LT1952EGN#PBF Linear Technology, LT1952EGN#PBF Datasheet - Page 16

• 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
LT1952/LT1952-1
Example calculation for (2)
Note 1: To achieve the same maximum duty cycle clamp at
100kHz as calculated for 200kHz, the SS_MAXDC voltage
should be reprogrammed by:
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:
Programming Soft-Start Timing
The LT1952/LT1952-1 have built-in soft-start capability to
provide low stress controlled start-up from a list of fault
conditions that can occur in the application (see Figure 1
and Figure 10). The LT1952/LT1952-1 provide true PWM
soft-start by using the SS_MAXDC pin to control soft-start
timing. The proportional 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. A capacitor C
pin and the resistor divider from V
16
For R
R
this gives SS_MAXDC(DC) = 1.84V, t
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%)
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)
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
DELAY
T
= 35.7k, R
= 40k, f
OSC
B
= 100k, V
= 200kHz and SD_V
REF
SS
= 2.5V,
REF
on the SS_MAXDC
used to program
DELAY
SEC
= 40ns
= 1.32V,
maximum switch duty cycle clamp, determine soft-start
timing (Figure 11).
A soft-start event is triggered for the following faults:
(1) V
(2) SD_V
(3) OC > 107mV (overcurrent 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
Soft-start latch reset requires all of the following:
SS_MAXDC
SS_MAXDC
SS_MAXDC
SS_MAXDC
C
IN
SOFT-START
SOFT-START
EVENT TRIGGERED
EVENT TRIGGERED
SS
< 8.75V, or
REF
SEC
Figure 11. Programming Soft-Start Timing
R
B
R
T
to be disabled and to fall to ground.
< 1.32V (UVLO), or
SS_MAXDC CHARGING MODEL
SS_MAXDC(DC) = V
R
Figure 10. Soft-Start Timing
SS_MAXDC
V
CHARGE
REF
LT1952-1
LT1952/
= [R
TIMING (A): SOFT START FAULT REMOVED
TIMING (A): SOFT START FAULT REMOVED
BEFORE SS_MAXDC FALLS TO 0.45V
BEFORE SS_MAXDC FALLS TO 0.45V
TIMING (B): SOFT-START FAULT REMOVED
TIMING (B): SOFT-START FAULT REMOVED
AFTER SS_MAXDC FALLS PAST 0.45V
AFTER SS_MAXDC FALLS PAST 0.45V
T
• R
B
SS_MAXDC(DC)
/(R
REF
T
[R
+ R
B
/(R
B
)]
R
CHARGE
T
+ R
0.8V (ACTIVE THRESHOLD)
0.8V (ACTIVE THRESHOLD)
0.45V (RESET THRESHOLD)
0.45V (RESET THRESHOLD)
0.8V (ACTIVE THRESHOLD)
0.8V (ACTIVE THRESHOLD)
0.45V (RESET THRESHOLD)
0.45V (RESET THRESHOLD)
0.2V
0.2V
B
)]
C
SS
SS_MAXDC
LT1952-1
LT1952/
1952 F10
1952 F10
19521fd
1952 F11