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PRODUCT SPECIFICATION
RC5035
9
A
selected. If this shorting bar is not utilized, the two power
islands will be joined solely via the internal bonding connec-
tions of the CPU and damage to the processor may result.
Design Procedure and Applications
Information
Simple Step-Down Converter
Figure 2. Simple Buck DC-DC Converter
Figure 2 shows a step-down DC-to-DC Converter with no
feedback controller. The derivation of the basic step-down
converter will serve as a basis for the design equations for
the RC5035 in Figure 1. In Figure 2, the basic operation
begins by closing the switch, S1. When S1 is closed the input
voltage V
B
is impressed across the inductor L1. The current
flowing in the inductor is given by the following equation:
I
L
=(V
B
- V
O
)Ton/L; where Ton is the time duration for S1 to
be closed. When S1 is open, the diode will conduct the
inductor current and the output current will be delivered to
the load according to the equation: I
L
=V
O
(T - T
ON
)/L;
where T- T
ON
is the time duration for S1 to be off. By solv-
ing these two equations we can arrive at the basic relation-
ship for the output voltage of a step-down converter:
Selecting the Inductor
The inductor is one of the most critical components to be
selected in the DC-to-DC converter application. The critical
parameters are inductance (L), max DC current (Imax), and
the coil resistance (Rl). The inductor core material is a
critical factor in determining the amount of current that the
inductor will be able to handle. As with all engineering
designs there are trade- offs for various types of inductor
core materials. In general, Ferrites are popular because of
their low cost, low EMI, and high frequency (>500kHz)
characteristics. Molypermalloy powder (MPP) materials
have good saturation characteristics and low EMI with low
hysteresis losses; however they tend to be expensive and are
more efficiently utilized at frequencies below 400kHz.
DC winding resistance is another critical parameter.
In general, the DC resistance should be kept as low as
possible. The power loss in the DC resistance will degrade
the efficiency of the converter by the relationship: Power
Loss = (Io)2*Rl.
The value of the inductor is a function of the switching
frequency (T
ON
) and the maximum inductor current.
The max inductor current can be calculated from the
relationship:
Where:
F
O
is the desired clock frequency
T
ON
is the max on time of the M1 FET
V
D
d is the forward voltage of the schottky diode D1
Then the inductor value can be calculated with the
relationship:
Where:
V
DSON
is the voltage across the drain-source of the M1 FET
when switched on (calculated by RDSon * I
MAX
)
Current-Sense Resistor
The current sense resistor will carry all of the peak current of
the inductor. This current will be more than the designed for
load current. The RC5035 will begin to limit the output cur-
rent to the load by turning off the top-side FET driver when
the voltage across the current-sense resistor exceeds 100mV.
When this happens the output voltage will temporarily go
out of regulation. As the voltage across the resistor becomes
larger, the top-side FET will turn off more and more until the
current limit value is reached and then the RC5035 will con-
tinuously deliver the limit current at a reduced output voltage
level. To insure that load transient conditions do not momen-
tarily cause deregulation of the output voltage, a 20% margin
in the limit voltage is advisable. Thus the resistor should be
set by the relationship:
R = 100 mV/Ipeak
Where:
Ipeak = Imax * 1.33
Since the value of the sense resistor is generally in the mil-
iohm region, care should be taken in the layout of the PCB.
Trace resistance can contribute significant errors. The traces
to the IFBH and IFBL pins of the RC5035 should be Kelvin
connected to the pads of the current-sense resistor as shown
in the sample layout Figure 4. To minimize the influence of
noise the two traces should be run next to each other and the
pins should be bypassed with a .1uF to GND as close to the
device pins as possible.
Filter Capacitors
Good ripple performance and transient response are func-
tions of the filter capacitors. Since the 5V input for a PC
motherboard can be located several inches away from the
65-5035-04
V
O
VB T
ON
T
¤
)
=
I
MAX
2I
F
O
T
ON
OUT
V
OUT
V
D
–
–
-V
è
1
+
--------------------V
=
L
V
----------I
V
MAX
–
è
T
ON
=