ltc4269cdkd-2 Linear Technology Corporation, ltc4269cdkd-2 Datasheet - Page 28

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

LTC4269-2
APPLICATIONS INFORMATION
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 16.
The ability to predict SS_MAXDC rise time between any two
voltages allows prediction of several key timing periods:
The time for SS_MAXDC to charge to a given voltage V
is found by re-arranging:
to give,
(1) No Switching Period (time from SS_MAXDC(DC) to
(2) Converter Output Rise Time (time from V
(3) Time For Maximum Duty Cycle Clamp within X% of
28
For an overcurrent fault (OC > 100mV), V
R
V
I
= 8e
SS_MAXDC(DC) = 1.84V
SS_MAXDC(t
= (1e
V
t = RC • (–1) • ln(1 – V
DIS
SS(MIN)
V
V
maximum duty cycle clamp equals the natural duty
cycle of the switch)
Target Value
SS
T
SS(MIN)
SS(REG)
= 35.7k, R
(t) = SS_MAXDC(DC) (1 – e
≅ 8e
–4
–7
+ (2.05)(–0.23e
/7.5e
–4
= 0.45V,
; V
OL
+ time from V
+ (2.5 – 0.45)[(1/2 • 100k) – (1/35.7k)]
–4
SS(REG)
for SS_MAXDC at 150μA is 0.2V.
B
FALL
) • (1.84 – 0.45)=1.85e
= 100k, C
)
is the level of SS_MAXDC where
SS
–4
SS(MIN)
SS
) = 7.5e
/SS_MAXDC(DC))
= 0.1μF and assume
(–t/RC)
to V
–4
SS(ACTIVE)
–4
)
s
REF
SS(ACTIVE
= 2.5V,
)
SS(MIN)
) to
SS
.
where,
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).
t
t
Step 1:
Step 2:
t(V
Step 3:
t(V
DISCHARGE
CHARGE
V
SS_MAXDC(DC) = programmed DC voltage setting
maximum duty cycle clamp = V
R = R
C = C
No Switching Period = t
t
0.45V
t
V
V
t
SS_MAXDC(DC) = 2.5[100k/(35.7k + 100k)] = 1.84V
R
t = R
= 2.63e
= 2.63e
t = R
= 2.63e
= 2.63e
SS
SS
DISCHARGE
CHARGE
CHARGE
SS
REF
SS(MIN)
CHARGE
= 0.45V) is calculated from:
= 0.8V) is calculated from:
= SS_MAXDC voltage at time t
CHARGE
CHARGE
SS
= 2.5V, R
CHARGE
is calculated by assuming the following:
4
–3
4
–3
(Figure 16)
= charge time from 0.45V to V
= t(V
was already calculated earlier as 185μs.
= 0.45V.
• 1e
• 1e
= (35.7k • 100k/135.7k) = 26.3k
• (–1) • ln(0.755) = 7.3e
• (–1) • ln(0.565) = 1.5e
= discharge time from SS_MAXDC(DC) to
• C
• C
–7
–7
SS
(Figure 16) = R
T
SS
SS
• (–1) • ln(1 – 0.45/1.84)
• (–1) • ln(1 – 0.8/1.84)
= 0.8V) – t(V
= 35.7k, R
• (–1) • ln(1 – V
• (–1) • ln(1 – V
DISCHARGE
B
T
= 100k, C
SS
• R
REF
= 0.45V)
B
SS
SS
–4
–3
/(R
+ t
(R
/SS_MAXDC(DC))
/SS–MAXDC(DC))
s
s
B
CHARGE
T
/(R
SS
SS(ACTIVE)
+ R
T
= 0.1μF and
B
+ R
)
B
)
42692f