Datasheet

LT3742
12
3742fa
applicaTions inForMaTion
The maximum input voltage is determined by the absolute
maximum ratings of the V
IN
and BIAS pins (30V and 40V, re-
spectively) and by the minimum duty cycle, DC
MIN
= 15%.
V
IN(MAX)
=
V
OUT
+ V
D
DC
MIN
⎛
⎝
⎜
⎞
⎠
⎟
+ V
SW
– V
D
The formula above calculates the maximum input voltage
that allows the part to regulate without pulse-skipping,
and is mainly a concern for applications with output volt-
ages lower than 3.3V. For example, for a 2.5V output, the
maximum input voltage is:
V
IN(MAX)
=
2.5V + 0.4V
0.15
⎛
⎝
⎜
⎞
⎠
⎟
+ 0.1V – 0.4V = 19V
If an input voltage higher than 19V is used, the 2.5V output
will still regulate correctly, but the part must pulse-skip
to do so. Pulse skipping does not damage the LT3742,
but it will result in erratic inductor current waveforms
and higher peak currents. Note that this is a restriction
on the operating input voltage only for a specific output
voltage; the circuit will tolerate inputs up to the absolute
maximum rating.
Figure 4. Example Waveforms for a Single Phase Dual Controller vs the 2-Phase LT3742
The Benefits of 2-Phase Operation
Traditionally, dual controllers operate with a single phase.
This means that both power MOSFETs are turned on at
the same time, causing current pulses of up to twice the
amplitude of those from a single regulator to be drawn
from the input capacitor. These large amplitude pulses
increase the RMS current flowing in the input capacitor,
require the use of larger and more expensive input capaci-
tors, increase EMI, and causes increased power losses in
the input capacitor and input power supply.
The two controllers of the LT3742 are guaranteed by design
to operate 180° out of phase. This assures that the current
in each power MOSFET will never overlap, always present-
ing a significantly low peak and RMS current demand to
the input capacitor. This allows the use of a smaller, less
expensive input capacitor, improving EMI performance
and real world operating efficiency.
Figure 4 shows example waveforms for a single phase dual
controller versus a 2-phase LT3742 system. In this case,
5V and 3.3V outputs, each drawing a load current of 2A,
are derived from a 12V supply. In this example, 2-phase
Single Phase
Dual Controller
2-Phase
Dual Controller
SW1 (V)
SW2 (V)
I
L1
I
L2
I
IN
3742 F04