LTC4212 LINER [Linear Technology], LTC4212 Datasheet - Page 21

• Manufacturer Part Number: LTC4212
• Description: Hot Swap Controller with Power-Up Timeout
• Manufacturer: LINER [Linear Technology]

OPERATIO
For example:
If a sense resistor with 7m
limiting, the nominal trip current I
Equations 11 and 12, I
9.02A respectively.
For proper operation and to avoid the circuit breaker
tripping unnecessarily, the minimum trip current
(I
load current. For reliability purposes, the operation at the
maximum trip current (I
carefully. If necessary, two resistors with the same R
can be connected in parallel to yield an R
that fits the circuit requirements.
Power MOSFET Selection Criteria
To start the power MOSFET selection process, choose the
maximum drain-to-source voltage, V
maximum drain current, I
V
voltage (including surges, spikes, ringing, etc.) and the
I
current in the system during a fault condition. In addition,
consider three other key parameters: 1) the required gate-
source (V
drain-to-source on resistance, R
mum junction temperature rating of the MOSFET.
Power MOSFETs are classified into two categories: stan-
dard MOSFETs (R
logic-level MOSFETs (R
absolute maximum rating for V
standard MOSFETs. However, the V
for logic-level MOSFETs ranges from 8V to 20V de-
pending upon the manufacturer and the specific part
number. The LTC4212’s GATE overdrive as a function of
V
level MOSFETs are recommended for low supply voltage
applications and standard MOSFETs can be used for appli-
cations 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 negative
V
D(MAX)
DS(MAX)
CC
GS
TRIP(MIN)
is illustrated in the Typical Performance curves. Logic-
voltage on the external MOSFET. Usually, the selected
rating must exceed the maximum short-circuit
GS
rating must exceed the maximum input supply
) must exceed the circuit’s maximum operating
) voltage drive, 2) the voltage drop across the
U
DS(ON)
TRIP(MIN)
DS(ON)
TRIP(MAX)
specified at V
D(MAX)
5% R
specified at V
GS
= 5.4A and I
DS(ON)
TRIP(NOM)
TOL
of the MOSFET. The
) must be evaluated
is typically 20V for
GS
is used for current
DS(MAX)
SENSE(NOM)
and 3) the maxi-
maximum rating
GS
GS
= 7.1A. From
= 10V) and
TRIP(MAX)
= 5V). The
, and the
value
TOL
=
external MOSFET should have a V
higher than the operating input supply voltage to ensure
that the external MOSFET is not destroyed by a negative
V
MOSFET must be higher than the gate overdrive voltage.
Lower V
LTC4212 if the GATE overdrive is clamped to a lower
voltage. The circuit in Figure 8 illustrates the use of zener
diodes to clamp the LTC4212’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
R
used with the LTC4212.
For reliable circuit operation, the maximum junction tem-
perature (T
the manufacturer’s recommended value. This includes
normal mode operation, start-up, current-limit and
autoretry mode in a fault condition. Under normal condi-
tions the junction temperature of a power MOSFET is given
by Equation 13:
GS
DS
DS(ON)
Figure 8. Optional Gate Clamp for Lower V
MOSFET Junction Temperature,
T
J(MAX)
CC
requirement can be relaxed in which case MOSFET
voltage. In addition, the V
DS(ON)
. At a V
. Table 6 lists some power MOSFETs that can be
*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
GS(MAX)
CC
J(MAX)
of the external pass transistor should be low
T
CC
A(MAX)
= 2.5V, V
R
) for a power MOSFET should not exceed
SENSE
rating MOSFETs can be used with the
+
DS(ON)
D
JA
and T
GATE
DS
Q1
• P
R
200
+ V
. At higher V
G
D1*
D
J
RSENSE
) may limit the value of
GS(MAX)
DS
D2*
GS(MAX)
) a small percentage
GS(MAX)
= 0.1V yields 4%
LTC4212
CC
rating of the
4212 F08
rating that is
voltages, the
V
OUT
MOSFETs
21
(13)
4212f