ltc4269cdkd-2 Linear Technology Corporation, ltc4269cdkd-2 Datasheet - Page 29

• 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
From Step 1 and Step 2
The total time of no switching for the converter due to a
soft-start event
Example (2) Converter Output Rise Time
The rise time for the converter output to reach regulation
can be closely approximated as the time between the start
of switching (SS_MAXDC = V
converter duty cycle is in regulation (DC(REG)) and no
longer controlled by SS_MAXDC (SS_MAXDC = V
Converter output rise time can be expressed as:
Step 1: Determine converter duty cycle DC(REG) for output
in regulation.
The natural duty cycle DC(REG) of the converter depends
on several factors. For this example it is assumed that
DC(REG) = 60% for power supply input voltage near the
power supply UVLO. This gives SD_V
Also assume that the maximum duty cycle clamp pro-
grammed for this condition is 72% for SS_MAXDC(DC)
= 1.84V, f
Step 2: Calculate V
To calculate the level of SS_MAXDC (V
longer clamps the natural duty cycle of the converter, the
equation for maximum duty cycle clamp must be used
(see previous section Programming Maximum Duty Cycle
Clamp).
The point where the maximum duty cycle clamp meets
DC(REG) during soft-start is given by:
This gives k = 1 and t
Rearranging the above equation to solve for SS_MAXDC
= V
t
= t
Output Rise Time = t(V
DC(REG) = Max Duty Cycle Clamp
0.6 = k • 0.522(SS_MAXDC(DC)/SD_V
• f
For SD_V
R
CHARGE
SS(REG)
DELAY
OSC
DISCHARGE
)
OSC
= 40k
= (1.5 – 0.73)e
SEC
= 200kHz and R
+ t
= 1.32V, f
CHARGE
SS(REG)
DELAY
OSC
SS(REG)
= 1.85e
–3
= 40ns.
SS(ACTIVE)
s = 7.7e
DELAY
= 200kHz and
) – t(V
–4
= 40k.
+ 7.7e
–4
) and the time where
SEC
SS(ACTIVE)
s
SS(REG)
SEC
= 1.32V.
–4
) – (t
= 9.55e
) that no
SS(REG)
)
DELAY
–4
s
).
Step 3: Calculate t(V
Recall the time for SS_MAXDC to charge to a given volt-
age V
(Figure 16 gives the model for SS_MAXDC charging)
For R
The rise time for the converter output:
Example (3) Time For Maximum Duty Cycle Clamp to
Reach Within X% of Target Value
A maximum duty cycle clamp of 72% was calculated previ-
ously in the section ‘Programming Maximum Duty Cycle
Clamp’. The programmed value used for SS_MAXDC(DC)
was 1.84V.
The time for SS_MAXDC to charge from its minimum value
V
From previous calculations, t(0.45) = 7.3e
Using previous values for R
Hence the time for SS_MAXDC to charge from its mini-
mum reset threshold of 0.45V to within 2% of its target
value is given by:
SS(MIN)
For X = 2 and V
= t(1.803) – t(0.45)
= [0.6 + (t
= [0.6 + (40ns • 200kHz)(1.32V)]/(1 • 0.522)
= (0.608)(1.32)/0.522 = 1.537V
t = R
For C
= t(V
= 2.63e
t(V
ln(1 – 1.66/1.84) = 2.63e
= t(V
= 3.5e
t(SS_MAXDC charge time within X% of target)
= t[(1 – (X/100) • SS_MAXDC(DC)] – t(V
t(1.803) = 2.63e
= 2.63e
t(1.803) – t(0.45) = 1.03e
T
SS
SS(REG)
= 35.7k, R
CHARGE
SS(0.8V)
SS(REG)
SS
is given by:
–3
to within X% of SS_MAXDC(DC) is given by:
–3
–3
= 0.1μF , this gives t(V
s
DELAY
) = t(V
• (–1) • ln(0.565) = 1.5e
• (–1) • ln(0.02) = 1.03e
) – t(V
) = 2.63e
• C
B
SS
SS(MIN)
• f
–4
= 100k, R
SS(1.537V)
SS(REG
• (–1) • ln(1 – V
OSC
SS(ACTIVE)
• 1e
4
)(SD_V
= 0.45V, t(0.98 • 1.84) – t(0.45)
• 1e
–7
)) – t(V
T
–3
• (–1) • ln(1 – 1.803/1.84)
–2
) = 26.3k • 0.1μF • –1 •
CHARGE
, R
–7
• (–1) • ln(0.146) = 5e
) = (5 – 1.5)e
– 7.3e
B
SEC
• (–1) • ln(1 – 0.8/1.84)
SS(ACTIVE)
and C
SS(ACTIVE)
)]/(k • 0.522)
LTC4269-2
SS
= 26.3k
–3
–2
–4
/SS_MAXDC(DC))
SS
s
s
= 9.57e
,
)
–4
SS(MIN)
)
–3
s.
s
–3
29
s
)
42692f
–3
s