ISL78211 Datasheet, PDF (22/35 Pages) Intersil Corporation – Automotive Single-Phase Core Regulator for IMVP-6™ CPUs

ISL78211
response vary a little bit from board-to-board. Compared
with potential long-term damage on CPU reliability, an
immediate system failure is worse. Thus, it is desirable to
avoid the waveforms shown in Figure 11. It is
recommended to choose the minimum C_n value based on
the maximum inductance so only the scenarios of
Figures 10 and 12 may happen. It should be noted that,
after calculation, fine-tuning of C_n value may still be
needed to account for board parasitics. C_n also needs to
be a high-grade cap like X7R with low tolerance. Another
good option is the NPO/COG (Class-I) capacitor, featuring
only 5% tolerance and very good thermal characteristics.
But the NPO/COG caps are only available in small
capacitance values. In order to use such capacitors, the
resistors and thermistors surrounding the droop voltage
sensing and droop amplifier need to be scaled up 10x to
reduce the capacitance by 10x. Attention needs to be
paid in balancing the impedance of droop amplifier.
Dynamic Mode of Operation - Compensation
Parameters
The voltage regulator is equivalent to a voltage source
equal to VID in series with the output impedance. The
output impedance needs to be 2.1mΩ in order to achieve
the 2.1mV/A load line. It is highly recommended to
design the compensation such that the regulator output
impedance is 2.1mΩ. A type-III compensator is
recommended to achieve the best performance. Intersil
provides a spreadsheet to design the compensator
parameters. Figure 13 shows an example of the
spreadsheet. After the user inputs the parameters in the
blue font, the spreadsheet will calculate the
recommended compensator parameters (in the pink
font), and show the loop gain curves and the regulator
output impedance curve. The loop gain curves need to be
stable for regulator stability, and the impedance curve
needs to be equal to or smaller than 2.1mΩ in the entire
frequency range to achieve good transient response.
The user can choose the actual resistor and capacitor
values based on the recommendation and input them in
the spreadsheet, then see the actual loop gain curves
and the regulator output impedance curve.
Caution needs to be used in choosing the input resistor to
the FB pin. Excessively high resistance will cause an error
to the output voltage regulation due to the bias current
flowing in the FB pin. It is recommended to keep this
resistor below 3k.
Droop using Discrete Resistor Sensing -
Static/Dynamic Mode of Operation
Figure 3 shows a detailed schematic using discrete
resistor sensing of the inductor current. Figure 14 shows
the equivalent circuit. Since the current sensing resistor
voltage represents the actual inductor current
information, R_s and C_n simply provide noise filtering. The
most significant noise comes from the ESL of the current
sensing resistor. A low ESL sensing resistor is strongly
recommended. The recommended R_s is 100Ω and the
recommended C_n is 220pF. Since the current sensing
resistance does not appreciably change with
temperature, the NTC network is not needed for thermal
compensation.
Droop is designed the same way as the DCR sensing
approach. The voltage on the current sensing resistor is
given by the following Equation 31:
V_rsen = R_sen ⋅ I _o
(EQ. 31)
Equation 24 shows the droop amplifier gain. So the
actual droop is given by Equation 32:
R_droop
=
R_sen
⋅ _{⎜⎜⎝⎛}1 +
R_{drp 2}
R_drp1
⎟⎟⎠⎞
(EQ. 32)
Solving for R_drp2 yields:
R_{drp 2}
=
R_drp1 ⋅ _{⎜⎜⎝⎛}
R_droop
R_sen
−1_{⎟⎟⎠⎞}
(EQ. 33)
For example: R_droop = 2.1mΩ. If R_sen = 1m and
R_drp1 = 1k, easy calculation gives that R_drp2 is 1.1k.
The current sensing traces should be routed directly to
the current sensing resistor pads for accurate
measurement. However, due to layout imperfections, the
calculated R_drp2 may still need slight adjustment to
achieve optimum load line slope. It is recommended to
adjust R_drp2 after the system has achieved thermal
equilibrium at full load.
22
FN7578.0
March 8, 2010