8
FN6487.1
August 23, 2007
VSW - This is the switched regulated low voltage output
supply derived from VIO. Bypass to GND with a 1.0μF
capacitor. Its output is adjustable from 0V to 15.0V using an
appropriate divider from VCONT to VADJ and GND. VSW is
nominally 7 x VSWADJ. Protection circuitry prevents the
output from exceeding 23V in the event of a fault on
VSWADJ (short high). The minimum output current
capability is 50mA with transient capability to > 80mA. VSW
may be soft-started by placing a capacitor from VSWADJ to
GND.
ENABLE - The positive logic on/off control input that
controls the VSW output. A logic high enables VSW.
VSWCOMP - A 220pF compensating capacitor is placed
between VSWCOMP and VSW to stabilize the control loop.
This value may vary depending on the output load and
capacitance applied between VSW and GND.
VSWADJ - The feedback adjustment pin for VSW. A divider
from VCONT to GND sets the output voltage for VSW.
VSWADJ has a node discharge device that activates
momentarily at power-up, when disabled (ENABLE low), and
during power-down sequences. VSW is nominally
7 x VSWADJ.
Functional Description
Features
The control circuitry used in Telecom/Datacom DC/DC
converters often requires an operating bias voltage
significantly lower than the source voltage available to the
converter. Many applications use a discrete linear regulator
from the input source to create the bias supply. Often an
auxiliary winding from the power transformer is used to
supplement or replace the linear supply once the converter
is operating. The auxiliary winding bias voltage may require
regulation, as well, to minimize the voltage variation inherent
in unregulated transformer winding outputs. When
implemented discretely, this circuitry occupies significant
PWB area, a considerable problem in today’s high density
converters.
The ISL6720A triple linear regulator simplifies the start-up
and operating bias circuitry needed in Telecom and Datacom
DC/DC converters by integrating these functions, and more,
in a small 4mmx4mm DFN package.
VIO
VIO is the primary output of the ISL6720A, providing bias
voltage whenever the input source voltage, VPWR, is above
its under voltage lockout (UVLO) threshold. VIO, which is an
abbreviation for “voltage input/output”, is adjustable from
0.5V to 20V using the VADJ input, and may be back-biased
up to 40V from an external source independent of VPWR.
The back-bias voltage must be higher than the VIO setpoint
to disable the internal VIO regulator. The transition from
internal VIO to external VIO is not abrupt. As the back-bias
voltage increases above the VIO setpoint, the load current
on VIO gradually transfers from the VIO regulator to external
back-bias source. Depending on load, the back-bias voltage
may have to exceed the VIO setpoint by as much as 3V
before the VIO regulator is off.
The output voltage of VIO is set by applying a reference
voltage to VADJ. The reference voltage may be set by using
a resistor and zener diode combination from the VPWR
input. VIO ranges from 0.5V to 4.5V below the voltage
applied to VADJ depending on the load on VIO. VIO can
source more than 125mA for short durations, limited only by
the device power dissipation and the thermal constraints of
the application.
where V
OFFSET
ranges from 0.5V to 4.5V.
VIO may be soft-started using the VADJ input. By limiting the
rate of rise of VADJ, the risetime of VIO may be controlled.
Soft-start may be accomplished by placing a capacitor to
ground from VADJ. The capacitor to ground and the resistor
from VPWR determine the RC charging characteristic for the
voltage at VADJ. Since VADJ is pulled low at power-up and
power-down, soft-start always starts from a known state.
The soft-start rate cannot exceed the intrinsic risetime set by
the current limit threshold of the output. As load capacitance
increases, the intrinsic risetime increases. In general,
placing large capacitance values on VIO should be avoided,
particularly if the source voltage applied to VPWR is high.
Having a large load capacitance and high input voltage
results in high power dissipation for a longer duration and
may activate the over-temperature protection before VIO
can be biased externally. Under such conditions, steady
state operation would not be achievable.
If the auxiliary transformer winding used to back-bias VIO
requires a large value of capacitance, it can be isolated from
VIO using a diode as shown in Figure 6.
VIO VADJ V
OFFSET
–= V
(EQ. 1)
FIGURE 5. SETPOINT ADJUSTMENT FOR VIO
1
VADJ
VPWR
ENABLE
VSW
VIO
2
7
9
10
11
3
4
8
VCONT
VSWCOMPGND
VSWADJ
56
VCOMP
N/C
VIN
ISL6720A
