Datasheet

LTC4370
11
4370f
in the absence of load current the differential input volt-
age to the error amplifier is zero and the COMP current is
g
m(EA)
• V
EA(OS)
. Before sharing can start, the COMP
voltage has to slew towards its operating point of 0.7V
(when V
IN1
< V
IN2
) or 1.24V (V
IN1
> V
IN2
). This delay is
determined by the differential input signal to the error
amplifier (which is ΔV
OUT
= OUT1 – OUT2 = (I
1
− I
2
) • R
S
),
its g
m
and the COMP capacitor value. Depending on how
the currents split before converging, the delay can vary
from 1 to 5 times:
C
C
• ΔV
COMP
g
m(EA)
• I
L
• R
S
Figure 4a shows the case where a 5.1V V
IN1
is turned
on while V
IN2
is at 4.9V supplying 10A. Initially, COMP
is railed low to 0.1V since ΔV
OUT
(−I
2
• R
S
) is negative,
and needs to rise to 1.24V as the final V
IN1
is higher than
V
IN2
. With V
IN1
off, ΔV
IN
is large and negative, causing the
forward regulation voltage of the second supply V
FR2
to be
folded back to the minimum V
FR(MIN)
(travelling from left
to right in Figure 2a). As the ΔV
IN
magnitude decreases,
V
FR2
rises to the maximum V
FR(MAX)
, lowering I
2
and the
load voltage. COMP is around 0.7V when V
FR2
is being
adjusted. When COMP reaches 1.24V, V
FR2
is kept at the
minimum and V
FR1
is adjusted appropriately to compensate
for the 0.2V of ΔV
IN
. The sharing closure is smoother for
the case where V
IN1
< V
IN2
since COMP only has to slew
to 0.7V to lower V
FR2
(Figure 4b).
MOSFET Selection
The LTC4370 drives N-channel MOSFETs to conduct the
load current. The important parameters of the MOSFET
are its maximum drain-source voltage BV
DSS
, maximum
gate-source voltage
V
GS(MAX)
, on-resistance R
DS(ON)
, and
maximum power dissipation P
D(MAX)
.
If an input is connected to ground, the full supply voltage
can appear across the MOSFET. To survive this, the BV
DSS
must be higher than the supply voltages. The V
GS(MAX)
rating of the MOSFET should exceed 14V since that is the
upper limit of the internal GATE to V
IN
clamp.
To obtain the maximum sharing capture range, the R
DS(ON)
should be low enough for the servo amplifier to regulate the
minimum forward regulation voltage across the MOSFET
while it’s conducting half of the load current. If it cannot,
the gate voltage will be railed high. Hence, the R
DS(ON)
value
in the MOSFET data sheet should be looked up for 10V or
4.5V gate drive depending on the V
IN
voltage. Since the
OUT voltages are equal, the breakpoint for exact sharing
in the higher R
DS(ON)
case is:
ΔV
IN(SH)
= V
FR(MAX)
– 0.5I
L
• R
DS(ON)
(2)
applicaTions inForMaTion
Figure 4. Start of Sharing at V
IN1
Turn-On
(4a) V
IN1
> V
IN2
(4b) V
IN1
< V
IN2
CURRENT
5A/DIV
VOLTAGE
2V/DIV
4370 F04a
25ms/DIV
V
IN1
= 5.1V
V
IN2
= 4.9V
I
L
= 10A
I
2
I
1
OUT
COMP
(1V/DIV)
V
IN1
CURRENT
5A/DIV
VOLTAGE
2V/DIV
4370 F04b
25ms/DIV
I
2
I
1
OUT
V
IN1
V
IN1
= 4.9V
V
IN2
= 5.1V
I
L
= 10A
COMP
(0.5V/DIV)