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參數(shù)資料
| 型號(hào)： | SiP2805 |
| 廠商： | Vishay Intertechnology,Inc. |
| 英文描述： | Low Power Consumption Current Mode Controller |
| 中文描述： | 低功耗電流模式控制器 |
| 文件頁數(shù)： | 7/8頁 |
| 文件大?。?/td> | 93K |
| 代理商： | SIP2805 |
SiP2800/01/02/03/04/05
Vishay Siliconix
New Product
Document Number: 72660
S-41623—Rev.C, 30-Aug-04
www.vishay.com
7
For the SiP2803 and SiP2805:
D
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
.
NotethattheSiP2801,SiP2804,andSiP2805haveaninternal
toggle flip-flop at the output of the oscillator, to ensure that the
output duty cycle never exceeds 50%.
frequencyappearingattheOUTpintoone-halfoftheoscillator
frequency for these three parts.
This divides the
Values ofR
T
below10 k are notrecommended. 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-
m
F
ceramic capacitor, as close to the part as possible.
electrolyticortantalumcapacitormaybeusedinadditiontothe
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 anoutputtoindicate thepart’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
m
A with V
CC
below the UVLO
voltage of the SiP280X, and can remain at or below 500-
m
A
total supply current once the part starts switching. To prevent
the ICfrombeing damaged by overvoltage conditions,each of
the SiP2800 family of parts has an internal clamp (effectively
An
a12.5-VZenerdiode)betweenV
CC
andGND. Ifthepart’sV
CC
pin is current-fed through an appropriate dropping resistor,the
V
CC
pin will never exceed its rated voltage, nor will the the
device as a whole exceed its rated power dissipation. This
does require knowing what the operating current of the IC will
be,so thatthe valueofthedropping resistorcan becalculated.
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.
Item (a) in the above list is a static dc value, and can generally
be calculated with good accuracy. Item (b) will increase with
operating frequency,butwillbe fixed fora given value ofF
OSC
.
Item (c) is usually the dominant term in the calculation of I
CC
,
as the power required to drive the external power switch will
typically increase as F
OUT
is increased. The most common
example ofthis is seen in driving thegate ofa powerMOSFET.
In such applications, the gate capacitances must be charged
once each switching cycle. This calculation is simplified by
using the gate charge term given by most MOSFET
manufacturers, allowing the use of the formula:
I
DRIVE
= F
OUT
×
Q
g
of the chosen MOSFET.
A first approximation of the necessary dropping resistor value
is then given by:
R = [(Nominal V
SUPPLY
) – 12 V]/(Nominal I
CC
)
Here R is in ohms and I
CC
is in amperes.
The resistor limiting the current into the V
CC
pin should be
selected such that I
CC(min)
equals the worst-case maximum
sum of the above currents, while holding I
CC(max)
to as low a
value above that number as practicable (for best overall
efficiency),and nevermore than 25 mA above that number(to
avoid exceeding the IC’s internal clamp diode ratings). V
CC
must be bypassed to GND with a good quality 0.1-
m
F ceramic
capacitor, as close to the part as possible. This will help avoid
problems created by high-frequency noise on the power
supply ofthepart. An electrolyticortantalumcapacitormaybe
placed in parallel with the ceramic capacitor if more
capacitance is needed or desired.
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