LTC4259ACGW-1#TR Linear Technology, LTC4259ACGW-1#TR Datasheet - Page 18

• Manufacturer Part Number: LTC4259ACGW-1#TR
• Description: IC,Power Control/Management,CMOS,SOP,36PIN,PLASTIC
• Manufacturer: Linear Technology

Specifications of LTC4259ACGW-1#TR

• Linear Misc Type: Negative Voltage
• Family Name: LTC4259A
• Package Type: SSOP
• Operating Supply Voltage (min): -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
LTC4259A-1
current to charge its bypass capacitance, slowing the rate
of port voltage increase.
Dual-Level Current Limit
A PD is permitted to draw up to 15.4W continuously and
up to 400mA for 50ms. The LTC4259A-1 has two corre-
sponding current limit thresholds, I
I
R
802.3af compliance. While the LTC4259A-1 allows the
port current to exceed I
t
I
monitoring the voltage across the sense resistor and re-
duces the MOSFET gate voltage as needed to keep the
current at or below I
I
the MOSFET resistance to a minimum.
t
Whenever more than I
the port’s sense voltage is above V
counts up. The t
OUT pin voltage is above V
persists and the t
turn off power to the port immediately and set the appro-
priate t
can be programmed via register 16h, bits 2 and 3 (Table
1).
The t
protect the external MOSFET from thermal stress caused
by repeatedly operating in current limit. The counter
counts up whenever the current is above I
down at 1/16th the rate when it is not. The counter will
bottom out at zero to prevent underflow. Full count indi-
cates that the t
turned off.
This count up/count down behavior implements duty cycle
protection, preventing intermittent current limit faults from
causing cumulative thermal stress in the MOSFET. If the port
18
LIM
ICUT
ICUT
LIM
LIM
S
I
CUT
is the sense resistor and should be 0.5Ω for IEEE
. The current limit circuit monitors the port current by
, the gate voltage is restored to the full value to keep
(425mA typ). These are given by the equations:
ICUT
timing below), it does not allow the current to exceed
Timing
ICUT
= V
timer is an up/down counter that is designed to
CUT
fault bit in register 06h/07h. The t
/R
ICUT
ICUT
S
ICUT
, I
U
timer has expired and the port will be
LIM
timer also counts up when the port’s
LIM
CUT
timer expires, the LTC4259A-1 will
= V
CUT
. When the current drops below
U
= V
PG
LIM
for a limited time period (see
. If either of these conditions
CUT
/R
/R
S
CUT
S
W
CUT
flows through a port,
and the t
(375mA typ) and
CUT
ICUT
and counts
U
ICUT
duration
timer
enters current limit but then exits before the timer expires,
the count will decrease slowly, giving the I
ability to turn off sooner in the case of a repetitive fault. If
the overcurrent duty cycle is less than 6.3% the t
will be fully reset.
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 after the t
returns to zero. In manual and semiauto modes the power
enable command must be received after the t
reaches zero. In auto mode the LTC4259A-1 must com-
plete a valid detection cycle after the t
zero.
t
To distinguish between normal turn-on current limit be-
havior and current limit faults which occur after power-up
is complete, the LTC4259A-1 starts a timer (the t
timer) 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 power without triggering a
t
entire t
instead of a t
the cause of an overcurrent fault.
As long as the PD draws less than I
and begins drawing the minimum current within t
t
be indicated.
START
START
START
START
ICUT
fault. Finally, if the device is in current limit for the
expires (if DC disconnect is enabled), no faults will
Timing
START
START
timer expires and causes the port to shut off, the
timer serves three functions. First and fore-
ICUT
period, a t
instead of t
fault. This can be useful for tracking down
INRUSH
START
to charge its input capacitance,
ICUT
) during turn-on (current
fault will be generated
CUT
ICUT
at the end of t
START
counter reaches
CUT
ICUT
ICUT
timer has
ICUT
timer the
DIS
counter
counter
START
START
4259a1fa
timer
after