Datasheet

LTC2974
83
2974fc
For more information www.linear.com/LTC2974
applicaTions inForMaTion
OV/UV faults. In addition, the high-speed current supervi-
sor OC and UC fault thresholds are configured using the
IOUT_OC
_FAULT_LIMIT and IOUT_UC_FAULT_LIMIT com-
mands, respectively. The IOUT_OC_FAULT_RESPONSE
and IOUT_UC_F
AULT_RESPONSE commands determine
the responses to OC/UC faults. Fault responses can range
from disabling the DC/DC converter immediately, waiting
to see if the fault condition persists for some inter
val be
-
fore disabling the DC/DC converter, or allowing the DC/DC
converter to continue operating in spite of the fault. If a
DC/DC converter is disabled, the L
TC2974 can be config
-
ured to retry one to six times, retry continuously without
limitation, or latch-off. The retr
y interval is specified using
the MFR_RETRY_DELAY command. Latched faults are
reset by toggling the CONTROL pin, using the OPERATION
command, or removing and reapplying the bias voltage to
the V
IN_SNS
pin. All fault and warning conditions result in
the ALERTB pin being asserted low and the corresponding
bits being set in the status registers. The CLEAR_FAULTS
command resets the contents of the status registers and
de-asserts the ALERTB output.
Output Overvoltage, Undervoltage, and Overcurrent
Warnings
OV, UV, and OC warning thresholds are processed by
the LTC2974’s ADC. These thresholds are set by the
VOUT_OV_WARN_LIMIT, VOUT_UV_WARN_LIMIT, and
IOUT_OC_WARN_LIMIT registers, respectively. Note that
there is no I
OUT
UC warning threshold. If a warning occurs,
the corresponding bits are set in the status registers and
the ALERTB output is asserted low. Note that a warning
will never cause a V
OUT_EN
output pin to disable a DC/DC
converter.
Configuring the AUXFAULTB Output
The AUXFAULTB output may be used to indicate an output
OV, OC, or UC fault. Use the MFR_CONFIG2_LTC2974
and MFR_CONFIG3_LTC2974 registers to configure the
AUXFAULTB pin to assert low in response to VOUT_OV,
IOUT_OC or IOUT_UC fault conditions. The AUXFAULTB
output will stop pulling low when the LTC2974 is com
-
manded to re-enter the ON state following a faulted-off
condition.
A charge-pumped 5µA pull-up to 12V is also available on
the AUXFAULTB output. Refer to the MFR_CONFIG_ALL_
LTC2974 register description in the PMBUS COMMAND
DESCRIPTION section for more information.
Figure 27 shows an application circuit where the AUX
-
FAULTB output is used to trigger a SCR crowbar on the
intermediate bus in order to protect the DC/DC converter’s
load from a catastrophic fault such as a stuck top-gate.
Figure 27. Application Circuit with Crowbar Protection on Intermediate Bus
0.1µF
V
PWR
AUXFAULTB
V
IN_SNS
V
DAC0
V
SENSEP0
V
SENSEM0
V
OUT_EN0
LTC2974*
4.5V < V
IBUS
< 15V
V
DD33
V
DD33
*SOME DETAILS OMITTED FOR CLARITY
ONLY ONE OF FOUR CHANNELS SHOWN
V
IN
C
BYPASS
0.1µF
0.01µF
0.22µF
0.01µF
220Ω
MCR12DC
0.1µF
V
DD25
V
DD25
V
OUT
2974 F27
RUN/SS
SGND
V
FB
GND
DC/DC
CONVERTER
LOAD
4.99k
10k
100Ω
68Ω
V
CC
SENSE
R
SENSE
0.007Ω
Q1
Si4894BDY
LTC4210-1
TIMER GND
GATE
24.3k
10k
ON
0.01µF
MMBT2907
REFP
REFM
GND