SMH4814 Datasheet, PDF (17/44 Pages) Summit Microelectronics, Inc. – Dual Feed Active-ORing Programmable Hot Swap Controller

APPLICATIONS INFORMATION (CONTINUED)
Circuit Breaker Operation
The SMH4814 provides a highly configurable method
for detecting and controlling over-current events. A
sustained over-current condition can cause physical
damage to the card edge connector, the load circuitry,
and may even disrupt operation of other cards in the
system. To detect such over-current conditions, a
series sense resistor (R_S) is connected between the
MOSFET source (which is tied to CBSENSE) and V_SS.
The board’s load current passes through the sense
resistor, and the CBSENSE input is monitored for
excessive voltage drop across R_S.
The SMH4814 compares the CBSENSE input against
three important voltage levels (V_CB, V_QCB, and V_CR) and
takes appropriate action as each successive level is
reached. The first voltage, V_CB, is the circuit breaker
trip point, which is determined by R0A[7:0]. V_CB may
be set to any one of 256 levels up to a maximum
voltage of V_CBMAX, which is a configurable voltage of
128mV, 256mV, 512mV or 1024mV, as determined by
R09[5:4]. For example, if V_CBMAX is set to 256mV, then
V_CB may be programmed to any value between 0 and
255mV, in 1mV increments. (Refer to the Register
Description for more information.) If CBSENSE
exceeds V_CB for a period of time longer than the glitch
filter delay associated with that input, t_CBD (set using
R06[1:0]), then the device is considered to be in an
over-current state. Once in an over-current state, the
SMH4814 will either shut down immediately, or if the
Current Regulation option is selected (R05[3]), the
device will begin another timer. Refer to the
description of Current Regulation for more information
on these actions.
Quick-Trip^TM Circuit Breaker
The second voltage level that the CBSENSE input is
compared against is the Quick-Trip™ Circuit Breaker
level, V_QCB. V_QCB is determined by the contents of
R0B[7:0], in a manner similar to V_CB. (Note that the
value stored in R0B is a 2’s complement number; refer
to the Register Description for more information.)
Unlike the V_CB comparator, the output of the V_QCB
comparator is a high-speed, non-filtered signal
designed to shutdown the MOSFET gate very quickly.
If the Current Regulation option is not selected, then
exceeding the Quick-Trip level causes an immediate
shutdown of the PUP outputs and MOSFET gate;
however, if Current Regulation is selected, the PUP
outputs will not be immediately shut off. Refer to the
description of Current Regulation for more information.
SMH4814
Preliminary Information
Figure 10 shows the circuit breaker ‘Quick Trip’
response. In this figure, the voltage rises above V_QCB,
causing VGATE to be deasserted.
CBSENSE
<t_CBD
V_QCB
V_CB
VGATE_HS
t_QTSD
Figure 10 - Circuit Breaker Quick Trip Response
without current regulation.
Current Regulation
The Current Regulation mode is an optional feature
that provides a means to regulate current through the
MOSFET for a programmable period of time. This
mode allows the system to “ride out” temporary
disruptions that might otherwise cause traditional
circuit breakers to trip. The Current Regulation trip
point, V_CR, is the third voltage level against which the
CBSENSE input is compared. V_CR is determined by
register R09[7:6] and is expressed as a percentage
above the V_CB level. There are four choices: 12.5%,
25%, 50% and 100%. Note that the Quick-Trip level
V_QCB should be chosen to fall above V_CR in order for
Current Regulation to be effective.
Current Regulation works by modulating VGATE_HS
so that CBSENSE is always less than or equal to V_CR.
In order to avoid overheating the MOSFET by
operating in its linear region for too long, a timer is
started whenever CBSENSE goes above V_CB or
VGATE_HS falls at least V_GT below V12. If either of
these conditions exist for the duration of the current
regulation timer, t_CR, then the PUP and VGATE
outputs are shut down. There are actually two
different Current Regulation timers; R00[7:4] controls
the timing for the initial VGATE_HS turn on, and
R04[7:4] controls the timing for all subsequent current
regulation events.
Summit Microelectronics, Inc
2080 2.0 07/21/05
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