NB669, 24V, HIGH CURRENT SYNCHRONOUS BUCK CONVERTER WITH LDO

NB669 Rev. 1.01

www.MonolithicPower.com

10

7/23/2013

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2013 MPS. All Rights Reserved.

OPERATION

PWM Operation

The NB669 is fully integrated synchronous

rectified step-down switch mode converter.

Constant-on-time (COT) control is employed to

provide fast transient response and easy loop

stabilization. At the beginning of each cycle, the

high-side MOSFET (HS-FET) is turned ON when

voltage. The ON period is determined by the

output voltage and input voltage to make the

switching frequency fairy constant over input

voltage range.

After the ON period elapses, the HS-FET is

turned off, or becomes OFF state. It is turned ON

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

will be a dead short between input and GND if

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

same time. It’s called shoot-through. In order to

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

internally generated between HS-FET off and LS-

FET on, or LS-FET off and HS-FET on.

An internal compensation is applied for COT

control to make a more stable operation even

when ceramic capacitors are used as output

capacitors, this internal compensation will then

improve the jitter performance without affect the

line or load regulation.

Heavy-Load Operation

Figure 2—Heavy Load Operation

When the output current is high and the inductor

current is always above zero amps, it is called

continuous-conduction-mode (CCM). The CCM

mode operation is shown in Figure 2 shown.

a fixed interval. When the HS-FET is turned off,

the LS-FET is turned on until next period.

In CCM mode operation, the switching frequency

is fairly constant and it is called PWM mode.

Light-Load Operation

With the load decrease, the inductor current

decrease too. Once the inductor current touch

zero, the operation is transition from continuous-

conduction-mode

(CCM)

to

discontinuous-

conduction-mode (DCM).

The light load operation is shown in Figure 3.

a fixed interval which is determined by one- shot

on-timer as equation 1 shown. When the HS-FET

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

inductor current reaches zero. In DCM operation,

current is approaching zero. The LS-FET driver

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

current reaches zero. A current modulator takes

over the control of LS-FET and limits the inductor

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

capacitors discharge slowly to GND through LS-

FET. As a result, the efficiency at light load

condition is greatly improved. At light load

condition, the HS-FET is not turned ON as

frequently as at heavy load condition. This is

called skip mode.

At light load or no load condition, the output

drops very slowly and the NB669 reduces the

switching frequency naturally and then high

efficiency is achieved at light load.