ISL68200 Datasheet, PDF (19/32 Pages) Intersil Corporation – Single-Phase R4 Digital Hybrid PWM Controller with Integrated Driver

ISL68200
Thermal compensation design procedure for inductor current
sensing is summarized as follows:
1. Properly choose the voltage divider for the NTC pin to match
the NTC voltage vs temperature curve with the recommended
curve in Figure 18 on page 18.
2. Run the actual board under the full load and the desired
airflow condition.
3. After the board reaches the thermal steady state (often takes
15 minutes), record the temperature (T_CSC) of the current
sense component (inductor) and the voltage at NTC and VCC
pins.
4. Use Equation 9 to calculate the resistance of the NTC, and
find out the corresponding NTC temperature T_NTC from the
NTC datasheet or using Equation 10, where is equal to 3380
for recommended NTC.
R_NTC@T_NTC = _V-V----C-^T---_CM-----x_–----R-V---^T-_T---^M-_M--
(EQ. 9)
T_NTC
=
---------------------------------------^----------------------------------------
ln



R^-R----_N^-N---_T^-T---_C^-C---^---@@^------T2^----5_N^---0-_T^---C-_C^----
+
2^----9^---8^----.^-1^----5^-
–
273.15
(EQ. 10)
5. Choose a number close to the result as in Equation 11 for the
“TCOMP” register.
T_COMP = T_CSC – T_NTC
(EQ. 11)
6. Run the actual board under full load again.
7. Record the IOUT pin voltage as V1 immediately after the
output voltage is stable with the full load. Record the IOUT pin
voltage as V2 after the VR reaches the thermal steady state.
8. If the IOUT pin voltage increases over 10mV as the
temperature increases, i.e., V2 - V1 > 10mV, reduce “TCOMP”
value. If the IOUT pin voltage decreases over 10mV as the
temperature increases, i.e., V1 - V2 > 10mV, increase
“TCOMP” value. “TCOMP” value can be adjusted via the series
bus for easy thermal compensation optimization.
I_OUT Calibration
The current flowing out of the IOUT pin is equal to the sensed
average current inside ISL68200. A resistor is placed from the
IOUT pin to GND to generate a voltage, which is proportional to
the load current and the resistor value, as shown in Equation 12:
R_IOUT = ²-₆---^.3--⁵--_.-_8V----₇-^x--₅-^R---_A--^I--^S_x----^E_R---^N-_x-- = -²---^.--⁵-₆--^V--₃---^x.--₈-^--₇--^R----₅---^1--x---_A-^0--x-----0-_x^-I----^O-^-_R-------^CA--_x^{------P-----}--
= -₆---₃---_.-²-₈--^.-₇-⁵---₅-^V--_A--^x---_xI--^O-₁----₀^C---₀-^P---_----_A-- = ²----₆₅---₃-^V--_.--_8x---₇-^I-^O--₅---^C_A----^P-- k
(EQ. 12)
63.875A load current. The IOUT voltage is linearly digitized every
108µs and stored in the READ_IOUT register (8Ch).
ISL68200
DIGITIZED
IOUT (8Ch)
VCC
EXTERNAL CIRCUIT
IOUT
R_{IOUT_UP}
R_{IOUT_DW}
FIGURE 20. IOUT NO LOAD OFFSET CALIBRATION
A small capacitor can be placed between IOUT and GND to
reduce the noise impact and provide averaging, > 200µs
(typically).
To deal with layout and design variation of different platforms,
ISL68200 is intentionally trimmed to negative at no load, thus,
an offset can easily be added to calibrate the digitized IOUT
reading (8Ch). Hence, the analog vs digitized current slope is set
by the equivalent impedance of R_{IOUT_UP}//R_{IOUT_DW} =R_IOUT
(as in Figure 20); the slope of the ideal curve should set to 1 A/A
with 0A offset.
For a precision digital I_OUT, follow the fine-tune procedure below
step-by-step; steps 1 to 5 must be completed before step 6.
1. Properly tune L/DCR or ESL/R_SEN matching as shown on
page 17 over the range of temperature operation. +25% over-
matching L/DCR at room temperature is needed for -40°C
operation.
2. Properly complete thermal compensation as shown on
“Thermal Monitoring and Compensation” on page 17.
3. Finalize R_ISEN resistor to set OCP for overall operating
conditions and board variations as shown in “Overcurrent and
Short-Circuit Protection” on page 20.
4. Collect no load I_OUT current with sufficient prototypes and
determine the mean of no load I_OUT current.
5. The pull-up impedance on IOUT pin should be
“VCC/IOUT_NO_LOAD”; for instance, a mean of -2.5µA I_OUT at
0A load, it will need R_{IOUT_UP} = 2MΩ.
6. Start with the value below and then fine tune the R_{IOUT_DW}
value until the average slope of various boards equals 1A/A.
R_{IOUT_DW} = ^R-_R----^II--^OO----^U-_U----^TT---^__---^UU----^PP----^x_–---RR-----_II--_OO----_UU----_T-^T-
(EQ. 13)
Where V_IOUT is the voltage at the IOUT pin, R_IOUT is the resistor
between the IOUT pin and GND, I_LOAD is the total output current
of the converter, R_ISEN is the sense resistor connected to the
CSRTN pin and R_X is the DC resistance of the current sense
element, either the DCR of the inductor or R_SENSE depending on
the sensing method. The R_IOUT resistor should be scaled to
ensure that the voltage at the IOUT pin is typically 2.5V at
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FN8705.1
March 7, 2016