lt3742 Linear Technology Corporation, lt3742 Datasheet - Page 9

• Manufacturer Part Number: lt3742
• Description: Dual, 2-phase Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

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 re-
duced 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 Benefi ts 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 amplifi er
5V/DIV
2A/DIV
V
Figure 1a. Start-Up Waveforms Without Soft-Start
OUT
I
L
0.5ms/DIV
3742 F01a
compares the FB pin to this ramp instead of to the 800mV
reference; this slowly and smoothly increases the output
voltage to its fi nal 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
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 ap-
plications, 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 maxi-
mum current limit every start-up cycle. Figures 1a and 1b
show start-up waveforms with and without soft-start for
the circuit of Figure 1. 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 1b. Start-Up Waveforms with 1nF Soft-Start Capacitor
5V/DIV
2A/DIV
V
OUT
I
L
IN
0.5ms/DIV
drops low enough to trigger
3742 F01b
LT3742
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3742f