MIC2310_08 Datasheet, PDF (23/34 Pages) Micrel Semiconductor – Single-FET, Constant Power-Limit Hot Swap Controller

Micrel, Inc.
Shorted R_SENSE Detector Trips CB and Asserts
HW_FLT
In order to protect the system from the result of the
installation of a shorted sense resistor on the PCB,
which would increase the effective OC detection
thresholds to unsafe levels, the controller incorporates
a shorted R_SENSE detection scheme whose operation is
described in Figure 11. The R_SENSE detection circuitry
is enabled upon the application of an ENABLE LOW-
to-HIGH transition, an elapsed POR timer, and the
V_GS of the external MOSFET being higher than the
controller’s V_GSPGH threshold voltage. Note that for the
case of a shorted sense resistor, dI_D/dt control of the
inrush current is disabled and the controller defaults to
dV_GATE/dt control of the GATE voltage.
An R_SENSE short is detected by comparing the V_RS
voltage drop across the sense resistor
(V_CCSENSE-V_SENSE) to the V_DS voltage drop across the
external MOSFET (V_SENSE-V_SOURCE). To avoid a false
MIC2310
R_SENSE short detection at low current, a minimum V_DS
of ∆V_DS(FET) must exist across the external MOSFET
for a shorted sense resistor to be detected. For larger
values of V_DS across the external MOSFET generated
by higher load currents, the ∆V_RSENSE threshold
voltage for the detection of an R_SENSE short follows the
equation ∆V_RSENSE = 0.5 * (V_DS - ∆V_DS(FET)). If there
exists a short across the sense resistor such that V_RS
drops below the ∆V_RSENSE threshold voltage, then an
internal circuit breaker is tripped, the GATE drive
circuit is disabled, the GATE fault-mode pull-down
current sink is enabled, the DISCH output goes high,
and the HW_FLT digital output is asserted. To repair
the damaged sense resistor and reset the HW_FLT
digital output, the service processor instructs the main
supply to turn off the V_CC supply voltage to the
controller such that V_REG falls below the controller’s
V_VREG(UVLOL) threshold voltage.
July 2008
23
M9999-070108-A