Datasheet

LT3742
9
3742fa
applicaTions inForMaTion
Soft-Start and Shutdown
The RUN/SS (Run/Soft-Start) pins are used to enable each
controller independently, and to provide a user-program-
mable soft-start function that reduces the peak input
current and prevents output voltage overshoot during
start-up. To disable either controller, pull its RUN/SS pin
to ground with an open-drain or open-collector device.
If both RUN/SS pins are pulled to ground, the LT3742
is placed in shutdown mode, and quiescent current is
reduced to 20µA. Internal 1µA current sources pull up on
each RUN/SS pin, and when either pin reaches 0.5V, that
controller is enabled, along with the internal bias supply,
gate drive boost regulator, voltage reference and master
oscillator. If both outputs are always enabled together,
one soft-start capacitor can be used with both RUN/SS
pins tied together.
The Benefits of Soft-Start
When a capacitor is tied from the RUN/SS pin to ground,
the internal 1µA pull-up current source generates a volt-
age ramp on this pin. During start-up, the error amplifier
compares the FB pin to this ramp instead of to the 800mV
reference; this slowly and smoothly increases the output
voltage to its final value, while maintaining control of the
inductor current. Always check the inductor current and
output voltage waveforms to ensure that the programmed
soft-start time is long enough. A new soft-start cycle will
be initiated whenever V
IN
drops low enough to trigger
undervoltage lockout (programmed using the UVLO pin),
or the LT3742 die temperature exceeds thermal shutdown.
A typical value for the soft-start capacitor is 1nF.
Soft-start is strongly recommended for all LT3742 appli-
cations, as it provides the least amount of stress on the
external power MOSFET and catch diode. Without soft-start,
both of these components will see the maximum current
limit every start-up cycle. Figures 1a and 1b show start-
up waveforms with and without soft-start for the circuit
on the front page. Notice the large inductor current spike
and the output voltage overshoot when soft-start is not
used. While this may be acceptable for some systems, the
addition of a single capacitor dramatically improves the
start-up behavior of each DC/DC controller.
Figure 1a. Start-Up Waveforms Without Soft-Start Figure 1b. Start-Up Waveforms with 1nF Soft-Start Capacitor
V
OUT
5V/DIV
I
L
2A/DIV
0.5ms/DIV
3742 F01a
V
OUT
5V/DIV
I
L
2A/DIV
0.5ms/DIV
3742 F01b