31
LTC4230
4230f
APPLICATIO S I FOR ATIO
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bias current 1N4691 zener diode is chosen to protect the
system. Here, the zener diode is connected from V
CC
to
the LTC4230’s FILTER pin. If the input voltage to the
system is greater than 6.8V during start-up, the voltage
on the FILTER pin is pulled higher than its 1.19V thresh-
old. As a result, the GATE
n
pin is not allowed to ramp and
the second timing cycle will not commence until the
supply overvoltage condition is removed. Should the
supply overvoltage condition occur during normal op-
eration, internal control logic would trip the electronic
circuit breaker and the GATE would be pulled to ground,
shutting off the external pass transistor. If a lower supply
overvoltage threshold is desired, use a zener diode with
a smaller breakdown voltage.
A timing diagram for illustrating LTC4230 operation under
a high side overvoltage condition is shown in Figure 23.
The start-up sequence in this case (between Time Points
1 and 2) is identical to any other start-up sequence under
normal operating conditions. At Time Point 2, the input
supply voltage causes the zener diode to conduct thereby
forcing V
FILTER
> 1.19V. At Time Point 3, FAULT is asserted
low and the TIMER pin voltage ramps down. At Time
Point␣ 4, the LTC4230 checks if V
FILTER
< 1.19V. FAULT is
asserted low (but not latched) to indicate a start-up failure.
Only if the input overvoltage condition is removed before
Time Point 5 does the start-up sequence resume at the
second timing cycle. At this point in time, the GATE
n
pin
voltage is allowed to ramp up, FAULT is pulled to logic high
and the circuit breaker is armed. Should, at any time after
Time Point 5, a supply overvoltage condition develop
(V
FILTER
> 1.26V), the electronic circuit breaker will trip,
the GATE
n
will be pulled low to turn off the external
MOSFET and FAULT will be asserted low and latched.
Low Side (Output) Overvoltage Protection
A zener diode can be used in a similar fashion to detect/
protect the system against a supply overvoltage condition
on the load (or low) side of the pass transistor. In this case,
the zener diode is connected from the load to the LTC4230’s
FILTER pin, as shown in Figure 24. An additional diode,
D1, prevents the FILTER pin from pulling low during
output short-circuit. Figure 25 illustrates the timing dia-
gram for a low side output overvoltage condition. In this
example, the LTC4230 can only sense the overvoltage
supply condition after Time Point 5 and the GATE
n
pin has
ramped up to its nominal operating value. After Time
Point␣ 5, a supply voltage fault occurs at the load and the
zener diode conducts, causing V
FILTER
to increase. At Time
Point 6, V
FILTER
is greater than 1.26V, the circuit breaker
trips, GATE pulls to ground and FAULT asserts low and is
latched.
In either case, the LTC4230 can be configured to auto-
matically initiate a start-up sequence. Please refer to the
section on AutoRetry After a Fault for additional
information.
PCB LAYOUT CONSIDERATIONS
For proper operation of the LTC4230’s circuit breaker
function, a 4-wire Kelvin connection to the sense resistors
is highly recommended. A recommended PCB layout for
the sense resistor, the power MOSFET and the GATE drive
components around the LTC4230 is illustrated in Fig-
ure␣ 26. In Hot Swap applications where load currents can
reach 10A or more, narrow PCB tracks exhibit more
resistance than wider tracks and operate at more elevated
temperatures. Since the sheet resistance of 1 ounce
copper foil is approximately 0.54mΩ/square, track resis-
tances add up quickly in high current applications. Thus,
to keep PCB track resistance and temperature rise to a
minimum, PCB track width must be appropriately sized.
Consult Appendix A of LTC Application Note 69 for details
on sizing and calculating trace resistances as a function of
copper thickness.
In the majority of applications, it will be necessary to use
plated-through vias to make circuit connections from
component layers to power and ground layers internal to
the PC board. For 1 ounce copper foil plating, a good
starting point is 1A of DC current per via, making sure the
via is properly dimensioned so that solder completely fills
any void. For other plating thicknesses, check with your
PCB fabrication facility.
