SMH4814
Preliminary Information
APPLICATIONS INFORMATION (CONTINUED)
Circuit Breaker Operation
Figure 10 shows the circuit breaker ‘Quick Trip’
The SMH4814 provides a highly configurable method
response. In this figure, the voltage rises above VQCB
,
for detecting and controlling over-current events.
A
causing VGATE to be deasserted.
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 (RS) is connected between the
MOSFET source (which is tied to CBSENSE) and VSS.
The board’s load current passes through the sense
resistor, and the CBSENSE input is monitored for
excessive voltage drop across RS.
<tCBD
VQCB
VCB
CBSENSE
tQTSD
VGATE_HS
The SMH4814 compares the CBSENSE input against
three important voltage levels (VCB, VQCB, and VCR) and
takes appropriate action as each successive level is
reached. The first voltage, VCB, is the circuit breaker
trip point, which is determined by R0A[7:0]. VCB may
be set to any one of 256 levels up to a maximum
voltage of VCBMAX, which is a configurable voltage of
128mV, 256mV, 512mV or 1024mV, as determined by
R09[5:4]. For example, if VCBMAX is set to 256mV, then
VCB may be programmed to any value between 0 and
255mV, in 1mV increments. (Refer to the Register
Description for more information.) If CBSENSE
exceeds VCB for a period of time longer than the glitch
filter delay associated with that input, tCBD (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
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, VCR, is the third voltage level against which the
CBSENSE input is compared. VCR is determined by
register R09[7:6] and is expressed as a percentage
above the VCB level. There are four choices: 12.5%,
25%, 50% and 100%. Note that the Quick-Trip level
VQCB should be chosen to fall above VCR in order for
Current Regulation to be effective.
device will begin another timer.
Refer to the
description of Current Regulation for more information
on these actions.
Current Regulation works by modulating VGATE_HS
so that CBSENSE is always less than or equal to VCR.
In order to avoid overheating the MOSFET by
operating in its linear region for too long, a timer is
started whenever CBSENSE goes above VCB or
VGATE_HS falls at least VGT below V12. If either of
these conditions exist for the duration of the current
regulation timer, tCR, 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.
Quick-TripTM Circuit Breaker
The second voltage level that the CBSENSE input is
compared against is the Quick-Trip™ Circuit Breaker
level, VQCB
.
VQCB is determined by the contents of
R0B[7:0], in a manner similar to VCB. (Note that the
value stored in R0B is a 2’s complement number; refer
to the Register Description for more information.)
Unlike the VCB comparator, the output of the VQCB
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.
Summit Microelectronics, Inc
2080 2.0 07/21/05
17
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
