SMH4802 Datasheet, PDF (12/20 Pages) Summit Microelectronics, Inc. – Programmable -48V Hot-Swap Controller with Forced Shut Down

SMH4802
Preliminary Information
APPLICATIONS INFORMATION (Continued)
of 250µA for IDMAX is used to illustrate the following
calculations.
With VOV (2.5V) being the over-voltage trip point, R1 is
calculated by the formula:
R1 = ^VOV
ID_MAX
Substituting:
Dropper Resistor Selection
The SMH4802 is powered from the high-voltage supply
via dropper resistor RD. The dropper resistor must
provide the SMH4802 (and its loads) with sufficient
operating current under minimum supply voltage condi-
tions, but must not allow the maximum supply current to
be exceeded under maximum supply voltage conditions.
The dropper resistor value is calculated from:
R1 = ^2.5V = 10kΩ
250µ A
Next the minimum current that flows through the resistive
divider, IDMIN, is calculated from the ratio of minimum and
maximum supply voltage levels:
ID_MIN
=
ID_MAX × VS_MIN
VS_MAX
Substituting:
ID_MIN
=
250µA × 36V
2.5V
=
125µA
Now the value of R3 is calculated from IDMIN:
R3 = ^{VSMIN × VUV}
ID_MIN
VUV is the under-voltage trip point, also 2.5V. Substitut-
ing:
R3 = ^{36V × 2.5V} = 286kΩ
125µ A
The closest standard 1% resistor value is 267kΩ
Then R2 is calculated:
R2 = ^2.5V – 10kΩ = 20kΩ – 10kΩ = 10kΩ
125µ A
or
R2 = ^VUV – R1
ID_MIN
Substituting:
R2 = ^2.5V – 10kΩ = 20kΩ – 10kΩ = 10kΩ
125µ A
An Excel spread sheet is available on Summit’s website
(www.summitmicro.com) to simplify the resistor value
calculations and tolerance analysis for R1, R2, and R3.
R_D
=
VS_MIN – V_DDMAX
I_DD – I_LOAD
where VSMIN is the lowest operating supply voltage,
VDDMAX is the upper limit of the SMH4802 supply voltage,
IDD is minimum current required for the SMH4802 to
operate, and ILOAD is any additional load current from the
2.5V and 5V outputs and between VDD and VSS.
Calculate the minimum wattage required for RD from:
( ) _{PRO ≥}
2
VS_MAX – V_DDMIN
R_D
where VDDMIN is the lower limit of the SMH4802 supply
voltage, and VSMAX 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 VDD and
VSS to handle the wide current range. The Zener voltage
should be below the nominal regulation voltage of the
SMH4802 so that it becomes the primary regulator.
MOSFET VDS(ON) Threshold
The drain sense input on the SMH4802 monitors the
voltage at the drain of the external power MOSFET switch
with respect to VSS. When the MOSFET’s VDS is below
the user-defined threshold the MOSFET switch is consid-
ered to be ON. The VDS(ON)THRESHOLD is adjusted using
the resistor RT 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 VDS(ON)THRESHOLD is calcu-
lated from:
( ) ( ) V_DS ON _THRESHOLD = V_SENSE – I_SENSE – R_T – V_DIODE
12
2062 2.3 6/19/03
SUMMIT MICROELECTRONICS, Inc.