LTC3789 Datasheet, PDF (18/28 Pages) Linear Technology – High Efficiency, Synchronous, 4-Switch Buck-Boost Controller

LTC3789
APPLICATIONS INFORMATION
In the boost region, the discontinuous current shifts
from the input to the output, 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 due to
charging and discharging the bulk capacitance is given by:
( ) Ripple (Boost,Cap) = IOUT(MAX) • VOUT – VIN(MIN) V
C_OUT • V_OUT • f
( ) Ripple (Buck,Cap) = IOUT(MAX) • VIN(MAX ) – VOUT V
C_OUT • V_IN(MAX) • f
where C_OUT is the output filter capacitor.
The steady ripple due to the voltage drop across the ESR
is given by:
∆V_BOOST,ESR = I_L(MAX,BOOST) • ESR
∆V_BUCK,ESR = I_L(MAX,BUCK) • ESR
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, such as OS-CON and POSCAP.
Power MOSFET Selection and
Efficiency Considerations
The LTC3789 requires four external N-channel power
MOSFETs, two for the top switches (switches A and
D, shown in Figure 1) and two for the bottom switches
(switches B and C, shown in Figure 1). Important param-
eters for the power MOSFETs are the breakdown voltage
V_BR,DSS, threshold voltage V_GS,TH, on-resistance R_DS(ON),
reverse transfer capacitance C_RSS and maximum current
I_DS(MAX).
The drive voltage is set by the 5.5V INTV_CC supply. Con-
sequently, logic-level threshold MOSFETs must be used
in LTC3789 applications.
In order to select the power MOSFETs, the power dissipated
by the device must be known. For switch A, the maximum
power dissipation happens in the boost region, when it
remains on all the time. Its maximum power dissipation
at maximum output current is given by:
P_A,BOOST
=


V_OUT
V_IN
²
• I_{OUT(MAX) }
• ρ_t
•
R_DS(ON)
where ρ_t is a normalization factor (unity at 25°C) ac-
counting for the significant variation in on-resistance
with temperature, typically about 0.4%/°C, as shown in
Figure 11. For a maximum junction temperature of 125°C,
using a value ρ_t = 1.5 is reasonable.
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3789 F11
Figure 11. Normalized R_DS(ON) vs Temperature
Switch B operates in the buck region as the synchronous
rectifier. Its power dissipation at maximum output current
is given by:
P_B,BUCK
=
V_IN
− V_OUT
V_IN
• I_OUT(MAX) ² • ρ_t • R_DS(ON)
Switch C operates in the boost region as the control switch.
Its power dissipation at maximum current is given by:
( ) _{PC,BOOST =}
V_OUT – V_IN V_OUT
V_IN ²
• I_OUT(MAX) ²
• ρ_t
•
R_DS(ON) + k • V_OUT ³
•
I_{OUT(MAX) •}
V_IN
C_RSS •
f
3789f
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