LTC4269-2
28
42692fb
applicaTions inForMaTion
The time for SS_MAXDC to fall to a given voltage can be
approximated as:
SS_MAXDC(t
FALL
)=
(C
SS
/I
DIS
) • [SS_MAXDC(DC) – V
SS(MIN)
]
where:
I
DIS
= net discharge current on C
SS
C
SS
= capacitor value at SS_MAXDC pin
SS_MAXDC(DC) = programmed DC voltage
V
SS(MIN)
= minimum SS_MAXDC voltage before
recharge
I
DIS
≅ 8e
–4
+ (V
REF
– V
SS(MIN)
)[(1/2R
B
) – (1/R
T
)]
For faults arising from (1) and (2):
V
REF
= 100mV.
For a fault arising from (3):
V
REF
= 2.5V.
SS_MAXDC(DC) = V
REF
[R
B
/(R
T
+ R
B
)]
V
SS(MIN)
= SS_MAXDC reset threshold = 0.45V
(if fault removed before t
FALL
)
Example
For an overcurrent fault (OC > 100mV), V
REF
= 2.5V,
R
T
= 35.7k, R
B
= 100k, C
SS
= 0.1µF and assume
V
SS(MIN)
= 0.45V,
I
DIS
≅ 8e
–4
+ (2.5 – 0.45)[(½ • 100k) – (1/35.7k)]
= 8e
–4
+ (2.05)(–0.23e
–4
) = 7.5e
–4
SS_MAXDC(DC) = 1.84V
SS_MAXDC(t
FALL
)
= (1e
–7
/7.5e
–4
) • (1.84 – 0.45)=1.85e
–4
s
If the OC fault is not removed before 185µs then SS_MAXDC
will continue to fall past 0.45V towards a new V
SS(MIN)
.
The typical V
OL
for SS_MAXDC at 150µA is 0.2V.
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 16.
The ability to predict SS_MAXDC rise time between any two
voltages allows prediction of several key timing periods:
(1)
No
Switching Period (time from SS_MAXDC(DC) to
V
SS(MIN)
+ time from V
SS(MIN)
to V
SS(ACTIVE)
)
(2) Converter Output Rise Time (time from V
SS(ACTIVE
) to
V
SS(REG)
; V
SS(REG)
is the level of SS_MAXDC where
maximum duty cycle clamp equals the natural duty
cycle of the switch)
(3)
T
ime For Maximum Duty Cycle Clamp within X% of
Target Value
The time for SS_MAXDC to charge to a given voltage V
SS
is found by re-arranging:
V
SS
(t) = SS_MAXDC(DC) (1 – e
(–t/RC)
)
to give,
t = RC • (–1) • ln(1 – V
SS
/SS_MAXDC(DC))
where,
V
SS
= SS_MAXDC voltage at time t
SS_MAXDC(DC) = programmed DC voltage setting
maximum duty cycle clamp = V
REF
(R
B
/(R
T
+ R
B
)
R = R
CHARGE
(Figure 16) = R
T
• R
B
/(R
T
+ R
B
)
C = C
SS
(Figure 16)
Example (1) No Switching Period
The period of no switching for the converter, when a soft-
start event has occurred, depends on how far SS_MAXDC
can fall before recharging occurs and how long a fault ex-
ists. It will be assumed that a fault triggering soft-start is
removed before SS_MAXDC can reach its reset threshold
(0.45V).
No Switching Period = t
DISCHARGE
+ t
CHARGE
t
DISCHARGE
= discharge time from SS_MAXDC(DC) to
0.45V
t
CHARGE
= charge time from 0.45V to V
SS(ACTIVE)
t
DISCHARGE
was already calculated earlier as 185µs.
