RC5042 View Datasheet(PDF) - Fairchild Semiconductor
Part Name
Description
Manufacturer
RC5042 Datasheet PDF : 20 Pages
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PRODUCT SPECIFICATION
RC5042
RC5042 Short Circuit Current Characteristics
The RC5042 has a short circuit current characteristic
that includes a hysteresis function that prevents the DC-DC
converter from oscillating in the event of a short circuit.
A typical V-I characteristic of the DC-DC converter output is
presented in the Typical Operating Characteristics section,
page 5. The converter performs with a normal load regulation
characteristic until the voltage across the resistor reaches the
internal short circuit threshold of 120mV. At this point, the
internal comparator trips and sends a signal to the controller
to turn off the gate drive to the power MOSFET. This causes
a drastic reduction in the output voltage as the load regula-
tion collapses into the short circuit mode of control. The out-
put voltage will not return to the normal load characteristic
until the output short circuit current is reduced to within the
safe range for the DC-DC converter.
Schottky Diode Selection
The application circuit diagram of Figure 1 shows two Schot-
tky 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 configured, it
allows the VCCQP supply pin of the RC5042 to be boot-
strapped 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 appli-
cation where output voltage is required to be less than 2.8V.
Table 9. Schottky Diode Selection Table
Manufacturer
Forward Voltage
Model #
Conditions
VF
Philips
PBYR1035
IF = 20A; Tj = 25°C
IF = 20A; Tj = 125°C
< 0.84V
< 0.72V
Motorola
IF = 20A; Tj = 25°C
MBR2035CT IF = 20A; Tj = 125°C
< 0.84V
< 0.72V
Motorola
IF = 15A; Tj = 25°C
MBR1545CT IF = 15A; Tj = 125°C
< 0.84V
< 0.72V
Motorola
IF = 20A; Tj = 25°C
MBR2015CTL IF = 20A; Tj = 150°C
< 0.58V
< 0.48V
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 RC5042.
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 appli-
cation 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
supply ESR data. A useful estimate of the ESR can be
obtained using the following equation:
ESR = 2---D-p---f-F--C--
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:
C(mF) = -D----V-----I–--O---I--´O-----D´----T-E----S---R---
Where DV is the maximum voltage deviation due load
transient
DT is reaction time of the power source (Loop
response time of the RC5042) and it is
approximately 8ms
IO is the output load current
For IO = 10A, and DV = 75mV, the bulk capacitor required
can be approximated as follows:
C(mF) = -D----V-----I–--O---I--´O-----D´----T-E----S---R--- = 7---5----m-----V-1---0--–--A--1---0´---A--8----m´---s--5---m-----W---- = 3200mF
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. We recommend a value of
around 2.5mH.
15
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