ISL68200 Datasheet, PDF (17/32 Pages) Intersil Corporation – Single-Phase R4 Digital Hybrid PWM Controller with Integrated Driver

ISL68200
A current sensing resistor has a distributed parasitic inductance,
known as ESL (equivalent series inductance, typically less than
1nH) parameter. A simple R-C network across the current sense
resistor extracts the R_SEN voltage, as shown in Figure 10 on
page 16.
The voltage on the capacitor V_C, can be shown to be proportional
to the inductor current I_L, see Equation 7.
V_Cs = -^--^s-----^---^R----E-----S----^S----^E-----^L_^---N--_s------+_---_R--¹---_C^----^-+--^---^R1----^S_----^E----^N-----^---^I--^L---^-
(EQ. 7)
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
inductor current. As an example, a typical 1mΩ sense resistor
can use R = 348 and C = 820pF for the matching. Figures 11 and
12 show the sensed waveforms without and with matching RC
when using resistive sense.
on current sensing will not provide a fast OCP response and hurt
system reliability.
LOAD
V_IOUT
FIGURE 13. DESIRED LOAD TRANSIENT RESPONSE WAVEFORMS
LOAD
V_IOUT
FIGURE 14. LOAD TRANSIENT RESPONSE WHEN R-C TIME
CONSTANT IS TOO SMALL
FIGURE 11. VOLTAGE ACROSS R WITHOUT RC
LOAD
FIGURE 12. VOLTAGE ACROSS C WITH MATCHING RC
Equation 8 shows that the ratio of the inductor 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--
(EQ. 8)
L/DCR OR ESL/R_SEN MATCHING
Figure 13 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 has a square change, the IOUT pin voltage (V_IOUT)
without a decoupling capacitor also has a square response.
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 at the board level to
improve overall transient performance and system reliability.
If the R-C timing constant is too large or too small, V_C(s) will not
accurately represent real-time output current and will worsen the
overcurrent fault response. Figure 14 shows the IOUT pin
transient voltage response when the R-C timing constant is too
small. V_IOUT will sag excessively upon load insertion and may
create a system failure or early overcurrent trip. Figure 15 shows
the transient response when the R-C timing constant is too large.
V_IOUT is sluggish in reaching its final value. The excessive delay
V_IOUT
FIGURE 15. LOAD TRANSIENT RESPONSE WHEN R-C TIME
CONSTANT IS TOO LARGE
Note that the integrated thermal compensation applies to the DC
current, but not the AC current; therefore, the peak current seen
by the controller will increase as the temperature decreases and
can potentially trigger an OCP event. To overcome this issue, the
RC should be over-matching L/DCR at room temperature by
(-40°C +25°C) * 0.385%/°C = +25% for -40°C operation.
Thermal Monitoring and Compensation
The block diagram of thermal monitoring function is shown in
Figure 16 on page 18. One NTC resistor should be placed close to
the respective power stage of the voltage regulator VR to sense
the operational temperature and pull-up resistors are needed to
form the voltage dividers for the NTC pin. As the temperature of
the power stage increases, the resistance of the NTC will reduce,
resulting in the reduced voltage at the NTC pin. Figure 18 on
page 18 shows the TM voltage over the temperature for a typical
design with a recommended 10kΩ NTC (P/N:
NCP15XH103J03RC from Murata,  = 3380) and 1.54kΩ resistor
R_TM. It is recommended to use those resistors for the accurate
temperature compensation since the internal thermal digital
code is developed based upon these two components. If a
different value is used, the temperature coefficient must be close
to 3380 and R_TM must be scaled accordingly. For instance, say
NTC = 20kΩ ( = 3380), then R_TM should be
20kΩ/10kΩ*1.54kΩ = 3.08kΩ.
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FN8705.1
March 7, 2016