SI9140DQ VISHAY [Vishay Siliconix], SI9140DQ Datasheet - Page 11

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

The functions of each circuit are explained in detail below.
Design equations are provided to optimize each application
circuit.
PWM Controller
There are generally two types of controllers, voltage mode or
current mode. In voltage mode control, an error voltage is
generated by comparing the output voltage to the reference
voltage. The error voltage is then compared to an artificial
ramp, and the result is the duty cycle necessary to regulate the
output voltage. In current mode, an actual inductor current is
used, in place of the artificial ramp, to sense the voltage across
the current sense resistor.
The logic and timing sequence for voltage mode control is
shown in Figure 5. The Si9140 offers voltage mode control,
which is better suited for applications requiring both fast
transient response and high output current.
Current mode control requires a current sense resistor to
monitor the inductor current. A 10-mW sense resistor in a 10-A
design will dissipate 1 W, decreasing efficiency by 3.5%. Such
a design would require a 2-W resistor to satisfy derating criteria,
besides requiring additional board space. Voltage mode control
is a second-order LC system and has a faster natural transient
response compared to current mode control (first-order RC
system). Current mode has the advantage of providing an
inherently good line regulation. But the situations where line
voltage is fixed, as in the point-of-use conversion for
microprocessors, this feature is wasted. Current mode control
also provides automatic pulse-to-pulse current limiting. This
feature requires a current sense resistor as stated above. These
characteristics make voltage mode control ideal for high-end
microprocessor power supplies.
Document Number: 70026
S-40699—Rev. H, 19-Apr-04
COMP
OSC
D
D
FIGURE 5. Voltage Mode Logic and Timing Diagram
S
R
The error amplifier of the PWM controller plays a major role in
determining the output voltage, stability, and the transient
response of the power supply. In the Si9140, the non-inverting
input of the error amplifier is available for use with an external
precision reference for tighter tolerance regulation. With a
two-pair lead-lag compensation network, it is easy to create a
stable 100-kHz closed loop converter with the Si9140 error
amplifier.
The Si9140 achieves the 5-mS 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.
The Si9140 solution requires only three 330-mF 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.
FIGURE 6. 100-kHz BW Synchronous Buck Converter
Frequency (Hz)
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