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PRODUCT SPECIFICATION
RC5050
15
Table 9. Schottky Diode Selection Table
Output Filter Capacitors
Optimal ripple performance and transient response are func-
tions of the filter capacitors used. Since the 5V supply of a
PC motherboard may be located several inches away from
the DC-DC converter, input capacitance can play an impor-
tant role in the load transient response of the RC5050. The
higher the input capacitance, the more charge storage is
available for improving the current transfer through the
FET(s). Low “ESR” capacitors are best suited for this type of
application and incorrect selection can influence the con-
verter’s overall performance. The input capacitor should be
placed as close to the drain of the FET as possible to reduce
the effect of ringing caused by long trace lengths.
The ESR rating of a capacitor is a difficult number to quan-
tify. ESR or Equivalent Series Resistance, is defined as the
resonant impedance of the capacitor. Since the capacitor is
actually a complex impedance device having resistance,
inductance and capacitance, it is quite natural for this device
to have a resonant frequency. As a rule, the lower the ESR,
the better suited the capacitor is for use in switching power
supply applications. Many capacitor manufacturers do not
supply ESR data. A useful estimate of the ESR can be
obtained using the following equation:
where:
DF is the dissipation factor of the capacitor
f is the operating frequency
C is the capacitance in farads.
With this in mind, correct calculation of the output capaci-
tance is crucial to the performance of the DC-DC converter.
The output capacitor determines the overall loop stability,
output voltage ripple and load transient response. The calcu-
lation is as follows:
where:
D
V is the maximum voltage deviation due to load
transients
D
T is the reaction time of the power source (Loop
response time of the RC5050), approximately 2
m
s
I
O
is the output load current.
For I
O
= 12.2A (0.8 to 13A) and
D
V = 100mV, the bulk
capacitance required can be approximated as follows:
Input Filter
It is recommended that the design include an input inductor
between the system +5V supply and the DC-DC converter
input described below. This inductor will serve to isolate the
+5V supply from noise occurring in the switching portion of
the DC-DC converter and also to limit the inrush current into
the input capacitors during power up. An inductor value of
around 2.5
m
H is recommended, as illustrated below.
PCB Layout Guidelines and
Considerations
PCB Layout Guidelines
1.
Placement of the MOSFETs relative to the RC5050 is
critical. The MOSFETs (M1 & M2), should be placed
such that the trace length of the HIDRV pin from the
RC5050 to the FET gates is minimized. A long lead
length on this pin will cause high amounts of ringing
due to the inductance of the trace combined with the
large gate capacitance of the FET(s). This noise will
radiate all over the board and will be very difficult to
suppress, especially when the oscillator frequency is
increased.
Figure 7 depicts an example of proper placement of the
MOSFETs in relation to the RC5050 as well as an
example of incorrect placement of the MOSFETs.
In general, all of the noisy switching lines should be
kept away from the quiet analog section of the RC5050.
That is to say, traces that connect to pins 12 and 13
(HIDRV and VCCQP) should be kept far away from the
traces that connect to pins 1 through 5, and pin 16.
Manufacturer
Model #
Philips
PBYR1035
Motorola
MBR2035CT
Motorola
MBR1545CT
Motorola
MBR2015CTL
Conditions
I
F
= 20A; T
j
=25
°
C
I
F
= 20A; T
j
=125
°
C
I
F
= 20A; T
j
=25
°
C
I
F
= 20A; T
j
=125
°
C
I
F
= 15A; T
=25
°
C
I
F
= 15A; T
j
=125
°
C
I
F
= 20A; T
j
=25
°
C
I
F
= 20A; T
j
=150
°
C
Forward Voltage
V
F
< 0.84V
< 0.72V
< 0.84V
< 0.72V
< 0.84V
< 0.72V
< 0.58V
< 0.48V
ESR
2
p
fC
-DF
=
C
m
F
(
)
I
–
D
V
I
O
ESR
′
-------------------------T
=
C
m
F
(
)
I
–
′
D
V
I
O
ESR
-------------------------T
–
2
m
s
′
100mV
12.2A
7.5m
W
′
-------------------12.2
3200
m
F
=
=
=
1000
μ
F, 10V
Electrolytic
0.1
μ
F
65-5050-09
2.5
μ
H
5V
Vin