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LT1952-1 Datasheet, PDF (18/24 Pages) Linear Technology – Single Switch Synchronous Forward Controller
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LT1952/LT1952-1
APPLICATIONS INFORMATION
VSS(t) = SS_MAXDC(DC) (1 – e(–t/RC))
to give:
t = RC • (–1) • ln(1 – VSS/SS_MAXDC(DC))
where:
VSS = SS_MAXDC voltage at time t
SS_MAXDC(DC) = programmed DC voltage setting
maximum duty cycle clamp =
VREF(RB/(RT + RB)
R = RCHARGE (Figure 11) = RT • RB/(RT + RB)
C = CSS (Figure 11)
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).
No Switching Period = tDISCHARGE + tCHARGE
tDISCHARGE = discharge time from SS_MAXDC(DC) to
0.45V
tCHARGE = charge time from 0.45V to VSS(ACTIVE)
tDISCHARGE was already calculated earlier as 185μs.
tCHARGE is calculated by assuming the following:
VREF = 2.5V, RT = 35.7k, RB = 100k, CSS = 0.1μF and
VSS(MIN) = 0.45V.
tCHARGE = t(VSS = 0.8V) – t(VSS = 0.45V)
Step 1:
SS_MAXDC(DC) = 2.5[100k/(35.7k + 100k)] = 1.84V
RCHARGE = (35.7k • 100k/135.7k) = 26.3k
Step 2:
t(VSS = 0.45V) is calculated from,
t = RCHARGE • CSS • (–1) • ln(1 – VSS/SS_MAXDC(DC))
= 2.63e4 • 1e–7 • (–1) • ln(1 – 0.45/1.84)
= 2.63e–3 • (–1) • ln(0.755) = 7.3e–4 s
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Step 3:
t(VSS = 0.8V) is calculated from:
t = RCHARGE • CSS • (–1) • ln(1 – VSS/SS_MAXDC(DC))
= 2.63e4 • 1e–7 • (–1) • ln(1 – 0.8/1.84)
= 2.63e–3 • (–1) • ln(0.565) = 1.5e–3 s
From Step 1 and Step 2:
tCHARGE = (1.5 – 0.73)e–3 s = 7.7e–4 s
The total time of no switching for the converter due to a
soft-start event:
= tDISCHARGE + tCHARGE = 1.85e–4 + 7.7e–4 = 9.55e–4 s
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 = VSS(ACTIVE)) and the time where
converter duty cycle is in regulation (DC(REG)) and no
longer controlled by SS_MAXDC (SS_MAXDC = VSS(REG)).
Converter output rise time can be expressed as:
Output Rise Time = t(VSS(REG)) – t(VSS(ACTIVE))
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_VSEC = 1.32V.
Also assume that the maximum duty cycle clamp
programmed for this condition is 72% for SS_MAXDC(DC)
= 1.84V, fOSC = 200kHz and RDELAY = 40k.
Step 2: Calculate VSS(REG)
To calculate the level of SS_MAXDC (VSS(REG)) that no 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:
DC(REG) = Max Duty Cycle clamp
0.6 = k • 0.522(SS_MAXDC(DC)/SD_VSEC) –
(tDELAY • fOSC)
19521fd
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