Datasheet

LTC3546
18
3546fc
For more information www.linear.com/3546
applicaTions inForMaTion
Keeping the current small (<20µA) in these resistors
maximizes efficiency, but making the current too small
may allow stray capacitance to cause noise problems and
reduce the phase margin of the error amp loop.
To improve the frequency response, a feedforward capacitor
C
FF
may also be used. Typical values used here are 10pf to
100pf. Great care should be taken to route the V
FB
node away
from noise sources, such as the inductor or an SW line.
Shutdown, Soft-Start and Tracking Start-Up
The LTC3546 start-up works by comparing two inputs, an
internal 1.2ms linear soft-start ramp and the TRACK/SS
pin. Whichever input is lower in voltage is the controlling
voltage used for start-up. The internal start-up ramps
to 0.6V in 1.2ms. If a slower start-up is desired, the
TRACK/SS pin has a 1.15µA pull up current so a start-up
ramp rate can be programmed with an external capacitor
,
or a
voltage divider from another signal can be applied
to the TRACK/SS pin.
During start-up the controlling voltage must rise above
120mV before the output will start switching.
When the RUN pin is low, both the internal 1.2ms soft-start
ramp and the TRACK/SS pin are pulled to ground. When
the RUN pin is pulled high, both the internal soft-start ramp
and the TRACK/SS pin are released. From the time when
the RUN pin is asserted until the controlling voltage reaches
0.6V, the regulator is in the start-up state. In this state, the
error amplifier will compare the feedback signal at V
FB
to
the controlling voltage (the lower of either the TRACK/SS
voltage or the internal ramp voltage) and the regulator
will force them to be equal. In this state, the mode of the
regulator is forced to pulse skipping. The regulator will
continue in this manner until the voltage on the control
-
ling voltage rises above 0.6V. Once the controlling ramp
signal is above 0.6V the error amplifier uses the internal
0.6V reference and the operational mode will switch to
the mode set by the SYNC/MODE pin.
If the TRACK/SS pin is ramped down after start-up, the
error amplifier will compare the feedback signal at V
FB
to the voltage on the TRACK/SS pin once the TRACK/SS
voltage drops 6% below the internal reference voltage of
0.6V (0.564V). The regulator will try to force the V
FB
voltage
to equal the TRACK/SS voltage if there is sufficient load
current to pull the output low at this rate, otherwise the
output will ramp down at the discharge rate of the output
capacitor. Once the TRACK/SS voltage drops below about
100mV all switching functions cease and the regulator is
forced back into pulse-skipping mode. The operational
mode while TRACK/SS is ramping down is set by the
MODE/SYNC pin.
To use the internal 1.2ms linear soft-start controlling
voltage leave the TRACK/SS pin floating. By floating the
TRACK/SS pin the internal 1.15µA pull up current will pull
the TRACK/SS pin up faster than the internal 1.2ms ramp.
Care must be taken to insure the TRACK/SS ramp up time
(from 0V to 0.6V) is much shorter than the internal 1.2ms
ramp time. Parasitic capacitance on this pin should be
much smaller than:
C
PARASITICTRACK/SS
<<
1.15µA•1.2ms
0.6V
or
C
PARASITICTRACK/SS
<< 2.3nF
An externally controlled soft-start ramp is obtained when
an external capacitor is connected from the TRACK/SS
pin to ground and its ramp rate is slower than the internal
soft-start ramp. In this configuration, soft-start times longer
than 1.2ms can be achieved. When RUN is pulled high, the
internal 1.15µA current source charges the external capaci
-
tor linearly from 0V. While the TRACK/SS pin is below 0.6V
the error amplifier forces the regulator to drive the V
FB
pin to
the voltage on the TRACK/SS pin. Once the V
FB
pin reaches
0.6V the regulator switches to the internal 0.6V reference.
The ramp-up time for the output is calculated as:
t
RAMP
=
C
TRACK/SS
• 0.6V
1.15µA
For this equation to be valid, the ramp time must be greater
than 1.2ms thus:
C
TRACK/SS
≥
1.15µA•1.2ms
0.6V
or
C
TRACK/SS
≥ 2.3nF