SMH4802 Datasheet, PDF (10/20 Pages) Summit Microelectronics, Inc. – Programmable -48V Hot-Swap Controller with Forced Shut Down

SMH4802
Preliminary Information
APPLICATIONS INFORMATION (Continued)
Quick-Trip^TM Circuit Breaker (Figure 7)
The SMH4802 provides a Quick-Trip feature that will
cause the circuit breaker to trip immediately if the voltage
drop across R_S exceeds V_QCB. The Quick-Trip can be
disabled or set to 60mV, 100mV (default) or 200mV.
CBSENSE
<T_CBD
V_QCB
50mV
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. It is generally enabled
in applications that have switched dual (A and B) distrib-
uted power sources. By using the current regulation
function unwarranted shutdowns can be avoided if one of
the dual supplies is switched in when it is at a more
negative potential than the currently operating supply.
When current regulation is selected it will be enabled
during soft start (power on period) and during normal
operation after the PG# output is enabled. If the voltage
monitored at the CBSENSE pin is greater than 50mV, but
less than VQCB, the SMH4802 will reduce the VGATE
voltage in order to maintain a CBSENSE potential less
than 60mV, effectively regulating the current through the
MOSFET.
Figures 8A and 8B illustrate the current regulation function.
The time period tPCR — selectable at 5, 80, or 320ms — is
the maximum time during which regulation will be enforced.
If either VQCB or tPCR are exceeded the VGATE and PG#
outputs will immediately be de-asserted. However, if
CBSENSE drops below 50mV before the timer ends, the
timer is reset and VGATE resumes normal operation. If
the Quick-Trip level is exceeded then the device will
bypass the current regulation timer and shut down imme-
diately. The Current Regulation feature is disabled in the
default configuration.
Operating at High Voltages
The breakdown voltage of the external active and passive
components limits the maximum operating voltage of the
SMH4802 hot-swap controller. Components that must be
able to withstand the full supply voltage are: the input and
output decoupling capacitors, the protection diode in
series with the DRAIN SENSE pin, the power MOSFET
switch and the capacitor connected between its drain and
gate, the high-voltage transistors connected to the power
good outputs, and the dropper resistor connected to the
controller’s VDD pin.
Over-Voltage and Under-Voltage Resistors
In Figure 9 the three resistors (R1, R2, and R3) connected
to the OV and UV inputs must be capable of withstanding
the maximum supply voltage of several hundred volts.
VGATE
T_QTSD
2062 Fig07
Figure 7. Circuit Breaker Quick Trip Response
V_QCB
CBSENSE
0V
12V
VGATE
t_PCR
50mV
t_CRD
0V
2062 Fig08A
Figure 8A. Current Regulation With Recovery
V_QCB
CBSENSE
0V
12V
VGATE
t_PCR
50mV
0V
2062 Fig08B
Figure 8B. Current Regulation Without Recovery
10
2062 2.3 6/19/03
SUMMIT MICROELECTRONICS, Inc.