SMH4804 Datasheet, PDF (7/41 Pages) Summit Microelectronics, Inc. – -48V Programmable Hot Swap Sequencing Power Controller

SMH4804
Functional Description
Current regulation is generally enabled in applications that
have switched dual (A and B) distributed 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 the
currently operating supply.
When current regulation is selected by programming bits
1:0 of Register 6 to a binary value of 01 (5 ms), 10 (80 ms),
or 11 (160 ms), it is enabled during a soft-start (power on
period) and during normal operation after the PG[4:1]#
outputs are enabled. If the voltage monitored at the
CBSENSE pin is greater than 50mV, but less than 60mV,
the SMH4804 reduces the VGATE voltage in order to
maintain a CBSENSE potential less than 60mV, effectively
regulating the current through the MOSFET.
Figure 8 and Figure 9 illustrate the current regulation
function. The time period t_PCR — selectable at 5, 80, or 160
ms — is the maximum time during which regulation is
enforced. If either V_QCB or t_PCR are exceeded the VGATE
and PG[4:1]# outputs are immediately deasserted.
However, if CBSENSE drops below 50mV before the timer
period ends, the timer is reset and VGATE resumes normal
operation (see Current Regulation With Recovery on page
7). If the Quick-Trip level is exceeded, the device bypasses
the current regulation timer and shuts down immediately.
The current regulation feature is disabled in the default
configuration.
V_QCB
CBSENSE
0V
12V
VGATE
t<_Pt_CPCRR
50mV
t_CRD
0V
2050 Fig06a
Figure 8. Current Regulation With Recovery
V_QCB
CBSENSE
0V
12V
VGATE
t_PCR
50mV
0V
2050 Fig06b
Figure 9. Current Regulation Without Recovery
Nonvolatile Fault Latch
The SMH4804 provides an optional nonvolatile fault latch
(NVFL) circuit breaker feature. The nonvolatile fault latch
essentially provides a programmable fuse on the circuit
breaker. When the latch is enabled by setting bit 5 of
Register 5, the nonvolatile fault latch is set whenever the
circuit breaker trips. Once set, it cannot be reset by cycling
power or through the use of the RESET# pin.
Note: The device remains disabled until Register C is
reprogrammed. Refer to Register C - Address 1100 on page
38 for more information.
As long as the NVFL is set, the FAULT# output remains
asserted. The Nonvolatile Fault Latch feature is disabled in
the default configuration.
Resetting FAULT#
When the circuit breaker trips, the VGATE output is turned
off and the SMH4804 drives FAULT# low. There are two
methods to reset the circuit breaker which are selectable
with the MODE pin:
• When the MODE is held high or left floating, the circuit
breaker is in the duty-cycle mode. In this case the
breaker resets automatically after a time of t_CYC.
• When the MODE pin is held low (or disabled in the
Configuration Register) FAULT# can be reset by
bringing RESET# low. The VGATE output attempts to
restart the MOSFET slew control circuitry t_PDD after
bringing RESET# back high again.
In either case, cycling power to the board resets the circuit
breaker. If the over current condition still exists after the
MOSFET switches back on, the circuit breaker will re-trip.
Summit Microelectronics
2050 3.7 10/30/02
7