LTC4211_12 Datasheet, PDF (23/40 Pages) Linear Technology – Hot Swap Controller

LTC4211
OPERATION
The maximum load current that trips the circuit breaker
is given in Equation 12.
I_{TRIP(MAX )}
=
V_{CB(M AX )}
R_SENSE(MIN)
=
60mV
R_SENSE(MIN)
(12)
where
R_SENSE(MIN)
=
R_SENSE(NOM)
s
⎡⎢⎢_1–
⎣
⎛⎝⎜_R1T0O0L
⎞
⎟
⎠
⎤⎥
⎥
⎦
For example:
If a sense resistor with 7mΩ ±5% 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 trip-
ping unnecessarily, the minimum trip current (I_TRIP(MIN))
must exceed the circuit’s 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 con-
nected 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 maxi-
mum 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 maximum
junction temperature rating of the MOSFET.
Power MOSFETs are classified into three categories:
standard MOSFETs (R_DS(ON) specified at V_GS = 10V)
logic-level MOSFETs (R_DS(ON) specified at V_GS = 5V), and
sub-logic-level MOSFETs (R_DS(ON) specified at V_GS = 2.5V).
The absolute maximum rating for V_GS is typically ± 20V for
standard MOSFETs. However, the V_GS maximum rating for
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_CC is
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_GS voltage on the external MOSFET. Usually, the
selected external MOSFET should have a ± V_GS(MAX) rat-
ing 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 ±V_GS(MAX) rating
of the MOSFET must be higher than the gate overdrive
voltage. Lower ±V_GS(MAX) rating MOSFETs can be used
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.
R_SENSE
Q1
V_CC
D1* D2*
V_OUT
R_G
200Ω
GATE
4211 F12
*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
Figure 12. 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
4211fb
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