20
LTC1430A
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
the sense resistors should be well matched. This is
accomplished by using trace resistors that are laid out
symmetrically. Since they are formed of the same material
and processed identically, they will inherently match very
well. Note that the absolute value of these resistors is not
important; only the match between them is of concern.
The second issue is related to the reference point for the
two sense voltages. In order to avoid the need to use a true
differential amplifier to measure input current, the circuit
is configured such that the input side of these resistors
must be at the exact same potential. If the layout is not
configured this way, the current sharing accuracy will
prove disappointing. With only 0.2Ω sense resistors, a
seemingly small error will produce a rather large current
mismatch between channels.
The last issue is related to having a very noisy sense
voltage. The current waveshape at the input to a buck
regulator is trapezoidal. Therefore, the sense amplifier
must integrate the two current measurements in order that
the average input currents be compared. The two-stage
RC filter on the sense amplifier provides an adequately
clean signal for the share circuit to operate correctly. High
speed is not required in the current sense loop. In balanced
operation any offsets in the slave regulator are dialed out
by the sense amplifier. If a sudden load change should
occur, both regulators will respond immediately and in the
right direction. If there are any gain differences in the two
loops there will need to be a small correction in the current
share error voltage. This can occur over a relatively long
time period with no adverse effects. As such, the share
amplifier’s bandwidth is on the order of a few hundred Hz,
ensuring good noise immunity.
Figure 18 demonstrates the high efficiency achieved with
this two-phase converter. An efficiency > 90% is realized
from a few amperes up to 30A. In theory and in practice,
this multiphase approach can be extended to even higher
current and output power levels. Consult Linear Technol-
ogy for further details.
Figure 18. Low Voltage 30A Power Supply Efficiency
LOAD CURRENT (A)
0
50
EFFICIENCY (%)
60
70
80
5
10
15 20
1430 F18
25
90
100
55
65
75
85
95
30
