ltc4269cdkd-2 Linear Technology Corporation, ltc4269cdkd-2 Datasheet - Page 27

• Manufacturer Part Number: ltc4269cdkd-2
• Description: Ieee 802.3at High Power Pd And Synchronous Forward Controller With Aux Support
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
SOUT
OUT
SS_MAXDC
SS_MAXDC
SS_MAXDC
SOFT-START
EVENT TRIGGERED
FAULTS TRIGGERING SOFT-START
V
OR
SD_V
OR
OC > 107mV (OVERCURRENT)
IN
< 8.75V
SEC
C
SS
< 1.32V (UVLO)
Figure 18. Programming Soft-Start Timing
R
B
R
t
T
DELAY
Figure 17. Soft-Start Timing
Figure 16. Timing Diagram
SS_MAXDC CHARGING MODEL
SS_MAXDC(DC) = V
R
SS_MAXDC
V
: PROGRAMMABLE SYNCHRONOUS DELAY
CHARGE
LTC4269-2
REF
TIMING (A): SOFT START FAULT REMOVED
BEFORE SS_MAXDC FALLS TO 0.45V
TIMING (B): SOFT-START FAULT REMOVED
AFTER SS_MAXDC FALLS PAST 0.45V
= [R
T
• R
B
SOFT-START
SS_MAXDC(DC)
/(R
LATCH SET
REF
T
[R
+ R
B
/(R
B
)]
R
CHARGE
T
+ R
42692 F17
B
)]
0.8V (ACTIVE THRESHOLD)
0.45V (RESET THRESHOLD)
0.8V (ACTIVE THRESHOLD)
0.45V (RESET THRESHOLD)
0.2V
SOFT-START LATCH RESET:
V
(> 8.75V IF LATCH SET BY OC)
AND
SD_V
AND
OC < 107mV
AND
SS_MAXDC < 0.45V
IN
0.8V (ACTIVE THRESHOLD)
0.45V (RESET THRESHOLD)
0.2V
C
> 14.25V
SS
SEC
> 1.32V
SS_MAXDC
LTC4269-2
42692 F18
42692 F16
Soft-start latch reset requires all of the following:
(A) V
(B) SD_V
(C) OC < 107mV, and
(D) SS_MAXDC < 0.45V (SS_MAXDC reset threshold)
*V
set by overcurrent condition in (3) above.
SS_MAXDC Discharge Timing
It can be seen in Figure 17 that two types of discharge
can occur for the SS_MAXDC pin. In timing (A) the fault
that caused the soft-start event has been removed be-
fore SS_MAXDC falls to 0.45V. This means the soft-start
latch will be reset when SS_MAXDC falls to 0.45V and
SS_MAXDC will begin charging. In timing (B), the fault that
caused the soft-start event is not removed until some time
after SS_MAXDC has fallen past 0.45V. The SS_MAXDC
pin continues to discharge to 0.2V and remains low until
all faults are removed.
The time for SS_MAXDC to fall to a given voltage can be
approximated as:
where:
For faults arising from (1) and (2):
IN
SS_MAXDC(t
(C
I
C
SS_MAXDC(DC) = programmed DC voltage
V
recharge
I
V
For a fault arising from (3):
V
SS_MAXDC(DC) = V
V
(if fault removed before t
DIS
DIS
SS
SS(MIN)
REF
REF
SS(MIN)
SS
IN
> 8.75V is okay for latch reset if the latch was only
= net discharge current on C
≅ 8e
= capacitor value at SS_MAXDC pin
/I
> 14.25V*, and
= 100mV.
= 2.5V.
DIS
SEC
–4
) • [SS_MAXDC(DC) – V
= minimum SS_MAXDC voltage before
= SS_MAXDC reset threshold = 0.45V
> 1.32V, and
+ (V
FALL
REF
)=
– V
REF
SS(MIN)
[R
FALL
B
/(R
)
)[(1/2R
T
+ R
LTC4269-2
SS
SS(MIN)
B
)]
B
) – (1/R
]
T
27
)]
42692f