

In CCM operation, the switching frequency is

fairly constant (PWM mode).

DCM Operation

When the load decreases, the inductor current

will decrease as well. Once the inductor current

reaches zero, the part transitions from CCM to

discontinuous conduction mode (DCM).

DCM operation is shown in Figure 3. When VFB is

below VREF, the HS-FET is turned on for a fixed

interval, which is determined by the one-shot on

timer. See Equation (1). When the HS-FET is

turned off, the LS-FET is turned on until the

inductor current reaches zero. In DCM operation,

VFB does not reach VREF when the inductor

current approaches zero. The LS-FET driver

turns into tri-state (high Z) when the inductor

current reaches zero. A current modulator takes

over the control of the LS-FET and limits the

inductor current to less than -1 mA. Hence, the

output capacitors discharge slowly to GND

through the LS-FET. As a result, the efficiency at

light-load is improved greatly. The HS-FET is not

turned on as frequently during a light-load

condition as it is during a heavy-load condition

(skip mode).

At a light-load or no-load condition, the output

drops very slowly, and the NB679 reduces the

switching frequency naturally, achieving high

efficiency at light load.

Figure 3

—DCM operation