LT1952IGN Linear Technology, LT1952IGN Datasheet - Page 18

• Manufacturer Part Number: LT1952IGN
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,SOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
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
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
REF
REF
SS(MIN)
CHARGE
= 0.45V) is calculated from,
= 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
SS
T
• (–1) • ln(1 – 0.45/1.84)
= 0.8V) – t(V
B
U
= 35.7k, R
)
• (–1) • ln(1 – V
SS
U
DISCHARGE
/SS_MAXDC(DC))
B
T
= 100k, C
SS
• R
= 0.45V)
SS
W
B
–4
/(R
+ t
/SS_MAXDC(DC))
s
CHARGE
T
SS
SS(ACTIVE)
+ R
= 0.1µF and
B
)
U
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
where converter duty cycle is in regulation (DC(REG)) and
no longer controlled by SS_MAXDC (SS_MAXDC =
V
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 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,
SS(REG)
t = R
= 2.63e
= 2.63e
t
= t
Output Rise Time = t(V
DC(REG) = Max Duty Cycle clamp
SS
CHARGE
DISCHARGE
SEC
= 0.8V) is calculated from,
CHARGE
OSC
). Converter output rise time can be expressed as,
= 1.32V.
4
–3
= (1.5 – 0.73)e
• 1e
= 200kHz and R
• (–1) • ln(0.565) = 1.5e
• C
–7
+ t
SS
• (–1) • ln(1 – 0.8/1.84)
CHARGE
SS(REG)
• (–1) • ln(1 – V
SS(REG)
–3
= 1.85e
DELAY
s = 7.7e
SS(ACTIVE)
) – t(V
–4
= 40k.
SS
+ 7.7e
–4
–3
/SS_MAXDC(DC))
s
SS(ACTIVE)
s
SS(REG)
) and the time
–4
= 9.55e
) that no
)
19521fb
–4
s