Datasheet

LTC4370
9
4370f
High availability systems often employ parallel-connected
power supplies or battery feeds to achieve redundancy
and enhance system reliability. ORing diodes have been
a popular means of connecting these supplies at the point
of load. System uptime improves further if these paralleled
supplies also share the load current.
applicaTions inForMaTion
Figure 1. 5V Diode-OR Load Share with Status Light
Current Sharing Characteristic
The LTC4370 load shares the two supplies by dropping
their voltage difference across the MOSFETs in series
with them (see Figure 1). The MOSFET on the lower sup-
ply drops the minimum servo voltage V
FR(MIN)
(12mV
or 25mV depending on supply voltage levels), while the
other MOSFET drops V
FR(MIN)
plus the supply voltage
difference. This equalizes both the OUT pin voltages, and
by Ohm’s law the current that flows through the sense
resistors. Figure2a illustrates this. It shows the higher
supply’s MOSFET forward voltage drop, V
FWD
, increasing
to compensate the supply difference up to ±500mV.
The upper limit of the servo command adjustment is the
minimum servo plus the RANGE pin voltage (500mV in
Figure 2). Hence, when the two supplies differ by a volt-
age equal to V
RANGE
, the higher supply’s V
FWD
is pinned
at the maximum servo voltage V
FR(MAX)
. If the supplies
diverge by more than V
RANGE
, the OUT pin voltages start
Figure 2. Load Sharing Characteristics
4370 F02
(2b) High R
DS(ON)
: Fully-On MOSFET
Drops 125mV at Half Load
–400mV 400mV0
525mV
25mV
MOSFET
FORWARD
DROP
V
IN1
– V
IN2
V
FWD1
V
FWD2
0.5I
L
• R
DS(ON)
125mV
100mV + I
L
• R
S
–400mV 400mV0
1
0
NORMALIZED
CURRENT
V
IN1
– V
IN2
I
2
I
1
I
1
I
2
0.5
SHARING CAPTURE RANGE
±∆V
IN(SH)
DRAWING IS NOT TO SCALE!
V
FR(MIN)
I
L
• R
DS(ON)
2R
S
+ R
DS(ON)
(2a) Low R
DS(ON)
: Can Servo 25mV Minimum
Forward Regulation Voltage at Half Load
–500mV 500mV0
525mV
25mV
MOSFET
FORWARD
DROP
MAXIMUM M2
MOSFET POWER
DISSIPATION
V
IN1
– V
IN2
V
FWD1
V
FWD2
V
FR(MAX)
V
FR(MIN)
I
L
• R
S
–500mV 500mV0
1
0
NORMALIZED
CURRENT
V
IN1
– V
IN2
I
2
I
1
I
1
I
2
0.5
SHARING CAPTURE RANGE ±∆V
IN(SH)
V
RANGE
= 500mV V
RANGE
= 500mV
= 2R
S
1
SLOPE
= 2R
S
1
SLOPE
MAXIMUM M1
MOSFET POWER
DISSIPATION
M2
SUM85N03-06P
D1: RED LED
LN1251C
SHARE
OFF
D1
M1
SUM85N03-06P
C2
39nF
GATE1CPO1
CPO2
GND
EN1
EN2
RANGE
4370 F01
V
INA
5V
V
INB
5V
C
VCC
0.1µF
R3
30.1k
R4
820Ω
C1
39nF
V
IN1
V
CC
FETON1
COMP
FETON2
OUT1
OUT2
GATE2V
IN2
LTC4370
R1
2.5mΩ
R2
2.5mΩ
OUT
10A
C
C
0.18µF