SMH4804 Datasheet, PDF (21/41 Pages) Summit Microelectronics, Inc. – -48V Programmable Hot Swap Sequencing Power Controller

SMH4804
Timing Diagram
not allow the maximum supply current to be exceeded
under maximum supply voltage conditions.
The dropper resistor value is calculated from:
R_D =
VS_MIN - VDD_MAX
I_DD - I_LOAD
where VS_MIN is the lowest operating supply voltage,
V_DDMAX is the upper limit of the SMH4804 supply voltage,
I_DD is minimum current required for the SMH4804 to
operate, and I_LOAD is any additional load current from the
2.5V and 5V outputs and between V_DD and V_SS.
Calculate the minimum wattage required for RD from:
P_R0
≥
(VS_MAX - VDD_MIN)²
R_D
where V_DDMIN is the lower limit of the SMH4804 supply
voltage, and VS_MAX is the highest operating supply voltage.
In circumstances where the input voltage may swing over a
wide range (e.g., from 20V to 100V) the maximum current
may be exceeded. In these circumstances it may be
necessary to add an 11V Zener diode between V_DD and V_SS
to handle the wide current range. The Zener voltage should
be below the nominal regulation voltage of the SMH4803A
so that it becomes the primary regulator.
MOSFET V_DS(ON) Threshold
The drain sense input on the SMH4804 monitors the voltage
at the drain of the external power MOSFET switch with
respect to V_SS. When the MOSFET’s V_DS is below the user-
defined threshold the MOSFET switch is considered to be
ON. The V_DS(ON)_THRESHOLD is adjusted using the resistor,
R_T, in series with the drain sense protection diode. This
protection, or blocking, diode prevents high voltage
breakdown of the drain sense input when the MOSFET
switch is OFF. A low leakage MMBD1401 diode offers
protection up to 100V. For high voltage applications (up to
500V) the Central Semiconductor CMR1F-10M diode
should be used. The V_DS(ON)_THRESHOLD is calculated from:
V_DS(ON)_THRESHOLD = V_SENSE - (I_SENSE x R_T) - V_DIODE
where VDIODE is the forward voltage drop of the protection
diode. The V_DS(ON)_THRESHOLD varies over temperature
due to the temperature dependence of V_DIODE and I_SENSE.
The calculation below gives the V_DS(ON)_THRESHOLD under
the worst case condition of 85°C ambient. Using a 68 kΩ
resistor for R_T gives:
V_DS(ON)_THRESHOLD = 2.5V - (15µA x 68kΩ) - 0.5V = 1V
The voltage drop across the MOSFET switch and sense
resistor, V_DSS, is calculated from:
V_DSS = I_D (R_S x R_ON)
where I_D is the MOSFET drain current, RS is the circuit
breaker sense resistor and R_ON is the MOSFET on
resistance.
The dropper resistor value should be chosen such that the
minimum and maximum I_DD and V_DD specifications of the
SMH4804 are maintained across the host supply’s valid
operating voltage range. First, subtract the minimum V_DD of
the SMH4804 from the low end of the voltage, and divide by
the minimum I_DD value. Using this value of resistance as R_D
find the operating current that would result from running at
the high end of the supply voltage to verify that the resulting
current is less than the maximum I_DD current allowed. If
some range of supply voltage is chosen that would cause
the maximum I_DD specification to be violated, then an
external zener diode with a breakdown voltage of ~12V
should be used across V_DD.
As an example of choosing the proper R_D value, assume
the host supply voltage ranges from 36 to 72V. The largest
dropper resistor that can be used is: (36V-11V)/3mA =
8.3kΩ. Next, confirm that this value of R_D also works at the
high end: (72V-13V)/8.3kΩ = 7.08mA, which is less than
8mA.
The FS# input can also be used in conjunction with a
secondary-side supervisory circuit providing a positive
feedback loop during the power up sequence. As an
example, assume the SMH4804 is configured to turn on
–48V to three DC/DC converters and then sequentially turn
on the converters with a 1.6ms delay. Further assume all of
the enable inputs are true and PG4# has just been
sequenced on. If FS# option 4 (100_BIN in register 5) has
been selected, then FS# must be driven high within 1.6ms
after PG4# goes low, otherwise all of the PG[4:1]# outputs
are disabled.
Ideally, there would be a secondary-side supervisor similar
to the SMS44 that would have its reset time-out period
programmed to be less than 1.6ms. After the last supply
turns on the RESET# output of the SMS44 would be
released and FS# pulled high. However, if for any reason
Summit Microelectronics
2050 3.7 10/30/02
21