SI9135 Datasheet, PDF (12/14 Pages) Vishay Siliconix – SMBus Multi-Output Power-Supply Controller

Si9135
Vishay Siliconix
New Product
DETAILED FUNCTIONAL BLOCK DIAGRAM
FIGURE 5. Complete Si9135 Block Diagram
DESCRIPTION OF OPERATION
Start-up Sequence
Si9135 is normally controlled by its SMBus interface after V_IN
is applied. Initially, if there is no incoming SMBus control
command, it comes up in its default power on sequence, first
the LDO 5 V will come up within its tolerance, and then the
precision 3.3-V reference will come up. Immediately
afterwards, the oscillator will begin and 3.3-V BUCK converter
will turn on and then 5-V BUCK converter and at last 12-V
FLYBACK converter. If Si9135 receives any SMBus controlling
command after LDO 5 V is established, the designated
converters will be allowed to turn on or off independently
depending on the command received. In the event of all three
converters are turned off, the oscillator will be turned off, the
total system would only draw 35-µA supply current.
Each converter can soft-start separately. The integrated
internal soft-start circuitry for each converter gradually
increases the inductor maximum peak current during soft-
start period (approximately 4 msec), preventing excessive
currents being drawn from the input during startup. The soft-
start is controlled by initial default start up sequence or
incoming SMBus command.
Si9135 converters a 5.5-V to 30-V input voltage to five
outputs, two BUCK (step-down) high current, PWM, switch-
mode supplies, one at 3.3 V and one at 5 V, one FLYBACK
12-V PWM switch-mode supply, one precision 3.3-V reference
and one 5-V Low Drop Out linear regulator output. Switch-
mode supply output current capabilities depend on external
components (can exceed 10 A). With typical application
shown on the application diagram, the two BUCK converters
deliver 4 A and the FLYBACK converters deliver 0.25 A. The
recommended load current for precision 3.3-V reference
output is less than 1 mA, the recommended load current for
5-V LDO output current is less than 30 mA. In order to
maximize the power efficiency, when the 5-V BUCK converter
supply is above 4.5 V, the BUCK converter’s output is internal
connected to LDO output.
Buck Converter Operation
The 3.3-V and 5-V buck converters are both current-mode
PWM and PSM (during light load operation) regulators using
high-side bootstrap n-channel and low-side n-channel
MOSFETs. At light load conditions, the converters switch at a
lower frequency than the clock frequency, seen like some
clock pulses between the actual switching are skipped, this
operating condition is defined as pulse-skipping. The
operation of the converter(s) switching at clock frequency is
defined as normal operation.
S-60752—Rev. B, 05-Apr-99
12
FaxBack 408-970-5600, request 70817
www.siliconix.com