LT1952EGN#PBF Linear Technology, LT1952EGN#PBF Datasheet - Page 17

• Manufacturer Part Number: LT1952EGN#PBF
• Description: IC CTLR PWM SGL SWITCH 16-SSOP
• Manufacturer: Linear Technology

Specifications of LT1952EGN#PBF

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 560kHz
• Duty Cycle: 90%
• Voltage - Supply: 9.25 V ~ 25 V
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 16-SSOP
• Frequency-max: 560kHz
• Number Of Pwm Outputs: 1
• On/off Pin: No
• Adjustable Output: No
• Topology: Flyback/Forward
• Switching Freq: 80 TO 560kHz
• Operating Supply Voltage (max): 25V
• Synchronous Pin: Yes
• Rise Time: 50ns
• Fall Time: 30ns
• Operating Temperature Classification: Automotive
• Mounting: Surface Mount
• Pin Count: 16
• Package Type: SSOP N
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
(A) V
(B) SD_V
(C) OC < 107mV, and
(D) SS_MAXDC < 0.45V (SS_MAXDC reset threshold)
*V
was only set by overcurrent condition in (3) above.
SS_MAXDC Discharge Timing
It can be seen in Figure 10 that two types of discharge
can occur for the SS_MAXDC pin. In timing (A) the fault
that caused the soft-start event has been removed before
SS_MAXDC falls to 0.45V. This means the soft-start
latch will be reset when SS_MAXDC falls to 0.45V and
SS_MAXDC will begin charging. In timing (B), the fault that
caused the soft-start event is not removed until some time
after SS_MAXDC has fallen past 0.45V. The SS_MAXDC
pin continues to discharge to 0.2V and remains low until
all faults are removed.
The time for SS_MAXDC to fall to a given voltage can be
approximated as:
where:
For faults arising from (1) and (2),
For a fault arising from (3),
IN
SS_MAXDC (t
(C
I
C
SS_MAXDC(DC) = programmed DC voltage
V
recharge
I
V
V
SS_MAXDC(DC) = V
V
(if fault removed before t
DIS
DIS
SS
SS(MIN)
REF
REF
SS(MIN)
> 8.75V (6.5V LT1952-1) is ok for latch reset if the latch
SS
IN
~ 8e
= net discharge current on C
= capacitor value at SS_MAXDC pin
/I
> 14.25* (7.75V LT1952-1), and
= 100mV.
= 2.5V.
DIS
SEC
–4
) • [SS_MAXDC(DC) – V
= minimum SS_MAXDC voltage before
= SS_MAXDC reset threshold = 0.45V
> 1.32V, and
+ (V
FALL
REF
) =
– V
REF
SS(MIN)
[R
FALL
B
/(R
)
)[(1/2R
T
+ R
SS
SS(MIN)
B
)]
B
) – (1/R
]
T
)]
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 11.
The ability to predict SS_MAXDC rise time between any two
voltages allows prediction of several key timing periods:
(1)No Switching Period
(2)Converter Output Rise Time
(3)Time For Maximum Duty Cycle Clamp within X% of
The time for SS_MAXDC to charge to a given voltage V
is found by re-arranging:
For an overcurrent fault (OC > 100mV), V
R
V
I
= 8e
SS_MAXDC(DC) = 1.84V
SS_MAXDC (t
= 1.85e–4 s
(time from SS_MAXDC(DC) to V
V
(time from V
level of SS_MAXDC where maximum duty cycle
clamp equals the natural duty cycle of the switch)
Target Value
DIS
T
SS(MIN)
SS(MIN)
= 35.7k, R
~ 8e
–4
+ (2.05)(–0.23e
–4
= 0.45V,
to V
OL
+ (2.5 – 0.45)[(1/2 • 100k) – (1/35.7k)]
SS(ACTIVE)
for SS_MAXDC at 150μA is 0.2V.
SS(ACTIVE)
B
FALL
= 100k, C
) = (1e – 7/7.5e
LT1952/LT1952-1
)
to V
–4
SS
) = 7.5e
SS(REG)
= 0.1μF and assume
SS(MIN)
–4
–4
; V
) • (1.84 – 0.45)
SS(REG)
+ time from
REF
is the
= 2.5V,
SS(MIN)
17
19521fd
SS
.