Datasheet

LTC4226
15
4226f
Fault Status
The FAULT status pin is active low with a 10µA current
source pull-up to a diode below its internal supply volt-
age, typically 5V for any V
CC
>7V. When a fault occurs,
the FAULT pin pulls to ground with a 5mA limit. Although
the FAULT pin has the same voltage rating as the supply
pin, sinking LED current as in Figure 9 requires a series
resistor to reduce pin power dissipation.
The FAULT pin is also an unlatched input to synchronize
the MOSFET GATE. Pulling this pin externally below 0.3V
causes the GATE to shutoff immediately. This pin can
optionally be wire-ORed with other LTC4226's FAULT
pins to turn off their GATEs when one of the LTC4226
has a circuit breaker fault with the FTMR pin asserted at
V
FTMR(H)
. The other LTC4226's FTMR pin is unaffected by
the low external FAULT input. When the LTC4226 with fault
is reset (see section on auto-retry and resetting faults), the
wire-ORed FAULT pins return high and the GATEs revert
to their prior states. It is not recommended to connect an
LED to wire-ORed FAULT pins.
Daisy Chained Ports
Figure 7, illustrates FireWire
power distribution with
LTC4226
Hot Swap circuits and supply diode-ORing.
The Firewire devices can be power providers or power
consumers and can be daisy chained together.
In Figure 8, a 2-port device allows either port to be powered
internally through diode D1 or to be powered from the op-
posite port. The higher voltage source delivers power to the
external port devices and the internal FireWire controller
interface. This permits the host power to be shutdown while
the FireWire controller remains active with external power
provided by the port. The port can relay actively current
limited power as long as there are power sources in the
chain. More than two ports per device are possible permit-
ting power consumption or distribution among multiple
ports. The ports allow live plugging and unplugging with
port load capacitances as large as 1mF at 33V for Figure8.
The output port step up surge current is actively limited.
Figure 9 shows a 12V host power source application that
can drive a remote load capacitance up to 100µF with a
small MOSFET like the Si2318DS. 2mA rated LEDs can be
used as FAULT indicators with resistors to reduce power
dissipation at the FAULT pins.
V
CC
Overvoltage Detection
The FTMR pin can be used to detect a V
CC
overvoltage
condition with a Zener diode Z2 as shown in Figure 6.
Resistor R5 and Zener Z3 protect the FTMR pin from
excessive voltage while R6 provides a ground path. An
overvoltage at V
CC
beyond 35V will pull the FTMR pin
above 1.23V through diode D2A and force a fault status.
If V
CC
has a transient suppressor as shown in Figure 10,
the overvoltage threshold should be set at 35V which
is below the transient suppressor SMCJ33A minimum
breakdown voltage of 36.7V.
R5
1k
FTMR
V
CC
D2A
1N4148
Z3
3V
Z2
33V
4226 F06
R6
1M
Figure 6. V
CC
Overvoltage Detection
applicaTions inForMaTion
Supply Transient Protection
All pins on the LTC4226 are tested for 44V operation with
the exception of FTMR and GATE. The GATE pins are volt-
age clamped either to OUT or GND while the FTMR pins
are low voltage. If greater than 44V supply transients are
possible, 33V transient suppressors are highly recom-
mended at the V
CC
pins to clamp the voltage below the
55V absolute maximum voltage rating of the pins.
Output Positive Overvoltage Isolation
Transient voltage suppressors are adequate for clamping
short overvoltage pulses at the ports, but they may over-
heat if forced to sink large currents for extended periods.
Figure 10 shows how series MOSFETs can be used to
isolate positive port voltages up to the MOSFET V
BVDSS
.
Q3 and Q4 are turned off when the overvoltage detection
Zener Z2 pulls both FTMR1 and FTMR2 high through D2A
and D2B. The resistors R7 and R8 with MOSFETs Q5 and
Q6 facilitate restart by pulling up through the body diodes
of Q1 and Q2, respectively.