LTC3784_15 Datasheet, PDF (19/38 Pages) Linear Technology – 60V PolyPhase Synchronous Boost Controller

LTC3784
Applications Information
C_IN and C_OUT Selection
The input ripple current in a boost converter is relatively
low (compared with the output ripple current), because this
current is continuous. The input capacitor C_IN voltage rating
should comfortably exceed the maximum input voltage.
Although ceramic capacitors can be relatively tolerant of
overvoltage conditions, aluminum electrolytic capacitors
are not. Be sure to characterize the input voltage for any
possible overvoltage transients that could apply excess
stress to the input capacitors.
The value of C_IN is a function of the source impedance, and
in general, the higher the source impedance, the higher the
required input capacitance. The required amount of input
capacitance is also greatly affected by the duty cycle. High
output current applications that also experience high duty
cycles can place great demands on the input supply, both
in terms of DC current and ripple current.
In a boost converter, the output has a discontinuous current,
so C_OUT must be capable of reducing the output voltage
ripple. The effects of ESR (equivalent series resistance) and
the bulk capacitance must be considered when choosing
the right capacitor for a given output ripple voltage. The
steady ripple voltage due to charging and discharging
the bulk capacitance in a single phase boost converter
is given by:
V_RIPPLE
=
I_OUT(MAX) •(V_OUT −
C_OUT • V_OUT
V_IN(MIN))
•f
V
where C_OUT is the output filter capacitor.
The steady ripple due to the voltage drop across the ESR
is given by:
∆V_ESR = I_L(MAX) • ESR
The LTC3784 is 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
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.
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)
3784 F03
Figure 3. Normalized Output Capacitor Ripple
Current (RMS) for a Boost Converter
For more information www.linear.com/LTC3784
3784fb
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