NB650/NB650H – 6A, 28V, FAST-TRANSIENT, SYNCHRONOUS STEP-DOWN CONVERTERS
following equation determines the critical level of
the output current:
Jitter and FB Ramp Slope
Figure 5 and Figure 6 show jitter in both PWM
and skip modes. When there is noise in the VFB
downward slope, the ON time of HS-FET
deviates from its intended level and produces
jitter. There is a relationship between a system’s
stability and the steepness of the VFB ripple’s
downward slope: The steepness of the VFB
ripple’s slope dominates in noise immunity. The
magnitude of the VFB ripple doesn’t directly affect
the noise immunity.
(VIN − VOUT)× VOUT
2×L× fSW × VIN
IOUT
=
(2)
When the output current exceeds the critical level,
light load mode turns into PWM mode, and the
switching frequency stays fairly constant over the
output current range.
Switching Frequency
The NB650/NB650H uses constant-on-time
(COT) control, and has no dedicated internal
oscillator. The input voltage is feed-forwarded to
the on-time one-shot timer through the resistor
RFREQ. The duty ratio is kept as VOUT/VIN. Hence,
the switching frequency is fairly constant over the
input voltage range. The switching frequency can
be set as follows:
−1
⎡
⎢
⎢
⎢
⎢
⎤
⎥
⎥
⎥
⎥
9.6 × RFREQ (kΩ)
VIN (V) − 0.4
VIN (V)
⎛
⎞
⎜
⎜
⎟
Figure 5: Jitter in PWM Mode
+ tDELAY1 (ns) ×
⎟
⎠
(3)
⎝
fSW (kHz) =
×106
+ tDELAY2 (ns)
V
OUT (V)
⎢
⎣
⎥
⎦
Where tDELAY2 is another comparator delay of
about 40ns.
Figure 6: Jitter in Skip Mode
Ramp with Large ESR Cap
When using POSCAPs or other types of
capacitors with larger ESR as output capacitors.
the ESR ripple dominates the output ripple, and
the slope on the FB is ESR-related. Figure 7
shows an equivalent circuit in PWM mode with
the HS-FET off and without an external ramp
circuit. The application section includes design
steps for large ESR capacitors.
Figure 4: Plot of VOUT as a Function of RFREQ and
the Frequency
NB650/NB650H is optimized to operate at high
switching frequencies at high efficiency. Higher
switching frequencies allow for smaller LC filter
components to reduce system PCB space.
NB650/NB650H Rev. 1.12
www.MonolithicPower.com
10
10/11/2012
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