Fairchild SEMICONDUCTOR RC5051 - Manual (Page 4)

AN50 APPLICATION NOTE
4
The HIDRV driver has a power supply, VCCQP, supplied
from a 12V source as illustrated in Figure 2. The resulting
voltage is sufficient to provide the gate to source voltage to
the external MOSFET that is required to achieve a low
R
DS,ON
. Since the low side synchronous FET is referenced
to ground, there is no need to boost the gate drive voltage,
and its VCCP power pin can be tied to VCC.
Internal Voltage Reference
The reference included in the RC5050 and RC5051 is a pre-
cision band-gap voltage reference. The internal resistors are
precisely trimmed to provide a near zero temperature coeffi-
cient (TC). Added to the reference input is the resulting out-
put from an integrated 5-bit DAC—provided in accordance
to the Pentium Pro specification guidelines. These guidelines
require the DC-DC converter output to be directly program-
mable via a 4-bit voltage identification (VID) code. This
code scales the reference voltage from 2.0V (no CPU) to
3.5V in 100mV increments. To target future generations of
low-voltage processors, the RC5050 and RC5051 incorpo-
rate a VID4 pin to allow additional programmability between
1.3V and 2.05V. For guaranteed stable operation under all
operating conditions, a 0.1
µ
F of decoupling capacitance
should be connected to the VREF pin. No load should be
imposed on this pin.
Power Good (PWRGD)
The RC5050 and RC5051 Power Good function is designed
in accordance with the Pentium Pro DC-DC converter speci-
fication to provide a constant voltage monitor on the VFB
pin. The circuit compares the VFB signal to the VREF volt-
age and outputs an active-low interrupt signal to the CPU
when the power supply voltage exceeds
±
12% of nominal.
The Power Good flag provides no other control function to
the RC5050 or the RC5051.
Output Enable (OUTEN)
The DC-DC converter accepts an open collector signal for
controlling the output voltage. The low state disables the out-
put voltage. When disabled, the PWRGD output is in the low
state.
Upgrade Present (UP#)
Intel specifications state that the DC-DC converter should
accept an open collector signal, used to indicate the presence
of an upgrade processor. The typical state is high (that is, a
standard processor is in the system). When in the low or
ground state (an OverDrive processor is present), the output
voltage must be disabled unless the converter can supply the
requirements of the OverDrive processor. When disabled, the
PWRGD output must be in the low state. Because the
RC5050 and RC5051 can supply the requirements of the
OverDrive processor, the #UP signal is not required.
Over-Voltage Protection
The RC5050 and RC5051 constantly monitor the output
voltage for protection against over voltage conditions. If the
voltage at the VFB pin exceeds 20% of the selected program
voltage, an over-voltage condition is assumed and the chip
disables the output drive signal to the external MOSFET(s).
Figure 2. RC5051 Block Diagram
DIGITAL
CONTROL
–
+
1.24v
REFERENCE
5-BIT
DAC
65-5051-01
POWER
GOOD
OSC
PWRGD
VREF
VID0
VID1
VID2 RSEL
VID3
–
+
–
+
–
+
+5V
VO
RC5051
+12V