Datasheet

LTC4216
19
4216fa
For more information www.linear.com/LTC4216
applicaTions inForMaTion
1. Maximum drain-to-source voltage, V
DS(MAX)
The V
DS(MAX)
rating must exceed the maximum load sup-
ply voltage including spikes and ringing.
2. Gate-to-sour
ce voltage, V
GS
, overdrive
The absolute maximum rating for V
GS
is typically ±8V for
“logic level” and “sub-logic level” MOSFETs.
3. Drain-to-source resistance, R
DS(ON)
The R
DS(ON)
should be low for low voltage applications
to allow its drain-to-source voltage, V
DS(ON)
, to be a very
small percentage of the supply voltage.
To begin a design, first specify the maximum operating load
current and load capacitance. Calculate the R
SENSE
value
from Equation (12). The minimum trip current, I
TRIP(MIN)
,
given by Equation (14) should be set to accommodate the
maximum operating load current.
During the start-up cycle, the LTC4216 may operate the
MOSFET in analog current limit, forcing ΔV
ACL(TH)
between
32mV to 48mV across R
SENSE
. The minimum inrush current
given by Equation (16) is calculated using the minimum
ΔV
ACL(TH)
and maximum R
SENSE
value.
I
V
R
mV
R
INRUSHMIN
ACL TH MIN
SENSE MAXSENSE MAX
()
(, )
()
()
=
∆
=
32
(16)
The maximum short-circuit current given by Equation (17)
is calculated using the maximum ΔV
ACL(TH)
and minimum
R
SENSE
value.
I
V
R
mV
R
SHORT CIRCUIT MAX
ACL TH MAX
SENSE MIN SENSE MIN
−
=
∆
=
()
(, )
()
()
48
(17)
Select the FILTER capacitor, C3, based on the slowest
expected charging rate; otherwise, FILTER might time-out
before the load capacitor is fully charged. A value for C3
is calculated based on the maximum time it takes the load
capacitor, C
LOAD
, to charge to its maximum value of load
supply (V
IN(MAX)
). That time is given by:
t
CV
I
CHARGE LOAD
LOAD IN MAX
INRUSHMIN
()
()
()
•
=
(18)
Rearranging Equation (2) for the circuit breaker response
time, the FILTER capacitor, C3, is given by:
C
tsA
V
CHARGE LOAD
3
20 60
1 253
=
µµ(–)•
.
()
(19)
Returning to Equation (2), the circuit breaker response
time is calculated with a chosen C3 and used in conjunc
-
tion with V
IN(MAX)
and I
SHORT-CIRCUIT(MAX)
to check the
SOA curves of a prospective MOSFET.
As a numerical design example for the Typical Application,
consider V
IN(MAX)
= 1.8V + 5%, maximum operating load
current = 5A, C
LOAD
= 1000µF. Equation (12) gives R
SENSE
= 4.3mΩ. Choose R
SENSE
= 4mΩ ± 1% tolerance. From
Equations (14) and (16), I
TRIP(MIN)
= 5.3A (> I
LOAD(MAX)
= 5A) and I
INRUSH(MIN)
= 7.9A respectively. Equation (19)
gives C3 = 10nF. To account for errors in C3, FILTER current
(60µA) and FILTER threshold (1.253V), the calculated value
should be multiplied by 1.5, giving the nearest standard
value of C3 = 18nF.
If a short-circuit occurs, a current of up to I
SHORT-
CIRCUIT(MAX)
= 12.1A will flow through the MOSFET for
400µs as dictated by C3 = 18nF in Equation (2). The
MOSFET must be selected based on this criterion and
checked against the SOA curve.
V
CC
Supply RC Network
The LTC4216 has two separate pins, V
CC
and SENSEP,
for supply input and sensing:
1. V
CC
pin for powering the internal circuitry.
2. SENSEP pin, together with the SENSEN pin, for sens-
ing the
current flowing from the load supply through the
external
sense resistor and N-channel MOSFET to the
output load.
In most Hot Swap devices, V
CC
and SENSEP are one
common pin, providing the device’s supply and external
MOSFET’s current sensing. However, supply dips due
to output short can potentially trigger the device into an
undervoltage lockout condition, causing the device to
disable and its internal latches to reset.
As bypass capacitors are not allowed on the powered
supply side of the external MOSFET switch residing on