Application Information: continued
The output current of each regulator can be calculated
from:
ILOAD
0.05
W =
IN = (VOUT(N) - VOUT) / RDROOP(N)
where:
where: VOUT(N) and RDROOP(N) are the output voltage and
droop resistance of a particular regulator and VOUT is the
system output voltage. Output current is the sum of each
regulator’s current:
W = minimum width (in mils) required for proper power
dissipation, and ILOAD Load Current Amps.
The Pentium®II maximum load current is 14.2A.
Therefore:
IOUT = I1 + I2 + … + IN
14.2A
0.05
W =
= 284 mils = 0.7213cm
Current sharing improves with increasing load current.
The increasing voltage drop across the droop resistor due
to increasing load current eventually swamps out the dif-
ferences in regulator output voltages. If a large enough
voltage can be developed across the droop resistors, cur-
rent sharing accuracy will be determined solely by their
matching. To realize the benefits of current sharing, it is
not necessary to obtain perfect matching. Keeping output
currents within +/- 10% is usually acceptable.
Droop Resistor Length Calculation
RDROOP × W × t
0.0030 × 284 × 1.37
L =
=
= 1626 mil = 4.13cm
ρ
717.86
Implementing current sharing using the “Droop
Resistor”
For microprocessor applications, the value of the droop
resistor must be selected to optimize adaptive voltage
positioning, current sharing, current limit and efficiency.
Current sharing is realized by simply connecting the
COMP pins of the respective buck regulators, as shown in
Figure 23.
In addition to improving load transient performance, the
CS5166 V2TM control method allows the droop resistor to
provide the additional capability to easily implement cur-
rent sharing. Figure 23 shows a simplified schematic of
two current sharing synchronous buck regulators.
Each buck regulator’s droop resistor is terminated at the
load. The PWM control signal from each Error Amp is con-
nected together, causing the inner PWM loop to regulate to
a common voltage. Since the voltage at each resistor termi-
nal is the same, this configuration results in equal voltage
being applied across each matched droop resistor. The
result is equal current flowing through each buck regula-
tor. An additional benefit is that synchronization to a com-
mon switching frequency tends to be achieved because
each regulator shares a common PWM ramp signal.
Figure 24 shows operation with no load. In this case, there
is insufficient output voltage ripple across the droop resis-
tors to produce complete synchronization. Duty Cycle is
close to the theoretical 56% (VOUT/VIN) resulting in a
switching frequency of approximately 275kHz.
Figure 25 shows operation with a 30 Amp load.
Synchronization between the two regulators is now
obtained due to increased ripple voltage. Increased losses
cause the V2TM control loop to increase on-time to compen-
sate. This results in a larger duty cycle and a correspond-
ing decrease in switching frequency to 233kHz.
In practice, each buck regulator will regulate to a slightly
different output voltage due to mismatching of the PWM
comparators, slope of the PWM ramp (output voltage rip-
ple), and propagation delays. At light loads, the result can
be very poor current sharing. With zero output current,
some regulators may be sourcing current while others may
be sinking current.
This results in additional power dissipation and lower effi-
ciency than would be obtained by a single regulator. This
is usually not an issue since efficiency is most important
when a supply is fully loaded.
This effect is similar to the difference in efficiency between
synchronous and non-synchronous buck regulators.
Synchronous Buck regulators have lower efficiency at light
loads because inductor current is always continuous, flow-
ing from the load to ground during switch off-time
through the synchronous rectifier. Under full load condi-
tions, the synchronous design is more efficient due to the
lower voltage drop across the synchronous rectifier.
Likewise, the efficiency of droop sharing regulators will be
lower at light loads due to the continuous current flow in
the droop resistors. Efficiency at heavy loads tends to be
higher due to reduced I2R losses.
Trace 1 Output voltage ripple
Trace 2 Buck regulator #1 inductor switching node
Trace 3 Buck regulator #2 inductor switching node
Figure 24: No load waveforms.
17
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
