LT8705
APPLICATIONS INFORMATION
The peak inductor current when operating in the buck
region is:
It is very important to consider power dissipation when
selecting power MOSFETs. The most efficient circuit will
use MOSFETs that dissipate the least amount of power.
Power dissipation must be limited to avoid overheating
that might damage the devices. For most buck-boost ap-
plications the M1 and M3 switches will have the highest
power dissipation where M2 will have the lowest unless
the output becomes shorted. In some cases it can be
helpful to use two or more MOSFETs in parallel to reduce
powerdissipationineachdevice.Thisismosthelpfulwhen
IL(MAX,BUCK) ≅ IOUT(MAX)
DC
(MAX,M2,BUCK
VOUT(MIN)
•
100%
+
A
2•L•f
2
power is dominated by I R losses while the MOSFET is
where DC
is the maximum duty cycle per-
(MAX,M2,BUCK)
centage of the M2 switch in the buck region given by:
“on”. The additional capacitance of connecting MOSFETs
inparallelcansometimesslowdownswitchingedgerates
and consequently increase total switching power losses.
V
OUT(MIN)
DC
≅ 1–
•100%
MAX,M2,BUCK
(
)
V
The following sections provide guidelines for calculating
power consumption of the individual MOSFETs. From a
known power dissipation, the MOSFET junction tempera-
ture can be obtained using the following formula:
IN(MAX)
Note that the inductor current can be higher during load
transients and if the load current exceeds the expected
maximum I . It can also be higher during start-
OUT(MAX)
T = T + P • R
J
A
TH(JA)
up if inadequate soft-start capacitance is used or during
output shorts. Consider using the output current limiting
topreventtheinductorcurrentfrombecomingexcessive.
Output current limiting is discussed later in the Input/
Output Current Monitoring and Limiting section. Care-
ful board evaluation of the maximum inductor current
is recommended.
where:
T is the junction temperature of the MOSFET
J
T is the ambient air temperature
A
P is the power dissipated in the MOSFET
R
TH(JA)
is the MOSFET’s thermal resistance from the
junction to the ambient air. Refer to the manufacturer’s
data sheet.
Power MOSFET Selection and Efficiency
Considerations
R
normally includes the R
for the device plus
TH(JC)
TheLT8705requiresfourexternalN-channelpowerMOS-
FETs,twoforthetopswitches(switchesM1andM4,shown
in Figure 3) and two for the bottom switches (switches
M2 and M3, shown in Figure 3). Important parameters for
TH(JA)
the thermal resistance from the case to the ambient tem-
perature R . Compare the calculated value of T to
TH(JC)
J
the manufacturer’s data sheets to help choose MOSFETs
that will not overheat.
the power MOSFETs are the breakdown voltage, V
,
BR,DSS
,reverse-
thresholdvoltage,V
,on-resistance,R
RSS
DS(MAX)
GS,TH
DS(ON)
Switch M1: The power dissipation in switch M1 comes
from two primary components: (1) I R power when the
switch is fully turned “on” and inductor current is flowing
through the drain to source connections and (2) power
transfercapacitance,C (gate-to-draincapacitance),and
2
maximum current, I
. The gate drive voltage is set
by the 6.35V GATEV supply. Consequently, logic-level
CC
threshold MOSFETs must be used in LT8705 applications.
8705p
25
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