Determine the trip voltages these values will actually
yield for rising and falling voltages. Rising voltages use
the VREF = 1.25V reference threshold, while falling volt-
ages use VLO = 1.125V reference threshold.
1) RTOT = R1+ R2+ R3+ R4
2) VOV,RISING = VREF x RTOT / R4
3) VOV,FALLING = VLO x RTOT / R4
4) VON,RISING = VREF x RTOT / (R3+ R4)
5) VON,FALLING = VLO x RTOT / (R3+ R4)
6) VOFF = VREF x RTOT / (R2+ R3+ R4)
The resulting voltage levels are VOV,RISING = 79.82V
±2.5%, VOV,FALLING = 71.84V ±5%, VON,RISING =
42.32V ±2.5%, VON,FALLING = 38.09V ±5%, and VOFF
= 38.26V ±2.5%. The voltage tolerance does not
account for the tolerance of the resistors.
Setting the Circuit-Breaker and
Short-Circuit Thresholds
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 VOUT, while at
the same time accommodating the AC response of
VOUT to the modulation of VIN. The AC response from
VIN to VOUT is dependent on the parasitics of the load,
especially the load capacitor, in conjunction with the
RDS(ON) of the power MOSFET. It behaves as a highly
dampened second-order system. As such, this system
functions as a bandpass filter from VIN to VOUT. The
response of VOUT to load-switching currents and volt-
age ripple and noise from the backplane power supply
must be taken into account. Adequate margin must be
provided between VCB, VSC, and the DC level of VOUT,
which depends on the RDS(ON) of the power MOSFET
(with VGS at 10V) and the maximum current the load is
expected to draw. While the circuit-breaker threshold
has glitch rejection for VOUT excursions lasting less
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 RCB_ADJ
resulting in a VCB that exceeds the product of RDS(ON)
and the maximum load current plus one half the peak-
to-peak AC response of VOUT to load-switching cur-
rents and the noise and ripple at VIN:
RDS(ON) (Ω) x ILOAD,MAX(mA) + 1/2x VOUTAC
< VCB(mV)
where
VCB(mV) = 1/2x ICB_ADJ(µA) x [RINT(kΩ)
+ RCB_ADJ(kΩ)]
RDS(ON) in a power MOSFET has a positive tempera-
ture coefficient and the MAX5938, when placed adja-
cent to the power MOSFET, tracks and compensates
for this temperature coefficient. In the MAX5938, VCB is
half of VSC, which is set by placing an external resis-
tance between CB_ADJ and VEE. The minimum
(default) short-circuit threshold voltage, VSC, is set by
an internal 2kΩprecision-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 VEE this gives a 50mV circuit-
breaker threshold. When an external resistor, RCB_ADJ,
is placed between CB_ADJ and VEE, the new circuit-
breaker threshold becomes:
VCB (mV) = 1/2x VSC (mV) = 1/2x ICB_ADJ (µA)
x (2kΩ+ RCB_ADJ)
and at +25°C, it becomes:
VCB (mV) = 1/2VSC (mV) = 1/2x 50µA
x (2kΩ+ RCB_ADJ)
The short-circuit and circuit-breaker voltages are sensed
at VOUT, which is the drain of the power MOSFET. The
RDS(ON) of the MOSFET is the current-sense resistance
and so the total current through the load and load
capacitance is the drain current of the power MOSFET.
Accordingly, the voltage at VOUT as a function of
MOSFET drain current is:
VOUT = ID,MOSFET x RDS(ON)
The temperature compensation of the MAX5938 is
designed to track the RDS(ON) of the typical power
MOSFET. Figure 14 shows the typical normalized temp-
co of the circuit-breaker threshold along with the nor-
malized tempco of RDS(ON) for several typical power
MOSFETS. When determining the circuit-breaker
threshold in an application go to the power MOSFET
manufacturer’s data sheet and locate the maximum
RDS(ON) at +25°C with a VGS of 10V. Next, find the fig-
ure presenting the tempco of normalized RDS(ON) or
on-resistance vs. temperature. Since this curve is in
MAX5938
-48V Hot-Swap Controller with VIN Step Immunity,
No RSENSE, and Overvoltage Protection
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