ltc1647-2 Linear Technology Corporation, ltc1647-2 Datasheet - Page 10

• Manufacturer Part Number: ltc1647-2
• Description: Dual Hot Swap Controllers
• Manufacturer: Linear Technology Corporation

LTC1647-1/LTC1647-2/LTC1647-3
APPLICATIO S I FOR ATIO
The circuit breaker trip voltage has a tolerance of 20%;
combined with a 5% sense resistor, the total tolerance is
25%. Therefore, calculate R1 based on a trip current I
of no less than 125% of the maximum operating current.
Do not neglect the effect of ripple current, which adds to
the maximum DC component of the load current. Ripple
current may arise from any of several sources, but the
worst offenders are switching supplies.
A switching regulator on the load side will attempt to draw
some ripple current from the backplane and this current
passes through the sense resistor. Similarly, output ripple
from a switching regulator supplying the backplane will
flow through the sense resistor and into the load capacitor.
Minimize the effects of ripple current by either filtering the
V
inductance of 1 H to 10 H inserted between Q1 and C
is adequate ripple current suppression in most cases.
Alternatively, a filter, consisting of R3 and C3(Figure 3),
simply filters the ripple component from the SENSE pin at
the expense of response time. The added delay is given by
10
OUT
t
DELAY
FAULT
line or adding an RC filter to the SENSE pin. A series
V
CC
V
CC
= – R3•C3•ln[1 – (V
– V
V
V
SENSE
FAULT
GATE
R3
10k
ON
V
Figure 1. Supply Control Circuitry
ON
Figure 2. Current Fault Timing
2
3
8
ON1
FAULT
GND
U
V
CC
1
0.01
R1
LTC1647-3
U
SENSE
CB
15
/R1 – I
IRF7413
t
FAULT
t
GATE
RESET
Q1
W
13
R2
10
AV
)/(I
+
1647-1/2/3 F02
PK
C1
10nF
1647-1/2/3 F01
– I
U
C
LOAD
V
OUT
AV
)]
LOAD
TRIP
Power MOSFET Selection
Power MOSFETs are classified into two catagories: stan-
dard MOSFETs (R
level MOSFETs (R
absolute maximum rating for V
standard MOSFETs. The maximum rating for logic-level
MOSFETs is lower and ranges from 8V to 16V depending
on the manufacturer and specific part number. Some
logic-level MOSFETs have a 20V maximum V
LTC1647 is primarily targeted for standard MOSFETs; low
supply voltage applications should use logic-level
MOSFETs. GATE overdrive as a function of V
trated in the Typical Performance Curves. If lower GATE
overdrive is desired, connect a diode in series with a Zener
between GATE and V
shown in Figure 4.
The R
make V
V
restricts the choice of MOSFETs to very low R
higher V
relaxed. MOSFET package dissipation (P
restrict the value of R
CB
= 0.1V yields 3% error at maximum load current. This
V
DS(ON)
CC
DS
V
CC
CC
V
Figure 3. Filtering Current Ripple/Glitches
a small percentage of V
CC
of the external pass transistor must be low to
voltages, the R
0.01
10nF
*USER SELECTED VOLTAGE CLAMP
1N4688 (5V)
1N4692 (7V): LOGIC-LEVEL MOSFET
1N4695 (9V)
1N4702 (15V): STANDARD-LEVEL MOSFET
R1
C3
Figure 4. Optional Gate Clamp
LTC1647
SENSE
D1*
DS(ON)
R1
DS(ON)
R3
1.5k
1N4148
DS(ON)
CC
D2
IRF7413
GATE
Q1
specified at V
or between GATE and V
R2
10
specified at V
Q1
.
DS(ON)
+
1N4148
D2
GS
CC
C
C1
10nF
LOAD
V
. At V
OUT
requirement can be
is typically 20V for
GS
D4*
I
I
I
t
= 10V) and logic-
PK
AV
TRIP
DELAY
CC
GS
D
= 7.5A
= 2.5A
1647-1/2/3 F04
= V
GS
= 3.3V, V
and T
= 10 s
V
CB
= 5V). The
OUT
1647-1/2/3 F03
CC
rating. The
/R1 = 5A
DS(ON)
is illus-
J
OUT
) may
DS
. At
as
+