6
FN9167.3
March 9, 2006
Functional Description
The ISL6421A is a single output voltage regulator controlled
by an I
2
C bus, making it an ideal choice for advanced
satellite set-top box and personal video recorder
applications. Both supply and control voltage outputs for a
low noise block (LNB) are available simultaneously in any
output configuration. The device utilizes a built-in DC/DC
step-converter which, from a single supply source ranging
from 8V to 14V, generates the voltage that enables the linear
post-regulator to work with a minimum of dissipated power.
An undervoltage lockout circuit disables the circuit when
V
CC
drops below a fixed threshold (7.5V typ).
DiSEqC Encoding
The internal oscillator is factory-trimmed to provide a tone of
22kHz in accordance with DiSeqC standards. No further
adjustment is required. The 22kHz oscillator can be
controlled either by the I
2
C interface (ENT bit) or by a
dedicated pin (DSQIN) that allows immediate DiSEqC data
encoding for the LNB. All the functions of this IC are
controlled via the I
2
C bus by writing to the system registers
(SR). The same registers can be read back, and two bits will
report the diagnostic status. The internal oscillator operates
the converters at ten times the tone frequency. The device
offers full I
2
C compatible functionality, 3.3V or 5V, and up to
400kHz operation.
If the Tone Enable (ENT) bit is set LOW through I
2
C, then
the DSQIN terminal activates the internal tone signal,
modulating the dc output with a 0.3V, 22kHz, symmetrical
waveform. The presence of this signal usually gives the LNB
information about the band to be received.
Burst coding of the 22kHz tone can be accomplished due to
the fast response of the DSQIN input and rapid tone
response. This allows implementation of the DiSEqC
(EUTELSAT) protocols.
When the ENT bit is set HIGH, a continuous 22kHz tone is
generated regardless of the DSQIN pin logic status. The
ENT bit must be set LOW when the DSQIN pin is used for
DiSEqC encoding.
Linear Regulator
The output linear regulator will sink and source current. This
feature allows full modulation capability into capacitive loads
as high as 0.25µF. In order to minimize the power
dissipation, the output voltage of the internal step-up
converter is adjusted to allow the linear regulator to work at
minimum dropout.
When the device is put in the shutdown mode (EN = LOW),
the PWM power block is disabled. When the regulator block
is active (EN = HIGH), the output can be logic controlled to
be 13V or 18V (typical) by means of the VSEL bit (Voltage
Select) for remote controlling of non-DiSEqC LNBs.
Additionally, it is possible to increment by 1V (typical) the
selected voltage value to compensate for the excess voltage
drop along the coaxial cable (LLC bit HIGH).
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
The current limiting block can operate either statically
(simple current clamp) or dynamically. The threshold is
between 500mA and 625mA. 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 shut down for a time T
OFF
, typically
VCC Main power supply to the chip.
GATE This is the device output of the PWM. This high current driver output is capable of driving the gate of a power FET.
This output is actively held low when Vcc is below the UVLO threshold.
VOUT Output voltage for the LNB.
ADDRESS Address pin to select two different addresses per voltage level at this pin.
COMP Error amp output used for compensation.
FB Feedback pin for the PWM.
CPVOUT, CPSWIN,
CPSWOUT
Charge pump connections.
SEL18V When connected HIGH, this pin will change the output of the PWM to 18V. Only available on the QFN package option.
Functional Pin Description (Continued)
SYMBOL FUNCTION
ISL6421A
