SI786 Vishay, SI786 Datasheet - Page 13

• Manufacturer Part Number: SI786
• Description: Dual-output Power-supply Controller
• Manufacturer: Vishay

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 V
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 V
A 5-V linear regulator supplies power to the internal logic
circuitry. The regulator is available for external use from pin V
able to source 5 mA. A 10-mF capacitor should be connected
between V
switching supply has voltage greater than 4.5 V, V
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.
V
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 V
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 V
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
L
and REF supplies stay on as long as V+ is greater than 4.5
L
L
and REF
and GND. To increase efficiency, when the 5 V
L
< 4.0 V or REF < 2.8 V.
L
to 5 V. This will keep the main
L
is internally
L
L
,
DESIGN CONSIDERATIONS
Inductor Design
Three specifications are required for inductor design:
inductance (L), peak inductor current (I
resistance (R
L +
When LIR is higher, smaller inductance values are acceptable,
at the expense of increased ripple and higher losses.
The peak inductor current (I
load current (I
(I
to be 30% of the steady-state current, which gives I
to 1.15 times I
The equation for computing peak inductor current is:
I
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:
C
ESR
LPEAK
Where:
and,
Where:
LPP
F
u
CF
). Typically, a designer will select the ac inductor current
V
V
+ I
OUT
V
IN(MAX)
t
OUT
OUT
V
V
V
f =
I
LIR = Ratio of inductor peak-to-peak ac current to
average dc load current, typically 0.3.
C
V
V
R
GBWP = Gain-bandwidth product, 60 kHz;
ESR
V
OUT
OUT
OUT
IN(MAX)
REF
OUT
F
CS
R
IN(MAX)
V
)
( f ) I
L
OUT
= Output filter capacitance (F)
CS
REF
OUT
). The equation for computing inductance is:
CF
V
= Sense resistor (W);
= Maximum dc load current (A);
Switching frequency, normally
300 kHz;
= Reference voltage, 3.3 V;
= Output voltage (3.3 V or 5 V);
R
= Output voltage, 3.3 V or 5 V;
V
OUT
REF
(2)(p)(GBWP)
) plus one half of the peak-to-peak ac current
OUT
CS
= Output filter capacitor ESR (W).
(2)(f)(L) V
.
–V
= Maximum input voltage (V);
( LIR )
OUT
V
IN(MAX)
IN(MAX)
LPEAK
–V
OUT
) is equal to the steady-state
Vishay Siliconix
LPEAK
www.vishay.com
LPEAK
), and coil
Si786
equal
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