ISL68201 Datasheet, PDF (17/32 Pages) Intersil Corporation – Single-Phase R4 Digital Hybrid PWM Controller

ISL68201
L
ISL68201
INTERNAL CIRCUIT
I_OUT
CURRENT
SENSE
CSEN
+
-
I_{SEN = IL R-}^R_----I-^S_-S---^E_-E---^N_-N--
CSRTN
IL
R_SEN ESL
R_SENSE
V_R
V_OUT
C_OUT
PLACE THESE IN CLOSE
PROXIMITY TO ISL68201
R _VC(s)
OPTIONAL
C
R_ISEN
FIGURE 10. SENSE RESISTOR IN SERIES WITH INDUCTORS
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 17.
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.
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 in 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
on current sensing will not provide a fast OCP response and hurt
system reliability.
LOAD
VIOUT
FIGURE 13. DESIRED LOAD TRANSIENT RESPONSE WAVEFORMS
LOAD
V_IOUT
FIGURE 14. LOAD TRANSIENT RESPONSE WHEN R-C TIME
CONSTANT IS TOO SMALL
LOAD
FIGURE 11. VOLTAGE ACROSS R WITHOUT RC
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)
VIOUT
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 diagram of thermal monitoring function block is shown in
Figure 16 on page 18. One NTC resistor should be placed close to
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FN8696.1
March 7, 2016