Datasheet

LTC2977
79
2977fa
For more information www.linear.com/LTC2977
APPLICATIONS INFORMATION
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
deasserts the ALERTB output.
Output Overvoltage and Undervoltage Warnings
OV and UV warning threshold voltages are processed by the
LTC2977’s ADC. These thresholds are set by the VOUT_OV_
WARN_LIMIT and VOUT_UV_WARN_LIMIT commands
respectively. 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 V
IN_EN
Output
The V
IN_EN
output may be used to disable the intermediate
bus voltage in the event of an output OV or UV fault.
Use the MFR_VINEN_OV_FAULT_RESPONSE and
MFR_VINEN_UV_FAULT_RESPONSE registers to
configure the V
IN_EN
pin to assert low in response to
VOUT_OV/UV fault conditions. The V
IN_EN
output will stop
pulling low when the LTC2977 is commanded to re-enter
the ON state following a faulted-off condition.
A charge-pumped 5µA pull-up to
12V is also available on the
V
IN_EN
output. Refer to the MFR_CONFIG_ALL_LTC2977
register description in the PMBus COMMAND DESCRIP-
TION section for more information.
Figure
19
shows an application circuit where the V
IN_EN
output is used to trigger an 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. The stuck top
gate causes an OV fault, which in turn causes the LTC2977
to pull V
IN_EN
low, thus deasserting the ON input to the
LTC4210 hot-swap controller, which opens the switch Q1
that supplies the DC/DC converter input. In addition, when
V
IN_EN
goes low it forces the MCR12DC SCR device into
the on-state via the 2N2907 PNP, thus quickly dropping
the voltage on the V
IN
input to the DC/DC converter, pre-
venting the stuck
top gate from damaging components
supplied by this converter. Note that the V
PWR
input to
the LTC2977 bypasses switch Q1, keeping the LTC2977
fully powered throughout the above sequence.
Figure 19. LTC2977 Application Circuit with Crowbar Protection on Intermediate Bus
V
PWR
V
IN_EN
V
IN_SNS
V
DACP0
V
SENSEP0
V
DACM0
V
SENSEM0
V
OUT_EN0
REFP
REFM
LTC2977*
V
IN
<15V
GNDV
DD33
V
DD33
V
DD25
*SOME DETAILS OMITTED FOR CLARITY
ONLY ONE OF EIGHT CHANNELS SHOWN
V
IN
C
BYPASS
0.1µF
0.01µF
0.22µF
0.01µF
0.1µF
220Ω
MCR12DC
0.1µF 0.1µF
V
OUT
2977 F19
RUN/SS
SGND
V
FB
GND
DC/DC
CONVERTER
LOAD
4.99k
BAT54
10k
100Ω
68Ω
V
CC
SENSE
R
SENSE
0.007Ω
Q1
Si4894BDY
LTC4210-3
TIMER GND
GATE
24.3k
10k
ON
2N2907