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

NB681 Rev. 1.04

www.MonolithicPower.com

12

12/28/2017

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OPERATION

PWM Operation

The NB681 is a fully integrated, synchronous,

rectified, step-down, switch-mode converter,

especially designed for IMVP8 applications—

VCCIO, PRIMCORE, EDRAM, EOPIO, and

V1.0A. 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 mode 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,

the 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 NB681 reduces the

switching frequency naturally, achieving high

efficiency at light load.

Figure 3—DCM operation