NB650/NB650H – 6A, 28V, FAST-TRANSIENT, SYNCHRONOUS STEP-DOWN CONVERTERS
OPERATION
PWM Operation
The NB650/NB650H is
a
fully-integrated,
synchronous, rectified, step-down, switch-mode
converter with dynamic output voltage control. It
offers a very compact solution to achieve a 6A
continuous output current over a wide input
supply range, with excellent load and line
regulation. The NB650/NB650H operates at high
efficiency over a wide output current load range.
Constant-on-time (COT) provides a fast transient
response and easy loop stabilization. At the
beginning of each cycle, the high-side MOSFET
(HS-FET) turns on when the feedback voltage
(VFB) falls below the reference voltage (VREF),
which indicates an insufficient output voltage.
The input voltage and the frequency-set resistor
determine the ON as follows:
Figure 2: Heavy-Load Operation
Light-Load Operation
When the load current decreases, the
NB650/NB650H automatically reduces the
switching frequency to maintain high efficiency.
Figure 3 shows the light-load operation. VFB does
not reach VREF when the inductor current
approaches zero. As the output current drops
from heavy-load condition, the inductor current
also decreases and eventually approaches zero.
The LS-FET driver enters a tri-state (high-Z)
whenever the inductor current reaches zero. A
current modulator takes control of the LS-FET
and limits the inductor current to less than 600μA
to slowly discharge the output capacitors to GND
through LS-FET as well as R1 and R2A, R2B
and R2C. The HS-FET does not turn ON as
frequently as in heavy-load condition. As a result,
the efficiency at light-load condition increases
greatly. This operation mode is also called skip
mode.
9.6×RFREQ (kΩ)
tON(ns) =
+ tDELAY1(ns)
(1)
V (V) − 0.4
IN
Where tDELAY1 is the 20ns delay of a comparator
in the tON module.
For best results, select tON ≥120ns.
After the ON period elapses, the HS-FET turns
off to enter the OFF state. The part turns ON
again when VFB drops below VREF. By repeating
this operation, the converter regulates the output
voltage. The integrated low-side MOSFET (LS-
FET) turns on when the HS-FET is OFF to
minimize conduction loss. There is a dead short
between input and GND (shoot-through) if both
HS-FET and LS-FET turn on at the same time.
An internally-generated dead-time (DT) between
HS-FET OFF and LS-FET ON, or LS-FET OFF
and HS-FET OFF avoids shoot-through.
Heavy-Load Operation
As shown in Figure 2, the HS-FET and LS-FET
repeatedly turn on/off when the output current is
high, and the inductor current never goes to zero.
It’s called continuous-conduction-mode (CCM)
operation. In CCM operation, the switching
frequency (fSW) is fairly constant.
Figure 3: Light-Load Operation
As the output current increases from the light-
load condition, the time period within which the
current modulator regulates becomes shorter.
As the part exits light-load mode, the HS-FET
turns on more frequently to increase the switching
frequency. The output current reaches critical
when the current modulator time is zero. The
NB650/NB650H Rev. 1.12
www.MonolithicPower.com
9
10/11/2012
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