21
LTC4212
4212f
OPERATIO
For example:
If a sense resistor with 7m& ?% R
TOL
 is used for current
limiting, the nominal trip current I
TRIP(NOM)
 = 7.1A. From
Equations 11 and 12, I
TRIP(MIN)
 = 5.4A and I
TRIP(MAX)
 =
9.02A respectively.
For  proper  operation  and  to  avoid  the  circuit  breaker
tripping  unnecessarily,  the  minimum  trip  current
(I
TRIP(MIN)
) must exceed the circuits maximum operating
load current. For reliability purposes, the operation at the
maximum  trip  current  (I
TRIP(MAX)
)  must  be  evaluated
carefully. If necessary, two resistors with the same R
TOL
can be connected in parallel to yield an R
SENSE(NOM)
 value
that fits the circuit requirements.
Power MOSFET Selection Criteria
To start the power MOSFET selection process, choose the
maximum  drain-to-source  voltage,  V
DS(MAX)
,  and  the
maximum  drain  current,  I
D(MAX)
  of  the  MOSFET.  The
V
DS(MAX)
 rating must exceed the maximum input supply
voltage (including surges, spikes, ringing, etc.) and the
I
D(MAX)
 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
GS
) voltage drive, 2) the voltage drop across the
drain-to-source on resistance, R
DS(ON)
 and 3) the maxi-
mum junction temperature rating of the MOSFET.
Power MOSFETs are classified into two categories: stan-
dard  MOSFETs  (R
DS(ON)
  specified  at  V
GS
  =  10V)  and
logic-level MOSFETs (R
DS(ON)
 specified at V
GS
 = 5V). The
absolute maximum rating for V
GS
 is typically ?0V for
standard MOSFETs. However, the V
GS
 maximum rating
for logic-level MOSFETs ranges from ?V to ?0V de-
pending  upon  the  manufacturer  and  the  specific  part
number. The LTC4212s GATE overdrive as a function of
V
CC
 is illustrated in the Typical Performance curves. Logic-
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
GS
 voltage on the external MOSFET. Usually, the selected
external MOSFET should have a 盫
GS(MAX)
 rating that is
higher than the operating input supply voltage to ensure
that the external MOSFET is not destroyed by a negative
V
GS
  voltage.  In  addition,  the  盫
GS(MAX)
  rating  of  the
MOSFET must be higher than the gate overdrive voltage.
Lower 盫
GS(MAX)
 rating MOSFETs can be used with the
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 LTC4212s GATE overdrive signal if
lower voltage MOSFETs are used.
V
CC
V
OUT
*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
4212 F08
R
SENSE
GATE
D2*
D1*
Q1
R
G
200&
Figure 8. Optional Gate Clamp for Lower V
GS(MAX)
 MOSFETs
The R
DS(ON)
 of the external pass transistor should be low
to make its drain-source voltage (V
DS
) a small percentage
of V
CC
. At a V
CC
 = 2.5V, V
DS
 + V
RSENSE
 = 0.1V yields 4%
error at the output voltage. This restricts the choice of
MOSFETs to very low R
DS(ON)
. At higher V
CC
 voltages, the
V
DS
 requirement can be relaxed in which case MOSFET
package dissipation (P
D
 and T
J
) may limit the value of
R
DS(ON)
. Table 6 lists some power MOSFETs that can be
used with the LTC4212.
For reliable circuit operation, the maximum junction tem-
perature (T
J(MAX)
) for a power MOSFET should not exceed
the manufacturers 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:
MOSFET Junction Temperature,
T
J(MAX)
 d T
A(MAX)