8
FN9176.1
February 8, 2005
Output Timing
The programmed output voltage rise and fall times can be
set by an external capacitor. The output rise and fall times
will be approximately 3400 times the TCAP value. For the
recommended range of 0.47µF to 2.2µF, the rise and fall
time would be 1.6ms to 7.6ms. Using a 0.47µF capacitor
insures the PWM stays below its overcurrent threshold when
charging a 120µF VSW filter cap during the worst case 13V
to 19V transition. A typical value of 1.0µF is recommended.
This feature affects the programmed voltage rise and fall
times.
Current Limiting (Only one ISEL option needed)
The current limiting block can operate either statically
(simple current clamp) or dynamically. The lower threshold is
between 425mA and 550mA (ISEL = L), while the higher
threshold is between 775mA and 950mA (ISEL = H). When
the DCL (Dynamic Current Limiting) bit is set to LOW, the
overcurrent protection circuit works dynamically. That is, as
soon as an overload is detected, the output is shutdown for a
time t
OFF
, typically 900ms. Simultaneously the overload flag
(OLF) bit of the system register is set to HIGH. After this time
has elapsed, the output is resumed for a time Ton = 20ms.
During Ton, the device output will be current limited to
between 575mA and 950mA. At the end of Ton, if the
overload is still detected, the protection circuit will cycle
again through Toff and Ton. At the end of a full Ton during
which no overload is detected, normal operation is resumed
and the OLF bit is reset to LOW. Typical Ton+Toff time is
920ms as determined by an internal timer. This dynamic
operation can greatly reduce the power dissipation in a short
circuit condition, still ensuring excellent power-on start-up in
most conditions.
However, there could be some cases in which a highly
capacitive load on the output may cause a difficult start-up,
when the dynamic protection is chosen. This can be solved
by initiating a power start-up in static mode (DCL = HIGH)
and then switching to the dynamic mode (DCL = LOW) after
a chosen amount of time. When in static mode, the OLF bit
goes HIGH when the current limit threshold at the CS pin
reaches 0.45V typ and returns LOW when the overload
condition is cleared. The OLF bit will be LOW at the end of
initial power-on soft-start.
Thermal Resistance
This IC is protected against overheating. When the junction
temperature exceeds 150°C (typical), the step-up converter
and the linear regulator are shut off and the overtemp flag
(OTF) bit of the SR is set HIGH. Normal operation is
resumed and the OTF bit is reset LOW, when the junction is
cooled down to 130°C (typical).
External Output Voltage Selection
The output voltage can be selected by the I
2
C bus.
Additionally, the QFN package offers a pin (SEL18V) for
independent 13V/18V output voltage selection. When using
this pin, the I
2
C bits should be initialized to 13V status.
I
2
C Bus Interface for ISL6425
(Refer to Philips I
2
C Specification, Rev. 2.1)
Data transmission from main microprocessor to the ISL6425
and vice versa takes place through the 2 wire I
2
C bus
interfaces, consisting of the two lines SDA and SCL. Both
SDA and SCL are bidirectional lines, connected to a positive
supply voltage via a pull up resistor. (Pull up resistors to
positive supply voltage must be externally connected). When
the bus is free, both lines are HIGH. The output stage of
ISL6425 will have an open drain/open collector in order to
perform the wired-AND function. Data on the I
2
C bus can be
transferred up to 100kbits/s in the standard-mode or up to
400kbits/s in the fast-mode. The level of logic “0” and logic
“1” is dependent of associated value of Vdd as per electrical
specification table. One clock pulse is generated for each
data bit transferred.
Data Validity
The data on the SDA line must be stable during the HIGH
period of the clock. The HIGH or LOW state of the data line
can only change when the clock signal on the SCL line is
LOW. (Refer to Figure 1.)
START and STOP Conditions
As shown in the Figure 2, START condition is a HIGH to
LOW transition of the SDA line, while SCL is HIGH. The
STOP condition is a LOW to HIGH transition on the SDA
line, while SCL is HIGH. A STOP condition must be sent
before each START condition.
TABLE 1.
I
2
C BITS SEL18V O/P VOLTAGE
13V Low 13V
13V High 18V
SDA
SCL
DATA LINE
STABLE
DATA VALID
CHANGE
OF DATA
ALLOWED
FIGURE 1. DATA VALIDITY
ISL6425
