ISL6398 Datasheet, PDF (19/57 Pages) Intersil Corporation – Programmable soft-start rate and DVID rate

ISL6398
If the R-C network components are selected such that the RC time
constant matches the ESL-R_SEN time constant (R*C = ESL/R_SEN),
the voltage across the capacitor V_C is equal to the voltage drop
across the R_SEN, i.e., proportional to the channel current. As an
example, a typical 1mΩ sense resistor can use R = 348 and
C = 820pF for the matching. Figures 6 and 7 show the sensed
waveforms with and without matching RC when using resistive
sense.
FIGURE 6. VOLTAGE ACROSS R WITHOUT RC
IOUT
VOUT
FIGURE 8. DESIRED LOAD TRANSIENT RESPONSE WAVEFORMS
If the R-C timing constant is too large or too small, V_C(s) will not
accurately represent real-time I_OUT(s) and will worsen the
transient response. Figure 9 shows the load transient response
when the R-C timing constant is too small. V_OUT will sag
excessively upon load insertion and may create a system failure
or early overcurrent trip. Figure 10 shows the transient response
when the R-C timing constant is too large. V_OUT is sluggish in
drooping to its final value. There will be excessive overshoot if
load insertion occurs during this time, which may potentially hurt
the reliability.
FIGURE 7. VOLTAGE ACROSS C WITH MATCHING RC
Equation 9 shows that the ratio of the channel current to the
sensed current, I_SEN, is driven by the value of the sense resistor
and the R_ISEN.
I_SEN = I_L  _R-^R----_I-^S-_S---^E-_E---^N-_N-- = I_L ^R-----^S-_R---^E--_S--^N--_E---^-_T--⁶----⁴-
(EQ. 9)
However, the R_ISEN resistor of each channel is integrated, while its
value is determined by the R_SET resistor. The RSET resistor value can
be from 3.84kΩ to 60.4kΩ and is 64x of the required I_SEN resistor
value. Therefore, the current sense gain resistor (Integrated R_ISEN)
value can be effectively set at 60Ωto 943Ω.
The inductor DCR value will increase as the temperature increases.
Therefore, the sensed current will increase as the temperature of
the current sense element increases. In order to compensate the
temperature effect on the sensed current signal, a Negative
Temperature Coefficient (NTC) resistor can be used for thermal
compensation, or the integrated temperature compensation
function of the ISL6398 should be utilized. The integrated
temperature compensation function is described in “Temperature
Compensation” on page 28.
Decoupling capacitor (C_T) on ISEN[6:1]- pins are optional and
might be required for long sense traces and a poor layout.
L/DCR OR ESL/R_SEN MATCHING
Assuming the compensator design is correct, Figure 8 shows the
expected load transient response waveforms if L/DCR or
ESL/R_SEN is matching the R-C time constant. When the load
current I_OUT has a square change, the output voltage V_OUT also
has a square response, except for the overshoot at load release.
However, there is always some PCB contact impedance of current
sensing components between the two current sensing points; it
hardly accounts into the L/DCR or ESL/R_SEN matching calculation.
Fine tuning the matching is necessarily done in the board level to
improve overall transient performance and system reliability.
I_OUT
V_OUT
FIGURE 9. LOAD TRANSIENT RESPONSE WHEN R-C TIME
CONSTANT IS TOO SMALL
I_OUT
V_OUT
FIGURE 10. LOAD TRANSIENT RESPONSE WHEN R-C TIME
CONSTANT IS TOO LARGE
R_SET AND L/DCR MATCHING FOR COUPLED INDUCTOR
The current sense circuitry operates in a very similar manner for
negative current feedback, where inductor current is flowing
from the output of the regulator to the PHASE node, opposite of
flow pictured in Figures 4 and 5. However, the range of proper
operation with negative current sensing has a limitation. The
worst-case peak-to-peak inductor ripple current should be kept
less than 80% of the OCP trip point (~80µA). Care should be
taken to avoid operation with negative current feedback
exceeding this threshold, as this may lead to momentary loss of
current balance between phases and disruption of normal circuit
operation. Note that the negative current can especially affect
coupled inductor designs, where the effective inductance is the
leakage between the two channels, much lower than the
specified mutual inductance (LM) and self inductance (L). To limit
the impact, a higher RSET value (1.5x to 2x) is often used to
reduce the effective negative current seen by the controller in
coupled inductor designs.
Refer to Intersil’s application note, AN1268 for detailed coupled
inductor discussion and ripple current calculation.
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FN8575.1
August 13, 2015