LTC3785 LINER [Linear Technology], LTC3785 Datasheet - Page 18

• Manufacturer Part Number: LTC3785
• Description: High Efficiency, Synchronous, 4-Switch Buck-Boost Controller
• Manufacturer: LINER [Linear Technology]

LTC3789
applicaTions inForMaTion
In the boost region, the discontinuous current shifts
from the input to the output, so C
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:
where C
The steady ripple due to the voltage drop across the ESR
is given by:
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
reverse transfer capacitance C
I
The drive voltage is set by the 5.5V INTV
sequently, logic-level threshold MOSFETs must be used
in LTC3789 applications.
18
Ripple (Boost,Cap) =
Ripple (Buck,Cap) =
DS(MAX)
BR,DSS
∆V
∆V
BOOST,ESR
BUCK,ESR
, threshold voltage V
OUT
.
is the output filter capacitor.
= I
= I
L(MAX,BUCK)
L(MAX,BOOST)
I
OUT(MAX)
I
OUT(MAX)
C
GS,TH
OUT
C
• ESR
RSS
OUT
• ESR
• V
• V
• V
, on-resistance R
(
(
and maximum current
• V
OUT
IN(MAX )
IN(MAX)
OUT
OUT
must be capable
– V
CC
• f
– V
• f
IN(MIN)
supply. Con-
OUT
DS(ON)
)
)
V
V
,
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:
where ρ
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
Switch B operates in the buck region as the synchronous
rectifier. Its power dissipation at maximum output current
is given by:
Switch C operates in the boost region as the control switch.
Its power dissipation at maximum current is given by:
P
P
P
• R
B,BUCK
C,BOOST
A,BOOST
DS(ON)
Figure 11. Normalized R
t
is a normalization factor (unity at 25°C) ac-
=
0.5
2.0
1.5
1.0
=
=
0
–50
V
+ k • V

 
IN
t
(
V
V
= 1.5 is reasonable.
− V
V
V
OUT
OUT
IN
IN
JUNCTION TEMPERATURE (°C)
OUT
OUT
0
– V
V
• I
IN
OUT(MAX)
3
IN
2
• I
•
)
50
V
DS(ON)
OUT(MAX)
I
OUT
OUT(MAX)
V

 
IN
vs Temperature
2
• I
100
• ρ
2
OUT(MAX)
t
• C
3789 F11
• ρ
• R
150
RSS
t
DS(ON)
• R
2
• f
DS(ON)
• ρ
3789f
t