Datasheet

LTC2928
19
2928f
Correcting for IR Drops
Sense feedback is common in high current applications.
Parasitic resistance coupled with high currents causes volt-
age drops. The sense pin of a power module is designed
to help regulate the voltage at a point in the distribution
circuitry beyond the voltage drops. The output of the power
module is raised until the desired voltage is achieved at
the sense point. During startup with sense feedback,
large inrush currents may cause the output of the power
module to exceed its maximum output. This may cause
the module to shutdown.
The LTC2928 is easily configured to enable a sense tran-
sistor after a power supply has been sequenced on and
inrush currents have diminished (Figure 7). The sense
transistor feeds the sequenced supply voltage back to the
sense line of the power module. The power module will
raise its output to compensate for voltage drops across
the sequencing transistor and other parasitics.
During the sequence-down phase, the sense transistor will
be disconnected before the supply sequencing transistor.
If the supply transistor were to be disconnected first, the
power module would sense the voltage drop and may
attempt to drive higher in order to compensate.
V
OUT
V
1
LTC2928
PARASITIC
RESISTANCE
EN2 V2
V
CC
POWER
MODULE
OUT +
SENSE +
RT1
RT2
I
OUT
R
DS(ON)
2928 F07
EN1
100 100
Figure 7. Correcting for IR drops
(RT1 > RT2)
Discharging Load Capacitance
It is often necessary to discharge load capacitance quickly.
Use the pull-down strength of the LTC2928 enable outputs
to achieve faster turn-off times. Sequence the enable
outputs in the same time position (RT resistors are iden-
tical). With the connections shown in Figure 8, EN1 is
used to discharge the load capacitance through its 100
Ω
on resistance. If shorter discharge time is required, use
external inverters as shown in Figure 9.
V
2
EN1
LTC2928
POWER
MODULE
RT2
V
1
V
CC
V
OUT
RT1
2928 F08
EN2
LOAD
SD
+
Figure 8. Load Capacitance Discharge
V
2
EN1
LTC2928
POWER
MODULE
RT2
V
1
V
CC
IRF530
2N7002
V
CC
V
OUT
RT1
2928 F09
EN2
LOAD
SD
+
Figure 9. Fast Load Capacitance Discharge
Using the Power Module Voltages to Gate the
Sequenced Supplies
The application shown in Figure 10 demonstrates how the
power module outputs can be enabled and monitored for
compliance, and then passed to various loads. Sequencing
up and down is controlled by the level of the 12V sup-
ply. Sequencing up begins at 10.76V. If the application is
disconnected from the 12V supply, an orderly shutdown
will commence when the 12V supply drops to 10.43V.
The blocking diode (D1) allows the 470µF capacitor (C1)
to retain enough energy to complete the sequence down
process.
APPLICATIO S I FOR ATIO
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