LTC3788 Datasheet, PDF (20/32 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788
APPLICATIONS INFORMATION
The LTC3788 can also be configured as a 2-phase single
output converter where the outputs of the two channels
are connected together and both channels have the same
duty cycle. With 2-phase operation, the two channels of
the dual switching regulator are operated 180 degrees out-
of-phase. This effectively interleaves the output capacitor
current pulses, greatly reducing the output capacitor ripple
current. As a result, the ESR requirement of the capacitor
can be relaxed. Because the ripple current in the output
capacitor is a square wave, the ripple current requirements
for the output capacitor depend on the duty cycle, the
number of phases and the maximum output current. Figure
3 illustrates the normalized output capacitor ripple current
as a function of duty cycle in a 2-phase configuration. To
choose a ripple current rating for the output capacitor,
first establish the duty cycle range based on the output
voltage and range of input voltage. Referring to Figure 3,
choose the worst-case high normalized ripple current as
a percentage of the maximum load current.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic
and ceramic capacitors are all available in surface mount
packages. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient.
Capacitors are now available with low ESR and high ripple
current ratings (i.e., OS-CON and POSCAP).
3.25
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
0.1
1-PHASE
2-PHASE
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY CYCLE OR (1-VIN / VOUT)
3788 F03
Figure 3. Normalized Output Capacitor Ripple
Current (RMS) for a Boost Converter
3788f
20