
RC5040
PRODUCT SPECIFICATION
16
Schottky Diode Selection
The application circuit diagrams of Figures 1 and 2 show two
Schottky diodes, DS1 and DS2. In synchronous mode, DS1
is used in parallel with M3 to prevent the lossy diode in the
FET from turning on. DS2 serves a dual purpose. As config-
ured, it allows the VCCQP supply pin of the RC5040 to be
bootstrapped up to 9V using capacitor C12. When the lower
MOSFET M3 is turned on, one side of capacitor C12 is
connected to ground while the other side of the capacitor is
being charged up to voltage VIN - VD through DS2. The
voltage that is then applied to the gate of the MOSFET is
VCCQP – VSAT, or typically around 9V. A vital selection
criteria for DS1 and DS2 is that they exhibit a very low
forward voltage drop, as this parameter can directly affect the
regulator efficiency. In non-synchronous mode, DS1 is used
as a flyback diode to provide a constant current path for the
inductor when M1 is turned off. Table 9 lists several suitable
Schottky diodes. Note that the MBR2015CTL has a very low
forward voltage drop. This diode is most ideal for application
where output voltage is required to be less than 2.8V.
Table 9. Schottky Diode Selection Table
Output Filter Capacitors
Optimal ripple performance and transient response are
functions 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 RC5040.
The higher the input capacitance, the more charge storage is
available for improving the current transfer through the FET.
Low “ESR" capacitors are best suited for this type of applica-
tion and can influence the converter's efficiency if not
chosen carefully. 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
quantify. 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
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
j
= 25
°
C
I
F
= 15A; T
j
= 125
°
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
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 load
transient
D
T is reaction time of the power source (Loop
response time of the RC5040) and it is
approximately 8
m
s
I
O
is the output load current
For I
O
= 10A, and
D
V = 75mV, the bulk capacitor required
can be approximated as follows:
Input filter
We recommend 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 to also limit the inrush current into
the input capacitors on power up. A value of around 2.5
m
H is
recommended.
Figure 8. Input Filter
ESR
p
fC
2
=
C
m
F
(
)
I
D
T
′
′
D
I
O
ESR
----V
=
C
m
F
(
)
I
–
′
D
V
I
O
ESR
-------------------------T
–
8
m
s
′
′
75mV
10A
5m
W
-------------10A
3200
m
F
=
=
=
1000
μ
F, 10V
Electrolytic
0.1
μ
F
65-5040-16
2.5
μ
H
5V
Vin