RC5042 View Datasheet(PDF) - Fairchild Semiconductor
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RC5042 Datasheet PDF : 20 Pages
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PRODUCT SPECIFICATION
RC5042
High Current Output Drivers
The RC5042 contains two identical high current output
drivers which utilize high speed bipolar transistors arranged
in a push-pull configuration. Each driver is capable of deliv-
ering 1A of current in less than 100ns. Each driver's power
and ground are separated from the overall chip power and
ground for additional switching noise immunity. The HIDRV
driver has a power supply, VCCQP, which is boot-strapped
from a flying capacitor as illustrated in Figure 2. Using this
configuration, C12 is alternately charged from VCC via the
Schottky diode DS2 and then boosted up when the FET is
turned on. This scheme provides a VCCQP voltage equal to
2•VCC – VDS(DS2), or approximately 9.5V with VCC =
5V. This voltage is sufficient to provide the 9V gate drive to
the external MOSFET required in order to achieve a low
RDS(ON). Since the low side synchronous FET is referenced
to ground (refer to Figure 3), there is no need to boost the
gate drive voltage and its VCCP power pin can be tied to
VCC. See Typical Operating Characteristics for typical full
load VCCQP waveform.
Internal Voltage Reference
The reference included in the RC5042 is a 1.24V precision
band-gap voltage reference. The internal resistors are pre-
cisely trimmed to provide a near zero temperature coefficient
(TC). Added to the reference input is the resulting output
from an integrated 4-bit DAC. The DAC is provided in
accordance with the Pentium Pro specification guideline,
which requires the DC-DC converter output to be directly
programmable via a 4-bit voltage identification (VID) code.
This code will scale the reference voltage from 2.0V (no
CPU) to 3.5V in 100mV increments. For guaranteed stable
operation under all loading conditions, a 10KW pull-up resis-
tor and 0.1mF of decoupling capacitance should be con-
nected to the VREF pin.
Power Good
The RC5042 Power Good function is designed in accordance
with the Pentium Pro DC-DC converter specification and
provides a constant voltage monitor on the VFB pin. The
circuit compares the VFB signal to the VREF voltage and
outputs an active-low interrupt signal to the CPU should the
power supply voltage exceed ±7% of its nominal setpoint.
The Power Good flag provides no other control function to
the RC5042.
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 con-
verter can supply the OverDrive processor’s specifications.
When disabled, the PWRGD output must be in the low state.
Since the RC5042 can supply the OverDrive processor spec-
ifications, the UP# signal is not required.
Over-Voltage Protection
The RC5042 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 RC5042 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 RC5042 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 RC5042 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 RC5042 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.
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