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LT4351CMS#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LT4351
13
4351fd
APPLICATIONS INFORMATION
The gate drive amplifier will attempt to regulate the voltage
across the MOSFETs to 15mV. Regulation will be achieved if:
R
DS
<
15mV
2 • I
LOAD
for two MOSFETs and
R
DS
<
15mV
I
LOAD
for a single MOSFET
This requires very low R
DS
values. This may be achieved
by paralleling MOSFETs, but be careful to keep intercon-
nection trace resistance low. In the event that regulation
cannot be achieved, the gate drive amplifier will drive GATE
to its clamp and achieve the best R
DS
possible at that level.
STATUS
The STATUS pin sinks current when the input (V
IN
) is
above output (OUT) by 15mV and GATE is above V
IN
by
0.7V. This will normally indicate that power is being passed
though the MOSFETs.
In the event of a nonfunctional MOSFET, the GATE voltage
will be driven high (to the GATE clamp voltage). If V
IN
is
greater than OUT by more than 0.21V, the FAULT pin will
sink current to signal the potential problem.
There is no direct measurement or confirmation of cur-
rent flowing in the MOSFETs. Current is shared between
sources based on their voltage and series resistance. If
precision load sharing is desired, the LTC4350 may be a
more suitable part.
Redundant Supplies
The LT4351 is an improved solution for ORing redundant
supplies because of its lower forward drop versus con-
ventional diodes. The lower forward drop significantly
improves overall efficiency, improves the voltage tolerance
at the load and provides for a more accurate transition
from supply to supply and more accurate load sharing
between supplies.
ORing can be done either at the load or at the source.
Figure 9 shows some examples. ORing at the load is usu-
ally the safest method since it protects against shorts in
interconnects.
The LT4351 tighter forward-voltage tolerance makes it
easier to balance current between similar supplies using
the droop method. The droop method uses the supply
voltage and series resistance in the power path to provide
load sharing. In this case, size the MOSFET’s R
DS(ON)
low
to allow for regulation.
LT4351
BOARD
LT4351
LOAD
SOURCE 1
BACKPLANE
SOURCE 2
LT4351
BOARD
LT4351
LOAD
4351 F09
SOURCE 1
BACKPLANE
SOURCE 2
Figure 9. Redundant Backplane Supplies
LT4351
14
4351fd
APPLICATIONS INFORMATION
ORing Disparate Supplies
The LT4351 provides an easy solution for connecting
together different types of power sources. Again, because
of the low forward drop, the efficiency of the system is
improved and the voltage transition between supplies is
more accurate. In addition, the undervoltage and overvolt-
age features of the LT4351 provide options for enabling
and disabling the supplies that are not available from a
common diode. Figure 10 shows some examples of con-
necting disparate supplies.
Once V
IN
is greater than 1.2V and V
DD
is up, the part then
operates normally. The UV and OV pins will control the
enabling of the gate driver and once enabled, the V
IN
to
OUT voltage controls MOSFET turn on.
If V
DD
is still being charged when the gate driver turns
on the MOSFET, the GATE pin tracks with the V
DD
in-
crease until it reaches either the gate clamp voltage or
the compliance of the gate driver. If V
DD
is present with-
out V
IN
or OUT, the GATE pin actively sinks low.
Power Dissipation
The internal power dissipation of the LT4351 is comprised
of the following four major components: DC power dis-
sipation from V
IN
, DC power dissipation from V
DD
, the
dissipation in the boost switch including the base drive, and
dynamic power dissipation due to current used to charge
and discharge the MOSFETs. The DC components are:
P
DCVIN
= I
VIN
• V
IN
P
DCVDD
= I
VDD
• V
DD
Figure 11 shows the internal dissipation of the boost
regulator as a function of V
IN
and inductor value. Figure
11 represents the worst-case condition with the regulator
on all the time, which does not occur in normal practice.
Figure 11. P
BOOST(MAX)
LT4351
Isolated System Supply
from Wall Adapter Isolated Battery Backup
Three Source ORing Provides Protection
Against Out of Range Supplies
WALL
ADAPTER
SYSTEM
SUPPLY
LOAD
LT4351
BATTERY
WALL
ADAPTER
LOAD
+
LT4351
BATTERY
LT4351
LT4351
WALL
ADAPTER
LOAD
4351 F10
+
SYSTEM
SUPPLY
Figure 10
V
IN
(V)
0
P
BOOST
(W)
0.20
0.25
L = 10µH
L = 4.7µH
20
4351 F11
0.15
0.10
5
10
15
0.30
Start-Up Considerations
There is no inherent shutdown in the part. As V
IN
ramps
up, the boost regulator starts at about 0.85V and becomes
fully operational by 1.1V. The undervoltage and overvolt-
age comparators become accurate by 1.2V. The gate drive
amplifier keeps GATE low during this period with either
a passive pull-down, a weak active pull-down if OUT is
greater than 0.8V or with the full gate drive sink if V
DD
is
above 2.2V.
LT4351
15
4351fd
APPLICATIONS INFORMATION
Since the boost regulator supplies current for V
DD
, the
current is the V
DD
supply current (3.5mA) plus the aver-
age current to charge the gate. For a gate charge of 50nC
at a 10kHz rate, this adds 0.5mA of current. The power
dissipated by the boost regulator to supply the 4mA is
shown in Figure 12, representing a more typical situation.
Finally, the gate driver dissipates power internally when
charging and discharging the gate of the MOSFETs. This
power depends on the input capacitance of the MOSFETs
and the frequency of charge and discharge. The power
associated with this can be approximated by:
P
GATE
= f
G
• V
DD
• Q
G
• 1–
V
IN
16
where Q
G
is the required gate charge to charge the MOSFET
to the clamp voltage (7.4V) and f
G
is the frequency at which
the gate is charged and discharged. Normally f
G
is low and
the resulting power would be very low. Figure 13 shows
P
GATE
for a 50nC gate charge at a 1kHz rate.
Total power dissipation is the sum of all of P
DCVIN
, P
DCVDD
,
P
BOOST
and P
GATE
. Figure 14 is representative of the total
power dissipation of a typical application at steady state.
The die junction temperature is then computed as:
T
J
= T
A
+ θ
JA
• P
TOTAL
where T
J
is the die junction temperature, T
A
is the ambi-
ent temperature, θ
JA
is the thermal resistance of the part
(120°C/W) and P
TOTAL
is ascertained from the above.
Therefore, a 0.1W power dissipation causes a 12° tem-
perature rise above ambient.
Figure 12. P
BOOST(TYP)
V
IN
(V)
0
P
BOOST
(W)
0.015
0.020
L = 4.7µH
20
4351 F12
0.010
0.005
5
10
15
0.025
Figure 13. P
GATE
vs V
IN
(V
DD
= V
IN
+ 10.7)
V
IN
(V)
0
P
GATE
(W)
0.002
0.003
20
4351 F13
0.001
0
5
10
15
0.004
f
GATE
= 1kHz
Q
G
= 50nC
V
IN
(V)
0
POWER (W)
0.10
0.12
20
4351 F14
0.08
0.06
5
10
15
0.16
0.14
L = 10µH
L = 4.7µH
V
DD
= V
IN
+ 10
0.5mA GATE CURRENT
Figure 14. Total Power (Typical)
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT4351CMS#TRPBF

Mfr. #:
Buy LT4351CMS#TRPBF
Manufacturer:
Analog Devices Inc.
Description:
Power Management Specialized - PMIC MOSFET Diode-OR Cntr
Lifecycle:
New from this manufacturer.
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