
RC5052
19
P
The output capacitance should also include a number of small
value ceramic capacitors placed as close as possible to the
processor; 0.1μF and 0.01μF are recommended values.
Input Filter
The DC-DC converter design may include an input inductor
between the system +5V supply and the converter input as
shown in Figure 8. This inductor serves to isolate the +5V
supply from the noise in the switching portion of the DC-DC
converter, and to limit the inrush current into the input capac-
itors during power up. A value of 2.5μH is recommended.
It is necessary to have some low ESR aluminum electrolytic
capacitors at the input to the converter. These capacitors
deliver current when the high side MOSFET switches on.
Figure 8 shows 3 x 1000μF, but the exact number required
will vary with the speed and type of the processor. For the
top speed Katmai and Coppermine, the capacitors should be
rated to take 9A and 6A RMS of ripple current respectively.
Capacitor ripple current rating is a function of temperature,
and so the manufacturer should be contacted to find out the
ripple current rating at the expected operational temperature.
For details on the design of an input filter, refer to Applica-
tions Bulletin AB-15.
Figure 8. Input Filter
Active Droop
The RC5052 includes active droop: as the output current
increases, the output voltage drops. This is done in order to
allow maximum headroom for transient response of the con-
verter. The current is sensed by measuring the voltage across
the high-side MOSFET during its on time. Note that this makes
the droop dependent on the temperature of the MOSFET.
However, when the formula given for selecting R
S
(current
limit) is used, there is a maximum droop possible (-40mV),
and when this value is reached, additional drop across the
MOSFET will not cause any increase in droop—until current
limit is reached.
Additional droop can be added to the active droop using a
discrete resistor (typically a PCB trace) outside the control
loop, as shown in Figure 1. This is typically only required for
the most demanding applications, such as for the next gener-
ation Intel processor (tolerance = +40/-70mV), as shown in
Figure 1.
PCB Layout Guidelines
¥ Placement of the MOSFETs relative to the RC5052 is
critical. Place the MOSFETs such that the trace length of
the HIDRV and LODRV pins of the RC5052 to the FET
gates is minimized. A long lead length on these pins will
cause high amounts of ringing due to the inductance of the
trace and the gate capacitance of the FET. This noise radiates
throughout the board, and, because it is switching at such
a high voltage and frequency, it is very difTcult to suppress.
¥ In general, all of the noisy switching lines should be kept
away from the quiet analog section of the RC5052. That
is, traces that connect to pins 9, 10, 11, 12 and 13 (LODRV,
VCCP, VCCQP, HIDRV and SW) should be kept far away
from the traces that connect to pins 4 through 6, and pin 14.
¥ Place the 0.1μF decoupling capacitors as close to the
RC5052 pins as possible. Extra lead length on these
reduces their ability to suppress noise.
¥ Each VCC and GND pin should have its own via to the
appropriate plane. This helps provide isolation between pins.
¥ Place the MOSFETs, inductor, and Schottky as close
together as possible for the same reasons as in the Trst
bullet above. Place the input bulk capacitors as close to
the drains of the high side MOSFETs as possible. In
addition, placement of a 0.1μF decoupling cap right on
the drain of each high side MOSFET helps to suppress
some of the high frequency switching noise on the input
of the DC-DC converter.
¥ Place the output bulk capacitors as close to the CPU as
possible to optimize their ability to supply instantaneous
current to the load in the event of a current transient.
Additional space between the output capacitors and the
CPU will allow the parasitic resistance of the board traces
to degrade the DC-DC converters performance under
severe load transient conditions, causing higher voltage
deviation. For more detailed information regarding
capacitor placement, refer to Application Bulletin AB-5.
¥ A PC Board Layout Checklist is available from Fairchild
Applications. Ask for Application Bulletin AB-11.
PC Motherboard Sample Layout and Gerber File
A reference design for motherboard implementation of the
RC5052 along with the PCAD layout Gerber file and silk
screen can be obtained from our marketing department at
650-968-9211 x7624.
RC5052 Evaluation Board
Fairchild provides an evaluation board to verify the system
level performance of the RC5052. It serves as a guide to per-
formance expectations when using the supplied external
components and PCB layout. Please call the marketing
department at 650-968-9211 x7624 for an evaluation board.
Additional Information
For additional information contact Fairchild Semiconductor’s
Analog & Mixed Signal Products Group Marketing Department
at 650-968-9211 x7624.
2.5
μ
H
5V
0.1
μ
F
1000
μ
F, 10V
Electrolytic
Vin