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Document Number: 70818
S11-0975-Rev. D, 16-May-11
Vishay Siliconix
Si9136
This document is subject to change without notice.
THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
DETAIL FUNCTIONAL BLOCK DIAGRAM
DESCRIPTION OF OPERATION
Start-up Sequence
Si9136’s outputs are controlled by three specific input control
lines; 3.3 ON/OFF
, 5 ON/OFF, and 12 ON/OFF. Once V
IN
is
applied, the V
L
, the 5 V LDO will come up within its tolerance.
When any one of these control lines becomes logic high, the
precision 3.3 V reference will also come up. Immediately
afterwards, the oscillator will begin and the corresponding
converter will come up with its own tolerance. In the event of
all three converters are turned off, the oscillator and the
reference output will be turned off, and the total system will
only draw 35 µA of supply current.
Each converter can soft-start independently. This internal
soft-start circuitry for each converter will gradually increases
the inductor maximum peak current during the soft-start
period (approximately 4 ms), preventing excessive currents
from being drawn from the input.
Si9136 converts a 5.5 V to 30 V input voltage to five different
output voltages; two buck (step-down) high current, PWM,
switch-mode supplies of 3.3 V and 5 V, one "flyback" PWM
switch-mode supply of 12 V, one precision 3.3 V reference
and one 5 V low drop out (LDO) linear regulator output.
Switch-mode supply output current capabilities depend on
external components (can be selected to exceed 10 A). In
the standard application circuit illustrated in Figure 1, each
buck converter is capable of delivering 5 A, with the flyback
converter delivering 250 mA. The recommended load
currents for the precision 3.3 V reference output is less than
1 mA, and the 5 V LDO output is less than 30 mA. In order to
maximize power efficiency of the converter, when the 5 V
buck converter output (FB5) voltage is above 4.5 V, the
internal 5 V LDO is turned off and V
L
is supplied by the 5 V
converter 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.
Figure 7. Complete Si9136 Block Diagram
5 V
Buck
Controller
FB
5
CS
5
BST
5
DH5
LX
5
DL5
5 V
Linear
Regulator
3.3 V
Reference
4 V
2.4 V
4.5 V
5 ON/OFF
3.3 V
Buck
Controller
FB
3
CS
3
BST
3
DH3
LX
3
DL3
3 ON/OFF
12 V
Flyback
Controller
FYBFY
ICSP
ICSN
BSTFY
DHFY
LXFY
12 ON/OFF
DLFY
V
IN
V
L
Logic
Control
