lt1941 Linear Technology Corporation, lt1941 Datasheet - Page 18

• Manufacturer Part Number: lt1941
• Description: Triple Monolithic Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT1941
In Figure 6a, a larger capacitor on RUN/SS2 delays chan-
nel 2 with respect to channel 1. The soft-start capacitor on
RUN/SS2 should be at least twice the value of the capacitor
on RUN/SS1. A larger ratio may be required, depending on
the output capacitance and load on each channel. Make
sure to test the circuit in the system before deciding on
final values for these capacitors.
The circuit in Figure 6b requires the fewest components,
with both channels sharing a single soft-start capacitor.
The power good comparator of channel 1 disables chan-
nel 2 until output 1 is in regulation.
For independent control of channel 2, use the circuit in
Figure 6c. The capacitor on RUN/SS1 is smaller than the
capacitor on RUN/SS2. This allows the LT1941 to start up
and enable its power good comparator before RUN/SS2
gets high enough to allow channel 2 to start switching.
Channel 2 only operates when it is enabled with the
external control signals and output 1 is in regulation.
The circuit in Figure 6a leaves both power good indicators
free. However, the circuits in Figures 6b and 6c have
another advantage. As well as sequencing the two outputs
at start-up, they also disable channel 2 if output 1 falls out
of regulation (due to a short circuit or a collapsing input
voltage).
18
OFF2 ON2
OFF
OFF
ON
ON
U
Figure 6. Several Methods of Sequencing Two Ouputs. Channel 1 Starts First
U
(6c) Independent Control of Channel 2
(6a) Channel 2 is Delayed
W
2.2nF
1.5nF
1nF
1nF
RUN/SS2
RUN/SS2
RUN/SS1
RUN/SS1
PG1
LT1941
LT1941
GND
GND
U
OFF
OFF
Finally, be aware that the circuit in Figure 6d does not
work, because the power good comparators are disabled
in shutdown.
PCB LAYOUT
For proper operation and minimum EMI, care must be
taken during printed circuit board (PCB) layout. Figure 7
shows the high current paths in the step-down regulator
circuit. Note that in the step-down regulators large, switched
currents flow in the power switch, the catch diode and the
input capacitor. In the inverter/boost regulator large,
switched currents flow through the power switch, the
switching diode, and either the output capacitor in boost
configuration, or the tank capacitor in the inverter configu-
ration. The loop formed by these components should be as
small as possible. Place these components, along with the
inductor and output capacitor, on the same side of the
circuit board and connect them on that layer. Place a local,
unbroken ground plane below these components and tie
this ground plane to system ground at one location, ideally
at the ground terminal of the output capacitor C2. Addi-
tionally, keep the SW and BOOST nodes as small as
possible.
ON
ON
(6b) Fewest Components
1nF
1nF
1.5nF
(6d) Doesn't Work !
RUN/SS1
RUN/SS2
RUN/SS2
RUN/SS1
PG1
LT1941
LT1941
GND
GND
PG1
1941 F06
V
C2
1941fa