LT1952
16
1952f
APPLICATIO S I FOR ATIO
WUU
U
Example calculation for (2)
For R
T
= 35.7k, R
B
= 100k, V
REF
= 2.5V,
R
DELAY
= 40k, f
OSC
= 200kHz and SD_V
SEC
= 1.32V,
this gives SS_MAXDC(DC) = 1.84V, t
DELAY
= 40ns
and k = 1
Maximum Duty Cycle Clamp
= 1 • 0.522(1.84/1.32) – (40ns • 200kHz)
= 0.728 – 0.008 = 0.72 (Duty Cycle Clamp = 72%)
Note 1: To achieve the same maximum duty cycle clamp
at 100kHz as calculated for 200kHz, the SS_MAXDC
voltage should be reprogrammed by,
SS_MAXDC(DC) (100kHz)
= SS_MAXDC(DC) (200kHz) • k (200kHz)/k (100kHz)
= 1.84 • 1.0/1.055 = 1.74V (k = 1.055 for 100kHz)
The second order effect of t
DELAY
should also be consid-
ered for final adjustment of SS_MAXDC(DC).
Note 2 : To achieve the same maximum duty cycle clamp
while synchronizing to an external clock at the SYNC pin,
the SS_MAXDC voltage should be re-programmed as,
SS_MAXDC (DC) (fsync)
= SS_MAXDC (DC) (200kHz) • [(fosc/fsync) +
0.09(fosc/200kHz)
0.6
]
For SS_MAXDC (DC) (200kHz) = 1.84V for 72%
duty cycle
SS_MAXDC (DC) (fsync = 250kHz)
for 72%
duty cycle
= 1.84 • [(200kHz/250kHz) + 0.09(1)
0.6
]
= 1.638V
Programming Soft-Start Timing
The LT1952 has built-in soft-start capability to provide low
stress controlled startup from a list of fault conditions that
can occur in the application (see Figure 1 and Figure 10).
The LT1952 provides true PWM soft-start by using the
SS_MAXDC pin to control soft-start timing. The propor-
tional relationship between SS_MAXDC voltage and switch
maximum duty cycle clamp allows the SS_MAXDC pin to
slowly ramp output voltage by ramping the maximum
switch duty cycle clamp — until switch duty cycle clamp
seamlessly meets the natural duty cycle of the converter.
Figure 10. Soft-Start Timing
A capacitor C
SS
on the SS_MAXDC pin and the resistor
divider from V
REF
used to program maximum switch duty
cycle clamp, determine soft-start timing (Figure 11).
A soft-start event is triggered for the following faults:
(1) V
IN
< 8.75V, or
(2) SD_V
SEC
< 1.32V (UVLO), or
(3) OC > 100mV (over-current condition)
When a soft-start event is triggered, switching at SOUT
and OUT is stopped immediately. A soft-start latch is set
and SS_MAXDC pin is discharged. The SS_MAXDC pin
can only recharge when the soft-start latch has been reset.
Note: A soft-start event caused by (1) or (2) above, also
causes V
REF
to be disabled and to fall to ground.
SOFT-START
EVENT TRIGGERED
TIMING (A): SOFT START FAULT REMOVED
BEFORE SS_MAXDC FALLS TO 0.45V
SS_MAXDC
0.8V (ACTIVE THRESHOLD)
0.45V (RESET THRESHOLD)
1952 F10
TIMING (B): SOFT-START FAULT REMOVED
AFTER SS_MAXDC FALLS PAST 0.45V
SS_MAXDC
0.8V (ACTIVE THRESHOLD)
0.45V (RESET THRESHOLD)
0.2V
Figure 11. Programming Soft-Start Timing
SS_MAXDC CHARGING MODEL
SS_MAXDC(DC) = V
REF
[R
B
/(R
T
+ R
B
)]
R
CHARGE
= [R
T
• R
B
/(R
T
+ R
B
)]
SS_MAXDC(DC)
1952 F11
SS_MAXDC
LT1952
R
CHARGE
SS_MAXDC
V
REF
LT1952
R
B
C
SS
R
T
C
SS
