SI9140DQ VISHAY [Vishay Siliconix], SI9140DQ Datasheet - Page 12

• Manufacturer Part Number: SI9140DQ
• Description: MP Controller For High Performance Process Power Supplies
• Manufacturer: VISHAY [Vishay Siliconix]

Si9140
Vishay Siliconix
a.
b.
Figure 7 is the measured transient response (time domain) for
the 10-A step response. The measured transient response
shows the processor voltage regulating to 70 mV, well within
the 0.145-V regulation.
The Si9140’s switching frequency is determined by the
external R
switching frequency of their choice. For applications where
space is the main constraint, the switching frequency can be
set as high as 2 MHz to minimize inductor and output capacitor
size. In applications where efficiency is the main concern, the
switching frequency can be set low to maximize battery life.
The switching frequency for high-performance processors
applications circuits are set for 400 kHz. The equation for
switching frequency is:
The precision reference is set at 1.5 V"1.5%. The reference
is capable of sourcing up to 1 mA. The combination of 1.5%
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12
FEEDBACK NETWORK COMPONENT VALUES
3 x 330 mF
6 x 100 mF
25 x 1 mF
2 x 330 mF
4 x 100 mF
25 x 1 mF
3 x 330 mF
4 x 100 mF
25 x 1 mF
Decoupling Capacitance
Power supply output capacitance.
mprocessor decoupling capacitance.
Current
Voltage
f
OSC
2.9 V
Total Output and
10 A
mP
mP
5 A
0 A
b
b
b
a
b
a
b
a
b
[
. . . . . . . . . .
. . . . . . . . . .
. . . . . . . . . .
OSC
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
. . . . . . . . .
R
a) Transient Response from 0- to 10-A Step Load
OSC
and C
0.75
Os-con
Tantalum
Ceramic
Os-con
Tantalum
Ceramic
Tantalum
Tantalum
Ceramic
C
OSC
OSC
TABLE 1.
values, allowing designers to set the
5.6 pF
10 pF
10 pF
C4
(at V
DD
180 pF
220 pF
100 pF
= 5.0 V)
C5
240 k
200 k
100 k
R5
FIGURE 7.
reference and 3.5% transient load regulation safely complies
with the "5% regulation requirement. If additional margin is
desired, an external precision reference can be used in place
of the internal 1.5-V reference.
Switching and Synchronous Rectification MOSFETs
The synchronous gate drive outputs of Si9140 PWM controller
drive the high-side p-channel switch MOSFET and the
low-side n-channel synchronous rectifier MOSFET. The
physical difference between the non-synchronous to
synchronous rectification requires an additional MOSFET
across the free-wheeling diode (D1). The inductor current will
reach 0 A if the peak-to-peak inductor current equals twice the
output current. In synchronous rectification mode, current is
allowed to flow backwards from the inductor (L1) through the
synchronous MOSFET (Q3) and to the output capacitor (C2)
once the current reaches 0 A. Refer to schematic on Figure 1.
In non-synchronous rectification, the diode (D1) prevents the
current from flowing in the reverse direction. This minor
difference has a drastic affect on the performance of a power
supply. By allowing the current to flow in the reverse direction,
it preserves the continuous inductor current mode, maintaining
the wide converter bandwidth and improving efficiency. Also,
maintaining the continuous current mode during light load to
full load guarantees consistent transient response throughout
a wide range of load conditions.
The transition from stop clock and auto halt to active mode is
a perfect example. The microprocessor current can vary from
0.5 A to 10 A or greater during these transitions. If the
converter were to operate in discontinuous current mode
during the stop clock and auto halt modes, the transfer function
of the converter would be different compared to operation in
the active mode. In discontinuous current mode, the converter
bandwidth can be 10 to 15 times lower than the continuous
current mode (Figure 8). Therefore, the response time will also
be 10 to 15 times slower, violating the microprocessor’s
regulator requirements. This could result in unreliable
operation of the microprocessor.
b) Transient Response from 10- to 0-A Step Load
S-40699—Rev. H, 19-Apr-04
Document Number: 70026