SI786 Datasheet, PDF (8/14 Pages) Vishay Siliconix – Dual-Output Power-Supply Controller

Si786
Vishay Siliconix
ORDERING INFORMATION
Lead (Pb)-Free
Part Number Part Number Temp Range
Si786CG
Si786CG-T1
Si786CRG
Si786CRG-T1
Si786CSG
Si786CSG-T1
Si786LG
Si786LG-T1
Si786LRG
Si786LRG-T1
Si786LSG
Si786LSG-T1
Si786DG
Si786DG-T1
Si786DRG
Si786DRG-T1
Si786DSG
Si786DSG-T1
Si786CG-T1—E3
Si786CRG-T1—E3
Si786CSG-T1—E3
Si786LG-T1—E3
Si786LRG-T1—E3
Si786LSG-T1—E3
Si786DG-T1—E3
Si786DRG-T1—E3
Si786DSG-T1—E3
C-Grade
0 to 70_C
L-Grade
−10_ to 90_C
D-Grade
−40_ to 85_C
V_OUT
3.3 V
3.45 V
3.6 V
3.3 V
3.45 V
3.6 V
3.3 V
3.45 V
3.6 V
Demo Board
Si786DB
Temp Range
0 to 70_C
Board Type
Surface Mount
DESCRIPTION OF OPERATION
The Si786 is a dual step-down converter, which takes a 5.5-V
to 30-V input and supplies power via two PWM controllers (see
Figure 1). These 5-V and 3.3-V supplies run on an optional
300-kHz or 200-kHz internal oscillator, or an external sync
signal. Amount of output current is limited by external
components, but can deliver greater than 6 A on either supply.
As well as these two main Buck controllers, additional loads
can be driven from two micropower linear regulators, one 5 V
(V_L) and the other 3.3 V (REF)—see Figure 2. These supplies
are each rated to deliver 5 mA. If the linear regulator circuits fall
out of regulation, both Buck controllers are shut down.
Two voltage comparators with adjustable output voltages are
included in the Si786. They can be used for gate drive in load
switching applications, where n-channel MOSFETs are used.
Logic level voltages can be generated as well, for instance to
serve as mP interfacing (e.g. a Power-good signal).
3.3-V Switching Supply
The 3.3-V supply is regulated by a current-mode PWM
controller in conjunction with several externals: two n-channel
MOSFETs, a rectifier, an inductor and output capacitors (see
Figure 1). The gate drive supplied by DH₃ needs to be greater
than V_L , so it is provided by the bootstrap circuit consisting of
a 100-nF capacitor and diode connected to BST₃ .
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A low-side switching MOSFET connected to DL₃ increases
efficiency by reducing the voltage across the rectifier diode. A
low value sense resistor in series with the inductor sets the
maximum current limit, to disallow current overloads at
power-on or in short-circuit situations.
The soft-start feature on the Si786 is capacitor programmable;
pin SS₃ functions as a constant current source to the external
capacitor connected to GND. Excess currents at power-on are
avoided, and power-supplies can be sequenced with different
turn-on delay times by selecting the correct capacitor value.
5-V Switching Supply
The 5-V supply is regulated by a current-mode PWM controller
which is nearly the same as the 3.3-V output. The dropout
voltage across the 5-V supply, as shown in the schematic in
Figure 1, is 400 mV (typ) at 2 A. If the voltage at V+ falls, nearing
5 V, the 5-V supply will lower as well, until the V_L linear regulator
output falls below the 4-V undervoltage lockout threshold.
Below this threshold, the 5-V controller is shut off.
The frequency of both PWM controllers is set at 300 kHz when
the SYNC pin is tied to REF. Connecting SYNC to either GND
or V_L sets the frequency at 200 kHz.
Document Number: 70189
S-40807—Rev. J, 26-Apr-04