CS5302
drive traces should be kept as short and wide as practical and
For ideal current sense compensation the ratio of L and
R is fixed, so the values of L and R will be a
L
should have a return path directly below the gate trace.
Output filter components should be placed on wide planes
connected directly to the load to minimize resistive drops
during heavy loads and inductive drops and ringing during
transients. If required, the planes for the output voltage and
return can be interleaved to minimize inductance between
the filter and load.
L
compromise typically with the maximum value R
L
limited by conduction losses or inductor temperature
rise and the minimum value of L limited by ripple
current.
3. For resistive current sensing choose L and R to
S
provide a steady state ramp greater than 25 mV.
Voltage feedback should be taken from a point of the
output or the output filter that doesn’t favor any one phase.
If the feedback connection is closer to one inductor than the
others the ripple associated with that phase may appear
larger than the ripple associated with the other phases and
poor current sharing can result.
The current sense signal is typically tens of milli–volts.
Noise pick–up should be avoided wherever possible.
Current feedback traces should be routed away from noisy
areas such as switch nodes and gate drive signals. The paths
should be matched as well as possible. It is especially
important that all current sense signals be picked off at
similar points for accurate current sharing. If the current
signal is taken from a place other than directly at the inductor
any additional resistance between the pick–off point and the
inductor appears as part of the inherent inductor resistance
and should be considered in design calculations. Capacitors
for the current feedback networks should be placed as close
to the current sense pins as practical.
LńR + (V * V
IN
) T
OUT
ń25 mV
ON
S
Again the ratio of L and R is fixed and the values of
L
L and R will be a compromise.
S
4. Calculate the high frequency output impedance
(ConverterZ) of the converter during transients. This
is the impedance of the Output filter ESR in parallel
with the power stage output impedance (PwrstgZ)
and will indicate how far from the original level
(∆VR) the output voltage will typically recover to
within one switching cycle. For a good transient
response ∆VR should be less than the peak output
voltage overshoot or undershoot.
DVR + ConverterZ ESR
PwrstgZ ESR
ConverterZ +
PwrstgZ ) ESR
where:
PwrstgZ + R CSA Gainń3.0
S
Multiply the converterZ by the output current step
size to calculate where the output voltage should
recover to within the first switching cycle after a
transient. If the ConverterZ is higher than the value
required to recover to where the adaptive positioning
is set the remainder of the recovery will be controlled
by the error amp compensation and will typically
recover in 10–20 µs.
DESIGN PROCEDURE
Current Sensing, Power Stage and
Output Filter Components
1. Choose the output filter components to meet peak
transient requirements. The formula below can be
used to provide an approximate starting point for
capacitor choice, but will be inadequate to calculate
actual values.
DVR + DI
OUT
ConverterZ
DV
+ (DIńDT) ESL ) DI ESR
PEAK
Make sure that ∆VR is less than the expected peak
Ideally the output filter should be simulated with
models including ESR, ESL, circuit board parasitics
and delays due to switching frequency and converter
response. Typically both bulk capacitance
(electrolytic, Oscon, etc.,) and low impedance
capacitance (ceramic chip) will be required. The bulk
capacitance provides “hold up” during the converter
response. The low impedance capacitance reduces
steady state ripple and bypasses the bulk capacitance
during slewing of output current.
transient for a good transient response.
5. Adjust L and R or R as required to meet the best
L
S
combination of transient response, steady state output
voltage ripple and pulse width jitter.
Current Limit
When the sum of the Current Sense amplifiers (V
)
ITOTAL
exceeds the voltage on the I
pin the part will enter hiccup
LIM
mode. For inductive sensing the I
pin voltage should be
LIM
set based on the inductor resistance (or current sense
resistor) at max temperature and max current. To set the level
2. For inductive current sensing (only) choose the
current sense network RC to provide a 25 mV
minimum ramp during steady state operation.
of the I
pin:
LIM
6. V
+ R I
CS to I Gain
LIM
I(LIM)
OUT(LIM)
where:
V
ńV
OUT IN
R is R or R
R + (V * V
IN
)
OUT
L
S;
F C 25 mV
I
is the current limit threshold.
OUT(LIM)
For the overcurrent to work properly the inductor
time constant (L/R) should be≤ the Current sense RC.
Then choose the inductor value and inherent
resistance to satisfy L/R = R × C.
L
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