LTC4259CGW Linear Technology, LTC4259CGW Datasheet - Page 17

• Manufacturer Part Number: LTC4259CGW
• Manufacturer: Linear Technology

Specifications of LTC4259CGW

• Linear Misc Type: Negative Voltage
• Operating Supply Voltage (typ): -48V
• Operating Supply Voltage (max): -57V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 36
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIO S I FOR ATIO
Dual-Level Current Limit
A PD is permitted to draw up to 350mA continuously and
up to 400mA for 50ms. The LTC4259 has two correspond-
ing current limit thresholds, I
typ) and I
given by the equations:
R
802.3af compliance. Two 1% 1 thick-film resistors in
parallel may be used to minimize cost. While the LTC4259
allows the port current to exceed I
period (see t
rent to exceed I
port current by monitoring the voltage across the sense
resistor and reduces the MOSFET gate voltage as needed
to keep the current at or below I
drops below I
value to keep the MOSFET resistance to a minimum.
t
Whenever more than I
the port’s sense voltage is above V
counts up. If the sense voltage is still above V
t
the port immediately and set the appropriate t
in register 06h/07h. The t
grammed via register 16h, bits 3 and 2 (Table 1).
The t
protect the external MOSFET from thermal stress caused
by operating in current limit for an extended duration. The
counter counts up whenever the current is above I
counts down at 1/16th the rate when it is not. The counter
will bottom out at zero to prevent underflow. Full count
indicates that the t
be turned off.
This count up/count down behavior creates duty cycle
protection, preventing intermittent current limit faults from
causing cumulative thermal stress in the MOSFET. If the port
enters current limit but then exits before the timer expires,
the count will decrease slowly, giving the I
ICUT
ICUT
S
I
CUT
is the sense resistor and should be 0.5
ICUT
timer expires, the LTC4259 will turn off the power to
Timing
= V
timer is an up/down counter that is designed to
LIM
CUT
ICUT
(400mA to 450mA, 425mA typ). These are
LIM
/R
LIM
S
timing below), it does not allow the cur-
, the gate voltage is restored to the full
, I
ICUT
. The current limit circuit monitors the
U
LIM
CUT
timer has expired and the port will
= V
ICUT
U
= V
LIM
CUT
CUT
/R
timer duration can be pro-
(350mA to 400mA, 375mA
/R
S
LIM
CUT
S
W
CUT
flows through a port,
. When the current
and the t
for a limited time
CUT
ICUT
CUT
U
ICUT
when the
for IEEE
Fault bit
CUT
timer a
timer
and
headstart with a repeated fault unless the repeat interval is
infrequent (less than 6.3% duty cycle typ).
If the t
timer will continue to run, counting down at the slow
1/16th rate and preventing the port from being repowered
until the count returns to zero. This protects the MOSFET
from damage due to a faulty PD that may still have a valid
signature, or from errant software that repeatedly writes to
the Power On bit.
The port will not repower until a valid detect occurs after
the count returns to zero in auto mode or the power on bit
is written to after the count returns to zero. Writing to the
power on bit before the count returns to zero has no effect.
t
To distinguish between normal turn-on current limit be-
havior and current limit faults which occur after power-up
is complete, the LTC4259 starts a timer (the t
whenever a power-up sequence begins.
The t
most, it allows the user to specify a different current limit
timeout (t
limit duty cycle protection remains functional). Second,
the DC disconnect timer is disabled during this period and
can only begin counting up after the t
expired. Together, these two features let the PD draw the
maximum current I
boot up and begin drawing the minimum load without
triggering a t
limit for the entire t
generated instead of a t
tracking down the cause of a current fault.
As long as the PD draws less than I
and begins drawing the minimum current within t
t
be indicated.
The t
and its duration can be programmed via register 16h, bits
5 and 4 (Table 1).
START
START
START
START
ICUT
expires (if DC disconnect is enabled) no faults will
Timing
START
timer expires and causes the port to shut off, the
timer also implements duty cycle protection
timer serves three functions. First and fore-
START
instead of t
INRUSH
fault. Finally, if the device is in current
START
ICUT
to charge its input capacitance,
ICUT
period, a t
fault. This can be useful for
) during turn-on (current
CUT
at the end of t
START
LTC4259
START
START
fault will be
timer has
DIS
17
timer)
START
after
4259i