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RC5035
PRODUCT SPECIFICATION
10
A
DC-to-DC converter, input capacitance can play an impor-
tant role in the load transient response of the RC5035.
In general, the higher the input capacitance, the more charge
storage is available for improving the current transfer
through the top-side FET. A good rule of thumb is that for
each watt of output power that you wish to deliver, there
should be around 10uF of input capacitance. Low “ESR”
capacitors are best suited for this application and can have an
influence on the converter’s efficiency. The input capacitor
should be placed as close to the drain of the top-side FET as
possible to reduce the effect of ringing that can be caused by
large trace lengths.
The ESR rating of a capacitor is a difficult number to pin
down. ESR or Equivalent Series Resistance, is defined at the
resonant impedance of that capacitor. Since the capacitor is
actually a complex impedance device having resistance,
inductance and capacitance, it is quite natural for it to have
an associated 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 with the following equation: ESR = DF/2
p
fC.
Where DF is the capacitor’s dissipation factor, f is the
frequency of measure and C is the capacitance in farads.
With this in mind, calculating the output capacitance cor-
rectly is crucial to the performance of the DC-to-DC con-
verter. The output capacitor determines the overall loop
stability, output voltage ripple, and the transient load
response. The calculation uses the following equation:
Where:
V
R
is the desired output ripple voltage
Schottky Diode Selection
The application circuit diagram shows two schottky diodes,
DS1 and DS2. As it is configured, DS2 provides the function
of bootstrapping the V
CCP
node during startup. It is possible
to cause the output stage to latchup if the V
CCP
supply is
brought up before the other V
CC
supplies of the RC5035.
It is therefore advisable that DS2 be connected. It is impor-
tant in the selection of DS1 and DS2 that they have a low
forward voltage drop as this directly affects the regulator
efficiency. During the off time of the power FET, M1,
the voltage on the inductor will drop until the diode DS1
clamps and conducts the full current in the inductor.
The power in DS1, Vf*IL, is a direct subtraction from the
overall efficiency of the DC-DC converter; therefore, it is
important for DS1 to have a low Vf in order to minimize the
power loss term.
MOSFET Switches
The MOSFET switch in the RC5035 applications circuit is
an N-channel “l(fā)ogic-level” FET. This means that it will be
fully on with a Vgs of 4V. Many manufacturers make logic-
level FETs and the trick is to choose the one with the lowest
Rdson at the given Imax current level. The value of Rdson
directly enters into the efficiency equation as a power loss.
Also influencing the efficiency is the gate charge of the FET
and the clock frequency of the RC5035. At higher clocking
rates the amount of charge needed to be delivered to the FET
is going to lower the overall efficiency.
PCB Layout and Grounding
As is the case with most analog circuitry, good layout
practices are necessary to achieve the optimum in the overall
performance of the DC-to-DC converter. In general, it is
always a good practice to have a tight layout that attempts to
minimize short low inductance wiring to the RC5035.
The use of multilayer PCB is recommended. In particular,
it is recommended to have a continuos ground plane beneath
the circuit, 2oz copper would be preferred in high current
applications. As was stated previously, the current-sense
resistor, R1, should be located as close to the RC5035 as
possible and all voltage and current feedback traces should
be Kelvin connected to the pads of R1. To minimize switch-
ing losses and noise, place M1, M2, L1 and DS2 as close
together as possible. Also try to keep the V
OSW
and V
OL
gate drive signal traces as short as possible. It is recom-
mended that the noisy switching part of the circuit be kept
away from the low current pins on the chip such as IF
BH
,
IF
BL
, V
FBSW
, V
FBSW2
and C
EXT
. Keep the 0.1uF bypass
capacitors as close to the chip pins as possible. All of the
ground pins should be connected to the ground plane directly
under the chip. A sample layout is provided in Figure 4.
C (
m
F)
T
-----------------------V
V
---------V
V
–
(
)
R
I
L
+
è
=