LT1681ESW Linear Technology, LT1681ESW Datasheet - Page 13

• Manufacturer Part Number: LT1681ESW
• Description: IC SYNCHRONOUS FWRD CTRLR 20SOIC
• Manufacturer: Linear Technology

Specifications of LT1681ESW

• Applications: Synchronous Forward Controller
• Voltage - Supply: 9 V ~ 18 V
• Current - Supply: 17mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-SOIC (7.5mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Voltage - Input: -

APPLICATIO S I FOR ATIO
to the minimum off time, the converter maximum duty
cycle can be forced using the SYNC input. For example, a
30% duty cycle SYNC pulse forces 30% maximum duty
cycle operation for the converter. Because the logic low
pulse width exceeds the logic high pulse width in < 50%
duty cycle operation, the oscillator free-run cycle time
must be programmed to exceed the logic low duration.
It is also possible to run the LT1681 in a SYNC-only mode
by disabling the oscillator completely. Connecting a resis-
tor divider from the 5V
voltage within the charge range of 1.5V to 2.5V, will allow
the oscillator to follow the SYNC input exclusively with no
provision for free-run. Setting values to force a voltage as
close to 2V as possible is recommended.
Shutdown
The LT1681 SHDN pin will support TTL and CMOS logic
signals and also analog inputs. The SHDN pin turn-on
(rising) threshold is 1.25V with 150mV of hysteresis. A
common use of the SHDN pin is for undervoltage detec-
tion on the input supply. Driving the SHDN pin with a
resistor divider connected from the input supply to ground
will prevent switching until the desired input supply volt-
age is achieved.
(INTERNAL)
Figure 5. Oscillator Connection for Sync-Only Mode Operation
SYSTEM
CLOCK
SYNC
FSET
2.5V
1.5V
Figure 4. Oscillator/SYNC Waveforms
75k
50k
U
REF
100pF
U
pin to the FSET pin, forcing a
5
6
5V
FSET
LT1681
REF
W
1681 F05
U
1681 F04
The LT1681 enters an ultralow current shutdown mode
when the SHDN pin is below 350mV. During this mode,
total supply current drops to a typical value of less than
1 A. When SHDN rises above 350mV, the IC will draw
increasing amounts of supply current until just before the
1.25V turn-on threshold is achieved, when the typical
supply current reaches 60 A.
The shutdown function can be disabled by connecting the
SHDN pin to V
through a 20k series input resistance and can therefore
draw almost 1mA when tied directly to the V
additional current can be minimized by making the con-
nection through an external series resistor (100k is typi-
cally used).
Soft-Start
The LT1681 current control pin (V
to zero at voltages less than 1.4V through full current limit
at V
The voltage on the V
or equal to SS + 0.7V. As such, the SS pin has a “dead
zone” between 0V and 0.7V, where a zero sensed current
condition is maintained. At SS voltages above 0.7V, the
sensed current limit threshold on pin V
needed up to the SS maintained current limit value. Once
the SS pin rises to the V
2.5V, the SS circuit has no effect.
The SS pin sources a typical current of 10 A. Placing a
capacitor (C
voltage on the SS pin to ramp up at a controlled rate,
allowing a graceful increase of maximum converter output
current during a start-up condition. The start-up delay
time to full available current limit is:
The LT1681 internally pulls the SS pin below the zero
current threshold during any fault condition to assure
graceful recovery. The SS circuit also acts as a fault control
latch to assure a full-range recovery from a short duration
fault. Once a fault condition is detected, the LT1681 will
suspend switching until the SS pin has discharged to
approximately 225mV.
t
SS
C
= 3.2V, yielding 1.8V over the full regulation range.
= 2.5 • 10
SS
) from the SS pin to ground will cause the
CC
5
. This pin is internally clamped to 2.5V
• C
C
SS
pin is internally forced to be less than
C
pin maximum value less 0.7V, or
(sec)
C
) limits sensed current
C
CC
may rise as
LT1681
supply. This
13
1681f