LTC3788-1 Datasheet, PDF (17/28 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788-1
APPLICATIONS INFORMATION
operating in continuous mode, the duty cycles for the top
and bottom MOSFETs are given by:
Main Switch Duty Cycle = ^{VOUT − VIN}
V_OUT
Synchronous Switch Duty Cycle = ^VIN
V_OUT
The MOSFET power dissipations at maximum output
current are given by:
( ) ^P_MAIN
=
(V_OUT
− V_IN)V_OUT
V
2
IN
• I_OUT(MAX)2 •
1+ δ
•
R_DS(ON)
+
k
•
V
3
OUT
•
I_OUT(MAX)
V_IN
• R_DR
• C_MILLER • f
( ) ^P_SYNC
=
V_IN
V_OUT
• I_OUT(MAX)2 •
1+ δ
• R_DS(ON)
where δ is the temperature dependency of R_DS(ON) and
R_DR (approximately 1Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. The constant k,
which accounts for the loss caused by reverse recovery
current, is inversely proportional to the gate drive current
and has an empirical value of 1.7.
Both MOSFETs have I²R losses while the bottom N-channel
equation includes an additional term for transition losses,
which are highest at low input voltages. For high V_IN the
high current efficiency generally improves with larger
MOSFETs, while for low V_IN the transition losses rapidly
increase to the point that the use of a higher R_DS(ON) device
with lower C_MILLER actually provides higher efficiency.
The synchronous MOSFET losses are greatest at high
input voltage when the bottom switch duty factor is low
or during overvoltage when the synchronous switch is on
close to 100% of the period.
The term (1+ δ) is generally given for a MOSFET in the
form of a normalized R_DS(ON) vs Temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
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 the 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 is given by:
V_RIPPLE
=
I_OUT(MAX) • (V_OUT − V_IN(MIN))
C_OUT • V_OUT • f
V
where C_OUT is the output filter capacitor.
The steady ripple due to the voltage drop across the ESR
is given by:
ΔV_ESR = I_L(MAX) • ESR
The LTC3788-1 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 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
37881f
17