max5938 Maxim Integrated Products, Inc., max5938 Datasheet - Page 15

• Manufacturer Part Number: max5938
• Description: Max5938 -48v Hot-swap Controller With Vin Step Immunity, No Rsense, And Overvoltage Protection
• Manufacturer: Maxim Integrated Products, Inc.

Determine the trip voltages these values will actually
yield for rising and falling voltages. Rising voltages use
the V
ages use V
1) R
2) V
3) V
4) V
5) V
6) V
The resulting voltage levels are V
±2.5%, V
42.32V ±2.5%, V
= 38.26V ±2.5%. The voltage tolerance does not
account for the tolerance of the resistors.
The MAX5938 can operate with a wide range of power
MOSFETs to meet the requirements of almost any appli-
cation. MOSFETs mentioned here are done to demon-
strate certain capabilities and features of the MAX5938.
They should not be construed as a recommendation or a
limitation of the interoperability of the MAX5938.
In the implementation of the circuit-breaker and short-
circuit functions, the MAX5938 eliminates the need for
an external current-sense resistor at the source of the
power MOSFET. As in any other hot-swap controller,
the proper circuit-breaker threshold for an application
must take into account the DC level of V
the same time accommodating the AC response of
V
V
especially the load capacitor, in conjunction with the
R
dampened second-order system. As such, this system
functions as a bandpass filter from V
response of V
age ripple and noise from the backplane power supply
must be taken into account. Adequate margin must be
provided between V
which depends on the R
(with V
expected to draw. While the circuit-breaker threshold
has glitch rejection for V
than 1.4ms, the short-circuit detection is designed to
respond very quickly (less than 330ns) to a short cir-
cuit. In the application, select a value for R
resulting in a V
-48V Hot-Swap Controller with V
OUT
IN
DS(ON)
to V
OV,RISING
OV,FALLING
ON,RISING
ON,FALLING
OFF
TOT
REF
to the modulation of V
GS
OUT
= V
= R
of the power MOSFET. It behaves as a highly
= 1.25V reference threshold, while falling volt-
OV,FALLING
at 10V) and the maximum current the load is
LO
REF
1
is dependent on the parasitics of the load,
+ R
= 1.125V reference threshold.
OUT
= V
= V
CB
Setting the Circuit-Breaker and
= V
x R
= V
ON,FALLING
2
REF
REF
______________________________________________________________________________________
that exceeds the product of R
+ R
to load-switching currents and volt-
TOT
LO
LO
CB
x R
x R
3
x R
, V
x R
= 71.84V ±5%, V
/ (R
Short-Circuit Thresholds
+ R
No R
DS(ON)
TOT
SC
TOT
TOT
TOT
OUT
2
4
, and the DC level of V
+ R
IN
= 38.09V ±5%, and V
/ R
/ (R
/ R
/ (R
. The AC response from
excursions lasting less
3
4
4
of the power MOSFET
3
3
+ R
+ R
OV,RISING
SENSE
+ R
4
4
)
IN
4
)
)
to V
OUT
ON,RISING
= 79.82V
OUT
, while at
CB_ADJ
DS(ON)
, and Overvoltage Protection
. The
OUT
OFF
=
,
and the maximum load current plus one half the peak-
to-peak AC response of V
rents and the noise and ripple at V
where
R
ture coefficient and the MAX5938, when placed adja-
cent to the power MOSFET, tracks and compensates
for this temperature coefficient. In the MAX5938, V
half of V
tance between CB_ADJ and V
(default) short-circuit threshold voltage, V
an internal 2kΩ precision-trimmed (±0.5%) resistor pro-
viding a minimum series resistance and a temperature-
compensated 50µA (+25°C) current source. When
CB_ADJ is connected to V
breaker threshold. When an external resistor, R
is placed between CB_ADJ and V
breaker threshold becomes:
and at +25°C, it becomes:
The short-circuit and circuit-breaker voltages are sensed
at V
R
and so the total current through the load and load
capacitance is the drain current of the power MOSFET.
Accordingly, the voltage at V
MOSFET drain current is:
The temperature compensation of the MAX5938 is
designed to track the R
MOSFET. Figure 14 shows the typical normalized temp-
co of the circuit-breaker threshold along with the nor-
malized tempco of R
MOSFETS. When determining the circuit-breaker
threshold in an application go to the power MOSFET
manufacturer’s data sheet and locate the maximum
R
ure presenting the tempco of normalized R
on-resistance vs. temperature. Since this curve is in
DS(ON)
DS(ON)
DS(ON)
OUT
V
R
CB
DS(ON)
V
, which is the drain of the power MOSFET. The
SC
CB
(mV) =
of the MOSFET is the current-sense resistance
at +25°C with a V
V
in a power MOSFET has a positive tempera-
CB
, which is set by placing an external resis-
(mV) =
V
(Ω) x I
(mV) =
OUT
IN
1
x (2kΩ + R
/
2
1
+ R
= I
LOAD,MAX
x V
/
2
Step Immunity,
1
x (2kΩ + R
DS(ON)
D,MOSFET
< V
x I
/
CB_ADJ
SC
2
V
CB_ADJ
SC
CB
DS(ON)
(mV) =
EE
GS
CB_ADJ
OUT
(mV)
(mV) =
(mA) +
for several typical power
this gives a 50mV circuit-
(kΩ)]
of 10V. Next, find the fig-
(µA) x [R
CB_ADJ
x R
to load-switching cur-
OUT
1
of the typical power
IN
/
)
2
DS(ON)
EE
EE
1
:
x I
1
/
2
, the new circuit-
as a function of
/
. The minimum
2
CB_ADJ
x 50µA
)
x VOUT
INT
SC
(kΩ)
, is set by
DS(ON)
(µA)
CB_ADJ
AC
CB
15
or
is
,