Document Number: 72660
S11-0598-Rev. E, 25-Apr-11
www.vishay.com
7
Vishay Siliconix
SiP2800, SiP2801, SiP2802, SiP2803, SiP2804, SiP2805
This datasheet is subject to change without notice.
THE PRODUCT DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
The SiP280X family incorporates internal leading-edge
blanking on the CS pin, to keep any spurious voltages on the
CS pin from reaching the comparator inputs during the
100 ns interval immediately following the rising edge on OUT
(for example, voltages due to capacitive charging currents).
Because of this internal leading-edge blanking, many
applications require no external RC filter on the CS input.
Compared to circuits requiring the use of an external RC filter
circuit, leading-edge blanking provides a shorter effective CS
to OUT propagation delay.
FB
FB is the inverting input of the VEA. Internally compared
against V
REF
/2 appearing on the VEA’s non-inverting input.
To avoid stability problems, keep lead lengths to FB as short
as possible, and use good layout practices to minimize the
stray capacitances of components connected to this pin.
GND
The GND pin is both the reference ground and the power
ground for this part.
OUT
OUT is the output of a high-current driver capable of peak
currents in excess of ± 750 mA. OUT is therefore well suited
to driving the gates of power MOSFETs. This pin is
specifically held low when V
CC
is below the SiP280X’s UVLO
threshold, to ensure a predictable system turn-on. Since the
OUT pin is internally connected to a low impedance CMOS
buffer, it is capable of rapid rail-to-rail transitions. This output
topology also mitigates the effects of undershoot and
overshoot. For this reason, external Schottky clamp diodes
are generally not required on this pin.
RC
RC is the oscillator frequency programming pin. F
OSC
is set
by the combination of R
T
and C
T
. The charging current for C
T
is provided through R
T
, which is normally connected
between REF and the SiP280X RC pin. C
T
then connects
from RC to GND. Due to the high impedances encountered
in low power control circuits, this connection must be a short
and quiet return to GND (preferably by means of a dedicated
signal trace, separated from all other circuit functions).
The oscillator frequency for the SiP280X family of parts is
approximated by the following formulas:
For the SiP2800, SiP2801, SiP2802, and SiP2804:
•F
OSC
(1.5)/R
T
C
T
For the SiP2803 and SiP2805:
•F
OSC
(1.0)/R
T
C
T
Here R
T
is in ohms and C
T
is in farads.
More accurate formulas for F
OSC
are:
For the SiP2800, SiP2801, SiP2802 and SiP2804:
•F
OSC
= 1/{[(C
T
+ C
STRAY
) x R
T
x 0.652] + [(C
T
+ C
STRAY
)
x R
DISCH
x 2.53] + T
DELAY
}
For the SiP2803 and SiP2805:
•F
OSC
= 1/{[(C
T
+ C
STRAY
) x R
T
x 0.93] + [(C
T
+ C
STRAY
)
x R
DISCH
x 2.53] + T
DELAY
}
Here R
T
is in ohms and C
T
is in farads, R
DISCH
is the value
of the resistor through which C
T
is discharged (normally an
on-chip 130 resistor, unless the circuit is configured with
additional external discharge-path resistance), and t
DELAY
is
an inherent internal comparator delay time of 100 ns. The
capacitance associated with the RC pin is approximately
7.5 pF, and should be included as a part of C
STRAY
.
Note that the SiP2801, SiP2804, and SiP2805 have an
internal toggle flip-flop at the output of the oscillator, to
ensure that the output duty cycle never exceeds 50 %. This
divides the frequency appearing at the OUT pin to one-half
of the oscillator frequency for these three parts.
Values of R
T
below 10 k are not recommended. Low values
of R
T
cause high circuit operating currents, and very low
values will prevent the oscillator from properly discharging
C
T
.
REF
The reference generator block of the Si280X provides an
accurate and stable 4.0 V or 5.0 V (depending upon part
number), which is available at this pin of the IC. This voltage
is also used internally for other functions on the IC. One of
these uses is as the logic power supply for high speed
switching logic on the IC; this, and stability concerns, make it
important to bypass V
REF
to GND with a good quality 0.1 µF
ceramic capacitor, as close to the part as possible. An
electrolytic or tantalum capacitor may be used in addition to
the ceramic capacitor. When 1 V < V
CC
< the UVLO
threshold, REF is pulled to ground through a 5 k resistor.
Hence, REF can also be used as an output to indicate the
part’s V
CC
status.
V
CC
V
CC
is the positive power connection for the SiP280X
controller IC, and should be the most positive terminal on the
part. In normal operation, V
CC
is powered through a current
limiting resistor. The required start-up supply current will
generally be on the order of 100 µA with V
CC
below the
UVLO voltage of the SiP280X, and can remain at or below
500 µA total supply current once the part starts switching.
To prevent the IC from being damaged by overvoltage
conditions, each of the SiP2800 family of parts has an
internal clamp (effectively a 13.5 V Zener diode) between
V
CC
and GND. If the part’s V
CC
pin is current-fed through an
appropriate dropping resistor, the V
CC
pin will never exceed
its rated voltage, nor will the device as a whole exceed its
rated power dissipation. This does require knowing what the
operating current of the IC will be, so that the value of the
dropping resistor can be calculated. A good estimate of the
actual operating current (I
CC
) may be made by summing
three components:
(a) Any external current loading on the V
CC
or REF pins
(b) The operating current required by the IC itself, and
(c) The drive current (I
DRIVE
) required by the external
power switch.
