SIP2802 datasheet(7/8 Pages) VISHAY

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SiP2800/01/02/03/04/05

Vishay Siliconix

New Product

Document Number: 72660

S-41623—Rev.C, 30-Aug-04

www.vishay.com

For the SiP2803 and SiP2805:

D FOSC = 1/{[(CT +CSTRAY)x RT x 0.93] + [(CT +CSTRAY)x

RDISCH x 2.53] + TDELAY}

Here RT is in ohms and CT is in farads, RDISCH is the value of

the resistor through which CT is discharged (normally an

on-chip 130-

Ω resistor, unless the circuit is configured with

additional external discharge-path resistance), and tDELAY 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 CSTRAY.

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 RT below 10 k are not recommended. Low values of

RT cause high circuit operating currents, and very low values

will prevent the oscillator from properly discharging CT.

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 VREF to GND with a good quality 0.1-mF

ceramic capacitor, as close to the part as possible.

electrolytic or tantalum capacitor may be used in addition to the

ceramic capacitor. When 1 V < VCC < the UVLO threshold,

REF is pulled to ground through a 5-k

Ω resistor. Hence, REF

can also be used as an output to indicate the part’s VCC status.

VCC

VCC is the positive power connection for the SiP280X

controller IC, and should be the most positive terminal on the

part. In normal operation, VCC is powered through a current

limiting resistor. The required start-up supply current will

generally be on the order of 100

mAwith VCC below the UVLO

voltage of the SiP280X, and can remain at or below 500-

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 12.5-V Zener diode) between VCC and GND. If the part’s VCC

pin is current-fed through an appropriate dropping resistor, the

VCC 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 that the value of the dropping resistor can be calculated.

A good estimate of the actual operating current (ICC) may be

made by summing three components:

(a) Any external current loading on the VCC or REF pins

(b) The operating current required by the IC itself, and

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, but will be fixed for a given value of FOSC.

Item (c) is usually the dominant term in the calculation of ICC,

as the power required to drive the external power switch will

typically increase as FOUT is increased. The most common

example of this is seen in driving the gate of a power MOSFET.

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:

IDRIVE =FOUT × Qg of the chosen MOSFET.

A first approximation of the necessary dropping resistor value

is then given by:

R = [(Nominal VSUPPLY) – 12 V]/(Nominal ICC)

Here R is in ohms and ICC is in amperes.

The resistor limiting the current into the VCC pin should be

selected such that ICC(min) equals the worst-case maximum

sum of the above currents, while holding ICC(max) to as low a

value above that number as practicable (for best overall

efficiency), and never more than 25 mA above that number (to

avoid exceeding the IC’s internal clamp diode ratings). VCC

must be bypassed to GND with a good quality 0.1-

mF ceramic

capacitor, as close to the part as possible. This will help avoid

problems created by high-frequency noise on the power

supply of the part. An electrolytic or tantalum capacitor may be

placed in parallel with the ceramic capacitor if more

capacitance is needed or desired.

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