ISL25700 Datasheet, PDF (15/18 Pages) Intersil Corporation – Programmable Temperature Controlled MOSFET Driver

ISL25700
Write Operation
A Write operation requires a START condition, followed by
a valid Identification Byte, a valid Address Byte, a Data
Byte, and a STOP condition. After each of the three
bytes, the ISL25700 responds with an ACK. At this time,
if the Data Byte is to be written only to volatile registers,
then the device enters its standby state. If the Data Byte
is to be written also to non-volatile memory, the
ISL25700 begins its internal write cycle to non-volatile
memory. During the internal non-volatile write cycle, the
device ignores transitions at the SDA and SCL pins, and
the SDA output is at a high impedance state. When the
internal non-volatile write cycle is completed, the
ISL25700 enters its standby state (see Figure 14).
The byte at address 08h determines if the Data Byte is to
be written to volatile and/or non-volatile memory (see
“Memory Map” on page 10). A 20ms delay is required
between two consecutive writes to the non-volatile
registers.
Polling Method
It is possible to check if the non-volatile memory write
cycle is finished or is in progress by polling (reading) the
BUSY bit in Control/Status Register, Reg.08h[0] (see
“Control/Status Register (Reg.08h [7:0])” on page 13).
The non-volatile write cycle is in progress when the BUSY
bit is 1. No other write attempt is allowed when the BUSY
bit is 1. The non-volatile write cycle is finished when the
BUSY bit is 0.
Read Operation
A Read operation consists of a three byte instruction
followed by one or more Data Bytes (see Figure 16). The
master initiates the operation issuing the following
sequence: a START, the Identification byte with the R/W
bit set to “0”, an Address Byte, a second START, and a
second Identification byte with the R/W bit set to “1”.
After each of the three bytes, the ISL25700 responds
with an ACK. Then the ISL25700 then transmits the Data
Byte. The master then terminates the read operation
(issuing a STOP condition) following the last bit of the
Data Byte (see Figure 16).
The byte at address 08h determines if the Data Bytes
being read are from volatile or non-volatile memory (see
“Memory Map” on page 10).
Application Information
In order to get the correct temperature setting, it is very
important to chose an appropriate thermistor and
calibrate the complete system. The following sequence
describes the thermistor selection and calibration
procedure.
1. Select a thermistor type and get the RT curve from
manufacturer.
2. Find out a thermistor resistance at the desired
temperature and select the coarse temperature
range from Table 2. The thermistor value at set
point should be in the middle of the coarse range.
Note, the coarse ranges overlap.
3. Set the Current Sense limit in Reg.01h[7:3] and
System Loop Gain in Reg. 03h[4:0] as high as
possible to enable fast settling yet prevent thermal
oscillation.
4. Perform system calibration and fine tuning using an
external temperature sensor.
Thermistor Selection
Chose the thermistor whose resistance at the desired
temperature set point T_SET will be within the range
specified in Table 2. The R/T characteristics of the
thermistor are usually provided by the manufacturer as a
table or as a curve.
For example, you are going to stabilize the temperature
at T_SET = +91.5°C. You decided to use a 100k NTC
thermistor from Vishay, NTCS0805E3104FXT. The
resistance at +91.5°C will be 8109.12Ω, according to the
manufacturer data provided online at
http://www.vishay.com/doc?29100. This resistance fits
in the coarse temperature range settings of 001b or
002b, as per Table 2.
Current Sense Resistor and MOSFET Selection
The current sense resistor should be selected with
consideration of system maximum power consumption,
maximum current, and minimum resolution. It is
recommended to select a current sense resistor in the
range from 0.1Ω to 10Ω. The 0.1Ω resistor allows
sensing and will limit the maximum current from 2A to
17.5A, while a 10Ω resistor allows selecting a current
limit from 20mA to 175mA. The type and size of current
sense resistor should have an appropriate power rating.
The ISL25700 will work only with the P-type of power
MOSFETs or Darlington pair. Since the driving capability is
limited, it is not recommended to drive a bipolar
transistor. The power MOSFET can be used as a heater
inside a micro temperature chamber.
System Loop Gain Setting
Assemble the application circuit including ISL25700, the
power MOSFET, heating element and thermistor. The
System Loop Gain allows controlling thermal stability of
the feedback system. In other words, current through
the power MOSFET should settle fast and without
oscillation when setpoint changes from one temperature
to another. Settling time of the current can be monitored
by oscilloscope with the current probe.
Another way to determine system stability is to obtain a
phase margin of the system by analyzing Bode plots.
Bode plots of the gain and phase margins can be
obtained by disconnecting the thermistor from R_TH -
breaking the loop, and connecting a low frequency signal
analyzer between the R_TH pin and the thermistor. That
will allow analysis of the system transfer function vs.
frequency. Note the thermistor has to be biased with
30μA DC current from an external supply. The signal
analyzer sweeps the frequency at input node (R_TH) and
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FN6885.0
September 3, 2010