SI9140 Datasheet, PDF (12/15 Pages) Vishay Siliconix – SMP Controller For High Performance Process Power Supplies

Si9140
Vishay Siliconix
The Si9140 achieves the 5-µS transient response by
generating a 100-kHz closed-loop bandwidth. This is possible
only by switching above 400 kHz and utilizing an error
amplifier with at least a 10-MHz bandwidth. The Si9140
controller has a 25-MHz unity gain bandwidth error amplifier.
The switching frequency must be at least four times greater
than the desired closed-loop bandwidth to prevent oscillation.
To respond to the stimuli, the error amplifier bandwidth needs
to be at least 10 times larger than the desired bandwidth.
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_OSC and C_OSC values, allowing designers to set the
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:
f_OSC
≈
-------------⁰---^.--⁷---⁵--------------
R_OSC × C_OSC
(at V_DD = 5.0 V)
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%
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.
FIGURE 6. 100-kHz BW Synchronous Buck Converter
The Si9140 solution requires only three 330-µF OS-CON
capacitors on the output of power supply to meet the 10-A
transient requirement. Other converter solutions on the
market with 20- to 50-kHz closed loop bandwidths typically
require two to five times the output capacitance specified
above to match the Si9140’s performance.
The theoretical issues and analytical steps involved in
compensating a feedback network are beyond the scope of
this application note. However, to ease the converter design
for today’s high-performance microprocessors, typical
component values for the feedback network are provided in
Table 1 for various combinations of output capacitance.
Figure 6 shows the Bode plot (frequency domain) of the 2.9-V
converter shown schematically in Figure 1.
TABLE 1. Feedback Network Component Values
Total Output and
Decoupling Capacitance
C4
C5
R5
3 x 330 µF^a . . . . . . . . . . .Os-con
6 x 100 µF^b . . . . . . . . . . .Tantalum
25 x 1 µF^b . . . . . . . . . . . .Ceramic
2 x 330 µF^a . . . . . . . . . . .Os-con
4 x 100 µF^b . . . . . . . . . . .Tantalum
25 x 1 µF^b . . . . . . . . . . . .Ceramic
3 x 330 µF^a . . . . . . . . . . .Tantalum
4 x 100 µF^b . . . . . . . . . . .Tantalum
25 x 1 µF^b . . . . . . . . . . . .Ceramic
5.6 pF 180 pF 240 k
10 pF 220 pF 200 k
10 pF 100 pF 100 k
Notes:
a. Power supply output capacitance.
b. µprocessor decoupling capacitance.
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 capacitors (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.
S-58034—Rev. G, 15-Mar-99
12
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