SI786 datasheet(13/14 Pages) VISHAY

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Si786

Vishay Siliconix

Document Number: 70189

S-40807—Rev. J, 26-Apr-04

www.vishay.com

Oscillator and SYNC

There are two ways to set the Si786 oscillator frequency: by

using an external SYNC signal, or using the internal oscillator.

The SYNC pin can be driven with an external CMOS level

signal with frequency from 240 kHz and 350 kHz to

synchronize to the internal oscillator. Tying SYNC to either VL

or GND sets the frequency to 200 kHz and to REF sets the

frequency to 300 kHz.

Operation at 300 kHz is typically used to minimize output

passive component sizes. Slower switching speeds of

200 kHz may be needed for lower input voltages.

Internal VL and REF

A 5-V linear regulator supplies power to the internal logic

circuitry. The regulator is available for external use from pin VL ,

able to source 5 mA. A 10-mF capacitor should be connected

between VL and GND. To increase efficiency, when the 5 V

switching supply has voltage greater than 4.5 V, VL is internally

switched over to the output of the 5-V switching supply and the

linear regulator is turned off.

The 5-V linear regulator provides power to the internal 3.3-V

bandgap reference (REF). The 3.3-V reference can supply 5

mA to an external load, connected to pin REF. Between REF

and GND connect a capacitor, 0.22 mF plus 1 mF per mA of load

current. The switching outputs will vary with the reference;

therefore, placing a load on the REF pin will cause the main

outputs to decrease slightly, within the specified regulation

tolerance.

VL and REF supplies stay on as long as V+ is greater than 4.5

V, even if the switching supplies are not enabled. This feature

is necessary when using the micropower regulators to keep

memory alive during shutdown.

Both linear regulators can be connected to their respective

switching supply outputs. For example, REF would be tied to

the output of the 3.3 V and VL to 5 V. This will keep the main

supplies up in standby mode, provided that each load current

in shutdown is not larger than 5 mA.

Fault Protection

The 3.3 V and 5 V switching controllers as well as the

comparators are shut down when one of the linear

regulators drops below 85% of its nominal value; that is, shut

down will occur when VL < 4.0 V or REF < 2.8 V.

DESIGN CONSIDERATIONS

Inductor Design

Three specifications are required for inductor design:

inductance (L), peak inductor current (ILPEAK), and coil

resistance (RL). The equation for computing inductance is:

L +

VOUT VIN(MAX)–VOUT

VIN(MAX) (f) IOUT (LIR)

Where:

VOUT = Output voltage (3.3 V or 5 V);

VIN(MAX) = Maximum input voltage (V);

f =

Switching frequency, normally

300 kHz;

IOUT = Maximum dc load current (A);

LIR = Ratio of inductor peak-to-peak ac current to

average dc load current, typically 0.3.

When LIR is higher, smaller inductance values are acceptable,

at the expense of increased ripple and higher losses.

The peak inductor current (ILPEAK) is equal to the steady-state

load current (IOUT) plus one half of the peak-to-peak ac current

(ILPP). Typically, a designer will select the ac inductor current

to be 30% of the steady-state current, which gives ILPEAK equal

to 1.15 times IOUT.

The equation for computing peak inductor current is:

ILPEAK + IOUT )

VOUT VIN(MAX)–VOUT

(2)(f)(L) VIN(MAX)

Output Capacitors

The output capacitors determine loop stability and ripple

voltage at the output. In order to maintain stability, minimum

capacitance and maximum ESR requirements must be met

according to the following equations:

CF u

VREF

VOUT RCS (2)(p)(GBWP)

Where:

CF = Output filter capacitance (F)

VREF = Reference voltage, 3.3 V;

VOUT = Output voltage, 3.3 V or 5 V;

RCS = Sense resistor (W);

GBWP = Gain-bandwidth product, 60 kHz;

ESRCF = Output filter capacitor ESR (W).

ESRCF t

VOUT RCS

VREF

and,

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