SI9140 Datasheet, PDF (11/15 Pages) Vishay Siliconix – SMP Controller For High Performance Process Power Supplies

Si9140
Vishay Siliconix
FIGURE 4. 1.5-V Converter for GTL⁺ Bus @ 5 A
Figure 4 is a schematic diagram of a converter which
produces 1.5 V for a GTL bus.
Each of these dc-to-dc converters has four major sections:
• PWM Controller-regulates the output voltage
• Switch and Synchronous Rectification MOSFETs-delivers
the power to the load
• Inductor-filters and stores the energy
• Input/Output Capacitor-filters the ripple
The functions of each circuit are explained in detail below.
Design equations are provided to optimize each application
circuit.
(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.
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-mΩ 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
FIGURE 5. Voltage Mode Logic and Timing Diagram
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.
FaxBack 408-970-5600, request 70026
www.siliconix.com
S-58034—Rev. G, 15-Mar-99
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