cs5323gdw20 ON Semiconductor, cs5323gdw20 Datasheet - Page 11

• Manufacturer Part Number: cs5323gdw20
• Description: Three-phase Buck Controller With 5-bit Dac
• Manufacturer: ON Semiconductor

Ramp Size and Current Sensing
and to sense current, the inductor and sense resistor values
will be constrained. A small ramp will provide a quick
transient response by minimizing the difference over which
the COMP pin must travel between light and heavy loads,
but a steady state ramp of 25 mV
required to prevent pulse skipping and minimize pulse width
jitter. For resistive current sensing the combination of the
inductor and sense resistor values must be chosen to provide
a large enough steady state ramp. For large inductor values
the sense resistor value must also be increased.
the requirement of L/R
and DC components of the current the signal. Again the
values for L and R
a large enough steady state ramp with a compensated current
sense signal. A smaller L, or a larger R
be required. But unlike resistive sensing, with inductive
sensing small adjustments can be made easily with the
values of R and C to increase the ramp size if needed.
portion of the current sensing signal will be scaled larger
than the DC portion. This will provide a larger steady state
ramp, but circuit performance will be affected and must be
evaluated carefully. The current signal will overshoot during
transients and settle at the rate determined by R × C. It will
eventually settle to the correct DC level, but the error will
decay with the time constant of R × C. If this error is
excessive it will effect transient response, adaptive
positioning and current limit. During transients the COMP
pin will be required to overshoot along with the current
signal in order to maintain the output voltage. The V
will also overshoot during transients and possibly slow the
response. Single phase overcurrent will trip earlier than it
would if compensated correctly and hiccup mode current
limit will have a lower threshold for fast rise step loads than
for slowly rising output currents.
current sense signal and the actual inductor current during
a positive step in load current with values of L = 500 nH,
R
signal compensation the value of R1 should be 31 kΩ. Due
to the faster than ideal RC time constant there is an
overshoot of 50% and the overshoot decays with a 200 μs
time constant. With this compensation the I
threshold must be set more than 50% above the full load
current to avoid triggering hiccup mode during a large
output load step.
L
Because the current ramp is used for both the PWM ramp
For inductive current sensing the RC network must meet
If RC is chosen to be smaller (faster) than L/R
The waveforms in Figure 9 show a simulation of the
= 1.6 mΩ, R1 = 20 k and C1 = .01 μF. For ideal current
L
will be constrained in order to provide
L
= R × C to accurately sense the AC
P−P
L
or greater is typically
than optimum might
L
, the AC
LIM
DRP
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pin
pin
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Current Limit
time the voltage on a Current Sense pin exceeds CS
more than the Single Phase Pulse by Pulse Current Limit, the
pwm comparator for that phase is turned off. This provides
fast peak current protection for individual phases. The
outputs of all the currents are also summed and filtered to
compare an averaged current signal to the voltage on the
I
the SS capacitor is discharged by a 5 μA source until the
COMP pin reaches 0.2 V. Then soft−start begins. The
converter will continue to operate in this mode until the fault
condition is corrected.
Overvoltage Protection
the normal operation of the enhanced V
with synchronous rectifiers. The control loop responds to an
overvoltage condition within 400 ns, causing the top
MOSFET’s to shut off, and the synchronous MOSFET’s to
turn on. This results in a “crowbar” action to clamp the
output voltage and prevents damage to the load. The
regulator will remain in this state until the overvoltage
condition ceases or the input voltage is pulled low.
Transient Response and Adaptive Positioning
filter is frequently sized larger than ripple currents require in
LIM
Figure 9. Inductive Sensing waveform during a Step
Two levels of overcurrent protection are provided. Any
Overvoltage protection (OVP) is provided as a result of
For applications with fast transient currents the output
pin. If this voltage is exceeded, the fault latch trips and
with Fast RC Time Constant (50 ms/div)
2
control topology
REF
by