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RC5102
PRODUCT SPECIFICATION
6
P
Applications Information
Theory of Operation
The RC5102 is a dual output, low dropout voltage regulator
controller intended for use in applications that require dual
voltages at moderate to high output currents. Using external
power transistors and precision trim resistors, a dual adjust-
able voltage regulator can be implemented. The choice of
the component(s) will depend heavily upon the type of appli-
cation. For load currents of up to 5A per output, the use of
low-cost power NPN transistors is sufficient, as illustrated in
figure 1. For applications requiring load currents greater
than 5A, either an NPN darlington pair or an N-channel
MOSFET is recommended due to the increased power
throughput. Because the gate/base drive voltage is derived
from the 12V input supply, the overall dropout voltage will
depend only on the load current and either the Vce,sat of the
NPN transistor or the Ron of the MOSFET. Therefore, very
low dropout voltages can be achieved when the load currents
are sufficiently low and a low Ron MOSFET or a low Vce,sat
transistor is used.
In applications using N-channel MOSFETs, the external R-C
network should be implemented as illustrated in figure 2.
Failure to include this circuitry can result in an unstable gate
drive signal. The actual component values may vary depend-
ing upon the individual application.
Minimum Load
The RC5102 regulator controller is specified over a finite
load range. If the output current becomes too small, leakage
currents will dominate and the output voltage(s) may drift
out of regulation. Maintaining a minimum of 1mA load
current on each output will assure that the load current will
dominate any leakage currents over the operating tempera-
ture range.
Adjustable Output Voltage Design
The RC5102 allows each of the two outputs to be adjusted
between the 1.5V reference voltage and 3.6V using two
external precision resistors. In order to maintain the 1%
output voltage accuracy, a minimum of 100uA should be fed
back to the VFB1 and VFB2 pin to correctly bias the internal
op-amp. For most applications, the sum value of the two
resistors should not exceed approximately 35K
ures 1 and 2, the resistor values can be calculated using the
following equations:
W
. For fig-
V
1
V
REF
R2
R3
+
R3
è
′
=
V
2
V
REF
R4
R5
+
R5
è
′
=
Output Capacitors
For stability and output noise reduction, the use of output
capacitors is required. The required amount of load capaci-
tance will depend upon the actual load current; higher loads
will require larger capacitors. Regardless, an absolute mini-
mum of 1uF is required to maintain stability under all load
conditions. It is not necessary to use expensive low ESR
type capacitors here; standard aluminum electrolytics are
generally sufficient and can actually provide increased
stability over extremely low ESR type devices. If possible,
solid tantalum capacitors should be used in applications
where transient response is critical.
Current Sense resistor
Over current protection is implemented using an external
current sense resistor between the 5V input and the VSC pin
that feeds the collector/drain of the pass transistors. This
resistor will need to carry currents in excess of the sum of the
two loads in order to perform correctly. The RC5102 will
begin to limit the output current to the load(s) by turning off
the output driver when the voltage across the sense resistor
exceeds the nominal 100mV threshold. When this happens,
the output voltage will temporarily go out of regulation.
As the voltage across the resistor increases, the switch will
continue to turn off until the current limit value is reached.
At this point, the RC5102 will continuously deliver the limit
current at a reduced output voltage level. To insure that load
transient conditions do not momentarily cause deregulation
of the output voltage, a 20% margin in the limit voltage is
recommended. Thus the current sense resistor should be
determined by the relationship:
R = 100mV/Ipeak ,
Where Ipeak = Imax 1.2
Since the value of the sense resistor is generally in the 10m
region, care should be taken in the layout of the PCB. Trace
resistance can contribute significant errors. The traces to
the VCC and VSC pins of the RC5102 should be Kelvin
connected to the outside pads of the sense resistor.
W
Dual Power Supply Application
Some CPU power applications such as the Intel Pentium
P55C will require separate voltages for the CPU core and I/O
circuitry. The circuit illustrated in figure 2 addresses this
requirement using a minimum of external components. In
this configuration, both linear regulator outputs can be easily
programmed between 1.5V and 3.6V to meet a variety of
dual voltage requirements. For loads of 4A or lower, the
power dissipation of the external MOSFET should not pose
any thermal design problems if it is chosen wisely. For loads
greater than 10W, an appropriate heatsink must be chosen to
assure the pass transistor remains within its Safe Operating
Area for the desired output current level.