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RC5040
PRODUCT SPECIFICATION
10
Output Enable (OUTEN)
The DC-DC converter accepts an open collector signal for
controlling the output voltage. A logic low on the ENABLE
pin disables the output voltage. When disabled, the PWRGD
output is in the low state. This feature is available for the
RC5040 only.
Upgrade Present (UP#)
Intel’s specifications state that the DC-DC converter must
accept an open collector signal, used to indicate the presence
of an upgrade processor. The typical state is high (standard
CPU). When in the low or ground state (OverDrive processor
present), the output voltage must be disabled unless the
converter can supply the OverDrive processor’s specifica-
tions. When disabled, the PWRGD output must be in the low
state. Since the RC5040 can supply the OverDrive processor
specifications, the UP# signal is not required.
Over-Voltage Protection
The RC5040 provides a constant monitor of the output
voltage for protection against overvoltage conditions. If the
voltage at the VFB pin exceeds 20% of the selected program
voltage, an overvoltage condition will be assumed, and the
RC5040 will disable the output drive signal to the
MOSFET(s).
Short Circuit Protection
A current sense methodology is implemented to disable the
output drive signal to the MOSFET(s) when an over-current
condition is detected. The voltage drop created by the output
current flowing across a sense resistor is presented to an
internal comparator. When voltage developed across the
sense resistor exceeds the comparator threshold voltage,
the RC5040 will disable the output drive signal to the
MOSFET(s).
The DC-DC converter returns to normal operation after the
fault has been removed, for either an overvoltage or a short
circuit condition.
Oscillator
The RC5040 oscillator section is implemented using a
fixed current capacitor charging configuration. An external
capacitor (CEXT) is used to preset the oscillator frequency
between 200KHz and 1MHz. This scheme allows maximum
flexibility in setting the switching frequency as well as
choosing external components.
In general, a lower operating frequency will increase the
peak ripple current flowing in the output inductor, and thus
require the use of a larger inductor value. Operation at lower
frequencies also increases the amount of energy storage that
must be provided by the bulk output capacitors during load
transients due to the slower loop response of the controller.
As the operating frequency is increased, the user should note
that the efficiency losses due to switching are relatively fixed
per switching cycle. Therefore, as the switching frequency is
increased, so is the contribution toward efficiency due to
switching losses.
Careful analysis of the RC5040 DC-DC controller has
resulted in an optimal operating frequency of 650KHz,
which allows the use of smaller inductive and capacitive
components while maximizing peak efficiency under all
operating conditions.
Design Considerations and
Component Selection
MOSFET Selection
This application requires N-channel
Logic Level
Enhance-
ment Mode Field Effect Transistors. Desired characteristics
are as follows:
Low Static Drain-Source On-Resistance, R
DS(on)
< 37
m
W
(lower is better)
Low gate drive voltage, V
GS
< 4V
Power package with low thermal resistance
Drain current rating of 20A minimum
Drain-Source voltage > 15V
The on-resistance (R
DS(ON)
) is the primary parameter for
MOSFET selection. The on-resistance determines the power
dissipation of the MOSFET and therefore significantly
affects the efficiency of the DC-DC Converter. Table 5
presents a list of suitable MOSFETs for this application.