Datasheet

LTC3873
8
3873fa
OPERATION
(nominally 8.4V) at least momentarily to enable LTC3873
operation. The V
CC
voltage is then allowed to fall to V
TURNOFF
(nominally 4V) before undervoltage lockout disables the
LTC3873. This wide UVLO hysteresis range supports the
use of trickle charger on the fl yback transformer to power
the LTC3873—see the section, V
CC
Bias Power. The RUN/SS
pin can be driven below V
SHDN
(nominally 0.7V) to force
the LTC3873 into shutdwn. When the chip is off, the input
supply current is typically only 55A.
Soft-Start
Leave the RUN/SS pin open to use the internal 3.3ms
soft-start. During the internal soft-start, a voltage ramp
limits the V
ITH
. 3.3ms is required for I
TH
to ramp from
zero current level to full current level. The soft-start can
be lengthened by placing an external capacitor from the
RUN/SS pin to the GND. A 3A current will charge the
capacitor, pulling the RUN/SS pin above the shutdown
threshold and a 15µA pull-up current will continue to ramp
RUN/SS to limit V
ITH
during the start-up. When RUN/SS
is driven by an external logic, a minimum of 2.75V logic
is recommended to allow the maximum I
TH
range.
Light Load Operation
Under very light load current conditions, the I
TH
pin volt-
age will be very close to the zero current level of 0.85V.
As the load current decreases further, an internal offset at
the current comparator input will assure that the current
comparator remains tripped (even at zero load current) and
the regulator will start to skip cycles in order to maintain
regulation. This behavior allows the regulator to maintain
constant frequency down to very light loads, resulting in low
output ripple as well as low audible noise and reduced RF
interference while providing high light load effi ciency.
Current Sense
During the switch on-time, the control circuit limits the
maximum voltage drop across the current sense com-
ponent to about 295mV, 110mV and 185mV at low duty
cycle with IPRG tied to V
IN
, GND or left fl oating respec-
tively. It is reduced with increasing duty cycle as shown
in Figure 4.
Figure 4. Maximum SENSE Threshold Voltage vs Duty Cycle
DUTY CYCLE (%)
1
0
MAXIMUM CURRENT SENSE VOLTAGE (mV)
50
100
150
200
250
300
20 40 60 80
3873 F04
100
IPRG = HIGH
IPRG = FLOAT
IPRG = LOW