LT1952-1 Linear Technology, LT1952-1 Datasheet - Page 18

• Manufacturer Part Number: LT1952-1
• Description: Single Switch Synchronous Forward Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LT1952/LT1952-1
to give:
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 exists. 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
18
DISCHARGE
CHARGE
V
t = RC • (–1) • ln(1 – V
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
SS
DISCHARGE
CHARGE
CHARGE
SS
SS
REF
REF
SS(MIN)
CHARGE
= 0.45V) is calculated from,
(t) = SS_MAXDC(DC) (1 – e
= SS_MAXDC voltage at time t
(R
CHARGE
= 2.5V, R
SS
CHARGE
is calculated by assuming the following:
B
4
–3
/(R
(Figure 11)
= charge time from 0.45V to V
= t(V
was already calculated earlier as 185μs.
= 0.45V.
= (35.7k • 100k/135.7k) = 26.3k
• 1e
• (–1) • ln(0.755) = 7.3e
T
= discharge time from SS_MAXDC(DC) to
• C
+ R
–7
(Figure 11) = R
SS
T
SS
• (–1) • ln(1 – 0.45/1.84)
= 35.7k, R
= 0.8V) – t(V
B
)
• (–1) • ln(1 – V
SS
DISCHARGE
/SS_MAXDC(DC))
B
= 100k, C
T
SS
• R
(–t/RC)
= 0.45V)
B
SS
–4
/(R
+ t
/SS_MAXDC(DC))
s
CHARGE
T
SS
)
SS(ACTIVE)
+ R
= 0.1μF and
B
)
Step 3:
t(V
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 system input voltage near the undervoltage
lockout threshold (UVLO). This gives SD_V
Also assume that the maximum duty cycle clamp
programmed for this condition is 72% for SS_MAXDC(DC)
= 1.84V, f
Step 2: Calculate V
To calculate the level of SS_MAXDC (V
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:
t = R
= 2.63e4 • 1e
= 2.63e–3 • (–1) • ln(0.565) = 1.5e
t
= t
Output Rise Time = t(V
DC(REG) = Max Duty Cycle clamp
0.6 = k • 0.522(SS_MAXDC(DC)/SD_V
(t
SS
CHARGE
DELAY
DISCHARGE
= 0.8V) is calculated from:
CHARGE
OSC
• f
= (1.5 – 0.73)e
OSC
= 200kHz and R
• CSS • (–1) • ln(1 – V
+ t
–7
)
CHARGE
• (–1) • ln(1 – 0.8/1.84)
SS(REG)
SS(REG)
= 1.85e
–3
SS(ACTIVE)
s = 7.7e
DELAY
) – t(V
–4
SS(REG)
+ 7.7e
= 40k.
SS
–4
) and the time where
–3
/SS_MAXDC(DC))
SS(ACTIVE)
s
s
SEC
–4
) that no longer
SEC
) –
= 9.55e
= 1.32V.
SS(REG)
)
19521fd
–4
s
).