NB621 – 8A, 25V, 600kHz SYNCHRONOUS STEP-DOWN CONVERTER WITH INTEGRATED LDO

NB621 Rev. 0.9

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10

12/10/2013

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BLOCK DIAGRAM

Figure 1—Function Block Diagram

OPERATION

The NB621 is a high frequency synchronous

rectified step-down switch mode converter with a

built in internal high side power MOSFET. It

offers a very compact solution to achieve 8A

continuous output current over a wide input

supply range with excellent load and line

regulation.

The NB621 operates in a fixed frequency, peak

current control mode to regulate the output

voltage. A PWM cycle is initiated by the internal

clock. The integrated high-side power MOSFET

is turned on and remains on until its current

reaches the value set by the COMP voltage.

When the power switch is off, it remains off until

the next clock cycle starts. If, in 90% of one PWM

period, the current in the power MOSFET does

not reach the COMP set current value, the power

MOSFET will be forced to turn off.

Error Amplifier

The error amplifier compares the FB pin voltage

with the internal 0.8V reference (REF) and

outputs a current proportional to the difference

between the two. This output current is then used

to charge or discharge the internal compensation

network to form the COMP voltage, which is used

to control the power MOSFET current. The

optimized

internal

compensation

network

minimizes the external component counts and

simplifies the control loop design.

Internal Regulator

Most of the internal circuitries are powered from

the 5V internal regulator. This regulator takes the

VIN input and operates in the full VIN range.

When VIN is greater than 5.0V, the output of the