ISL25700 Datasheet, PDF (16/18 Pages) Intersil Corporation – Programmable Temperature Controlled MOSFET Driver

ISL25700
measures the phase change of the output node
(thermistor). As a rule of thumb, phase margin should be
at least 45° when the system gain reaches 0dB.
It is not recommended to change the system loop gain
when it is determined.
Calibration Procedure
Power-up the circuit and program Reg.01h[2:0] = 001b
of ISL25700. Use an externally calibrated temperature
sensor to measure the temperature of the thermistor.
When the thermistor temperature is stabilized, it can be
adjusted to the desired point by changing the Fine
Temperature Control settings in Reg.02h[7:0] until the
external thermometer indicates +91.5°C (for this
example).
The decimal FTC code can be calculated by solving for
Equations 1 and 2 as follows:
Code = ^K-----¹-----^×-----^R----^I--^N----^T----⁽--^T----⁾----–----^K-----²-----^×-----^R----^T----^H----⁽--^T----⁾- × 255
K3 × R_TH(T)
(EQ. 3)
Note, the final application system temperature depends
on the allowed power consumption and the level of
insulation from the ambient temperature.
Two different calibration point settings can be stored in
General Purpose non-volatile registers. More calibrated
temperature points can be stored and used as a look-up
table in an external memory.
Placement and Layout Consideration
It is recommended to place the ISL25700 and thermistor
as close as possible to each other, inside the insulated
area to achieve higher accuracy and keep the ground
trace between the thermistor and the device as short as
possible. The device ground pin and the system ground
should be connected at a single point, known as the
“single point ground”, preventing picking up the common
ground noise into the R_TH feedback pin.
The ISL25700 also can be placed outside of the insulated
area and far away from the thermistor, but the setpoint
accuracy will decrease, due to the wide ambient
temperature range. It is recommended to use a look-up
table based on Equation 2 to compensate for the setpoint
shift.
Put a 1μF capacitor in parallel with 0.1μF decoupling
capacitor close to the VDD pin.
It is recommended to have a pull-up resistor of 500k or
higher, or a high-pass RC filter, as shown in the “Block
Diagram/Application Circuit” on page 2, to the gate of a
P-MOSFET in order to prevent a high current spike
through the MOSFET at power-up. The time constant of
the high-pass filter should be in the range of 1μs to 20μs.
16
FN6885.0
September 3, 2010