LTC4253/LTC4253A
25
425353aff
For more information www.linear.com/4253
APPLICATIONS INFORMATION
V
UVHI
 (V
UV
 for the LTC4253A). In addition, the internal logic 
checks for OV < V
OVHI
 (V
OV
 for the LTC4253A), RESET < 
0.8V , GATE < V
GATEL
,
 
SENSE < V
CB
, SS < 20 " V
OS
 and 
TIMER?燰
TMRL
. When all conditions are met, initial timing 
starts and the TIMER capacitor is charged by a 5礎(chǔ) current 
source pull-up. At time point 3, TIMER reaches the V
TMRH
 
threshold and the initial timing cycle terminates. The TIMER 
capacitor is quickly discharged. At time point?, the V
TMRL
 
threshold is reached and the conditions of GATE?燰
GATEL
, 
SENSE?燰
CB 
and SS??0 " V
OS
 must be satisfied before 
the GATE start-up cycle begins. SS ramps up as dictated 
by R
SS 
" C
SS
; GATE is held low by the analog current 
limit amplifier until SS crosses 20 " V
OS
. Upon releasing 
GATE, 50礎(chǔ) sources into the external MOSFET gate and 
compensation network. When the GATE voltage reaches 
the MOSFETs threshold, current begins flowing into the 
load capacitor at time point 5. At time point 6, load current 
reaches SS control level and the analog current limit loop 
activates. Between time points 6 and 8, the GATE voltage 
is servoed and the SENSE voltage is regulated at V
ACL
(t) 
and soft-start limits the slew rate of the load current. If the 
SENSE voltage (V
SENSE
  V
EE
) reaches the V
CB
 threshold 
at time point 7, the circuit breaker TIMER activates. The 
TIMER capacitor, C
T
 is charged by a (200礎(chǔ)???營(yíng)
DRN
) 
current pull-up. As the load capacitor nears full charge, 
load current begins to decline. At point?, the load cur-
rent falls and the SENSE voltage drops below V
ACL
(t). 
The analog current limit loop shuts off and the GATE pin 
ramps further. At time point 9, the SENSE voltage drops 
below V
CB
 and the fault TIMER ends, followed by a 5礎(chǔ) 
discharge current source (cool-off). When GATE ramps 
past V
GATEH
 threshold at time point A, PWRGD1 pulls low, 
starting off the PWRGD sequence. PWRGD2 pulls low at 
time point C when EN2 is high and PWRGD1 is low for 
more than one t
SQT
. PWRGD3 pulls low at time point D 
when EN2 and EN3 is high and PWRGD2 is low for more 
than one t
SQT
. At time point B, GATE reaches its maximum 
voltage as determined by V
IN
.
Undervoltage Timing
In Figure 10 when the UV pin drops below V
UVLO
 (V
UV
  
V
UVHST
 for the LTC4253A) at time point 1, the LTC4253/
LTC4253A shut down with TIMER, SS and GATE pulled 
low. If current has been flowing, the SENSE pin voltage 
decreases to zero as GATE collapses. When UV recovers 
and clears V
UVHI
 (V
UV
 for the LTC4253A) at time point 2, 
an initial time cycle begins followed by a start-up cycle.
V
IN
 Undervoltage Lockout Timing
V
IN
 undervoltage lockout comparator, UVLO has a similar 
timing behavior as the UV pin timing except it looks at 
V
IN
 < (V
LKO
?燰
LKH
) to shut down and V
IN
 > V
LKO
 to start. 
In an undervoltage lockout condition, both UV and OV 
comparators are held off. When V
IN
 exits undervoltage 
lockout, the UV and OV comparators are enabled.
Overvoltage Timing
During normal operation, if the OV pin exceeds V
OVHI
 
(V
OV
 for the LTC4253A) as shown at time point? of Fig-
ure 11, the TIMER and PWRGD status are unaffected; SS 
and GATE pull down; load disconnects. At time point 2, 
OV recovers and drops below the V
OVLO
 (V
OV
  V
OVHST
 
for the LTC4253A) threshold; GATE start-up begins. If 
the overvoltage glitch is long enough to deplete the load 
capacitor, time points? through 7 may occur.
Circuit Breaker Timing
In Figure 12a, the TIMER capacitor charges at 200礎(chǔ) if 
the SENSE pin exceeds V
CB
 but V
DRN
 is less than 5V . If 
the SENSE pin returns below V
CB
 before TIMER reaches 
the V
TMRH
 threshold, TIMER is discharged by 5礎(chǔ). In 
Figure?12b,  when  TIMER  exceeds  V
TMRH
,  GATE  pulls 
down immediately and the chip shuts down. In Figure?2c, 
multiple momentary faults cause the TIMER capacitor to 
integrate and reach V
TMRH
 followed by GATE pull down 
and  the  chip  shuts  down.  During  chip  shutdown,  the 
LTC4253/LTC4253A latch TIMER high with a 5礎(chǔ) pull-up 
current source.