LT1952 Linear Technology, LT1952 Datasheet - Page 17

• Manufacturer Part Number: LT1952
• Description: Single Switch Synchronous Forward Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Soft-start latch reset requires all of the following:
(A) V
(B) SD_V
(C) OC < 100mV, and
(D) SS_MAXDC < 0.45V (SS_MAXDC reset threshold)
*V
by over-current condition in (3) above
SS_MAXDC Discharge Timing
It can be seen in Figure 10 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 before
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 re-
mains 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),
For a fault arising from (3),
IN
SS_MAXDC (t
(C
recharge
(if fault removed before t
I
C
SS_MAXDC(DC) = programmed DC voltage
V
I
V
V
SS_MAXDC(DC) = V
V
DIS
DIS
SS
SS(MIN)
REF
REF
SS(MIN)
SS
> 8.75V is ok for latch reset if the latch was only set
IN
~ 8e
/I
= net discharge current on C
= capacitor value at SS_MAXDC pin
> 14.25*, 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
U
) =
– V
REF
SS(MIN)
U
[R
FALL
B
/(R
)[(1/2R
)
T
+ R
W
SS
SS(MIN)
B
B
) – (1/R
)]
]
U
T
)]
Example
If the OC fault is not removed before 185µs then SS_MAXDC
will continue to fall past 0.45V towards a new V
The typical V
SS_MAXDC Charge Timing
When all faults are removed and the SS_MAXDC pin has
fallen to its reset threshold of 0.45V or lower, the
SS_MAXDC pin will be released and allowed to charge.
SS_MAXDC will rise until it settles at its programmed DC
voltage — setting the maximum switch duty cycle clamp.
The calculation of charging time for the SS_MAXDC pin
between any two voltage levels can be approximated as an
RC charging waveform using the model shown in
Figure 11.
The ability to predict SS_MAXDC rise time between any
two voltages allows prediction of several key timing
periods:
(1) No Switching Period
(2) Converter Output Rise Time
(3) Time For Maximum Duty Cycle Clamp within X% of
The time for SS_MAXDC to charge to a given voltage V
is found by re-arranging,
For an over-current fault (OC > 100mV), V
R
V
I
= 8e
SS_MAXDC(DC) = 1.84V
SS_MAXDC (t
= 1.85e
V
DIS
(time from SS_MAXDC(DC) to V
V
(time from V
level of SS_MAXDC where maximum duty cycle
clamp equals the natural duty cycle of the switch)
Target Value
SS(MIN)
SS
T
SS(MIN)
= 35.7k, R
(t) = SS_MAXDC(DC) (1 – e
~ 8e
–4
+ (2.05)(–0.23e
–4
–4
= 0.45V,
to V
s
OL
+ (2.5 – 0.45)[(1/2 • 100k) – (1/35.7k)]
for SS_MAXDC at 150µA is 0.2V.
B
SS(ACTIVE)
FALL
SS(ACTIVE)
= 100k, C
) = (1e – 7/7.5e
–4
)
to V
SS
) = 7.5e
= 0.1µF and assume
SS(REG)
www.DataSheet4U.com
(–t/RC)
–4
SS(MIN)
–4
; V
) • (1.84 – 0.45)
)
SS(REG)
LT1952
+ time from
REF
is the
= 2.5V,
SS(MIN)
17
1952f
SS
.