LTC1647 LINER [Linear Technology], LTC1647 Datasheet - Page 23

• Manufacturer Part Number: LTC1647
• Description: Hot Swap Controller
• Manufacturer: LINER [Linear Technology]

OPERATION
The maximum load current that trips the circuit breaker
is given in Equation 12.
where
For example:
If a sense resistor with 7mΩ ±5% R
limiting, the nominal trip current I
Equations 11 and 12, I
9.02A respectively.
For proper operation and to avoid the circuit breaker trip-
ping unnecessarily, the minimum trip current (I
must exceed the circuit’s maximum operating load current.
For reliability purposes, the operation at the maximum
trip current (I
necessary, two resistors with the same R
nected in parallel to yield an R
the circuit requirements.
POWER MOSFET SELECTION CRITERIA
To start the power MOSFET selection process, choose the
maximum drain-to-source voltage, V
mum drain current, I
rating must exceed the maximum input supply voltage
(including surges, spikes, ringing, etc.) and the I
rating must exceed the maximum short-circuit current in
the system during a fault condition. In addition, consider
three other key parameters: 1) the required gate-source
(V
to-source on resistance, R
junction temperature rating of the MOSFET.
Power MOSFETs are classified into three categories:
standard MOSFETs (R
logic-level MOSFETs (R
sub-logic-level MOSFETs (R
The absolute maximum rating for V
I
TRIP(MAX)
GS
R
) voltage drive, 2) the voltage drop across the drain-
SENSE(MIN)
=
TRIP(MAX)
R
SENSE(MIN)
V
= R
CB(MAX)
SENSE(NOM)
D(MAX)
TRIP(MIN)
) must be evaluated carefully. If
DS(ON)
DS(ON)
DS(ON)
of the MOSFET. The V
DS(ON)
=
specified at V
R
specified at V
SENSE(NOM)
SENSE(MIN)
⎡
⎢
⎢
⎣
= 5.4A and I
1–
TRIP(NOM)
specified at V
60mV
GS
and 3) the maximum
TOL
DS(MAX)
⎛
⎜
⎝
is typically ± 20V for
R
100
is used for current
TOL
TOL
⎞
⎟
⎠
, and the maxi-
value that fits
GS
= 7.1A. From
⎤
⎥
⎥
⎦
can be con-
TRIP(MAX)
GS
= 5V), and
GS
TRIP(MIN)
DS(MAX)
= 2.5V).
= 10V)
D(MAX)
(12)
=
)
standard MOSFETs. However, the V
logic-level MOSFETs ranges from ±8V to ±20V depend-
ing upon the manufacturer and the specific part number.
The LTC4211’s GATE overdrive as a function of V
illustrated in the Typical Performance curves. Logic-level
and sub-logic-level MOSFETs are recommended for low
supply voltage applications and standard MOSFETs can
be used for applications where supply voltage is greater
than 4.75V.
Note that in some applications, the gate of the external
MOSFET can discharge faster than the output voltage
when the circuit breaker is tripped. This causes a nega-
tive V
selected external MOSFET should have a ± V
ing that is higher than the operating input supply voltage
to ensure that the external MOSFET is not destroyed by
a negative V
of the MOSFET must be higher than the gate overdrive
voltage. Lower ±V
with the LTC4211 if the GATE overdrive is clamped to a
lower voltage. The circuit in Figure 12 illustrates the use
of Zener diodes to clamp the LTC4211’s GATE overdrive
signal if lower voltage MOSFETs are used.
The R
to make its drain-source voltage (V
of V
error at the output voltage. This restricts the choice of
MOSFETs to very low R
V
package dissipation (P
Figure 12. Optional Gate Clamp for Lower V
DS
CC
requirement can be relaxed in which case MOSFET
GS
DS(ON)
. At a V
*USER SELECTED VOLTAGE CLAMP
(A LOW BIAS CURRENT ZENER DIODE IS RECOMMENDED)
1N4688 (5V)
1N4692 (7V): LOGIC-LEVEL MOSFET
1N4695 (9V)
1N4702 (15V): STANDARD-LEVEL MOSFET
V
voltage on the external MOSFET. Usually, the
CC
GS
of the external pass transistor should be low
CC
voltage. In addition, the ±V
R
= 2.5V, V
SENSE
GS(MAX)
DS(ON)
D
GATE
and T
Q1
DS
rating MOSFETs can be used
R
200Ω
G
D1*
+ V
. At higher V
J
RSENSE
) may limit the value of
GS
DS
D2*
maximum rating for
) a small percentage
LTC4211
GS(MAX)
= 0.1V yields 4%
CC
GS(MAX)
4211 F12
V
voltages, the
GS(MAX)
OUT
MOSFETs
23
rating
CC
4211fb
rat-
is