LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 18

• Manufacturer Part Number: LTC3780EG
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,SSOP,24PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3780
(Switch B and C shown in Figure 1). Important parameters
for the power MOSFETs are the breakdown voltage V
threshold voltage V
transfer capacitance C
The drive voltage is set by the 6V INTV
quently, logic-level threshold MOSFETs must be used in
LTC3780 applications. If the input voltage is expected to
drop below 5V, then the sub-logic threshold MOSFETs
should be considered.
In order to select the power MOSFETs, the power dissi-
pated by the device must be known. For Switch A, the
maximum power dissipation happens in Boost mode,
when it remains on all the time. Its maximum power
dissipation at maximum output current is given by:
where ρ
ing for the significant variation in on-resistance with
temperature, typically about 0.4%/°C as shown in Fig-
ure 9. For a maximum junction temperature of 125°C,
using a value ρ
Switch B operates in Buck mode as the synchronous
rectifier. Its power dissipation at maximum output current
is given by:
18
P
P
B BUCK
A BOOST
,
,
T
is a normalization factor (unity at 25°C) account-
=
=
V
⎛
⎜
⎝
IN
T
V
= 1.5 is reasonable.
V
–
OUT
V
IN
IN
U
V
GS,TH
OUT
RSS
•
I
OUT MAX
, on-resistance R
•
U
and maximum current I
I
OUT MAX
(
(
)
⎞
⎟
⎠
2
W
)
2
•
ρ
•
CC
Figure 9. Normalized R
T
ρ
T
2.0
1.0
0.5
1.5
•
DS(ON)
supply. Conse-
0
R
–50
•
DS ON
R
DS ON
U
(
(
, reverse
DS(MAX)
JUNCTION TEMPERATURE (°C)
BR,DSS
)
0
)
,
.
50
DS(ON)
Switch C operates in Boost mode as the control switch. Its
power dissipation at maximum current is given by:
where C
turers. 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.
For Switch D, the maximum power dissipation happens in
Boost mode, when its duty cycle is higher than 50%. Its
maximum power dissipation at maximum output current
is given by:
For the same output voltage and current, Switch A has the
highest power dissipation and Switch B has the lowest
power dissipation unless a short occurs at the output.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
P
C BOOST
,
P
T
D BUCK
J
100
vs Temperature
,
= T
RSS
A
3780 F09
=
+ P • R
=
150
+
(
is usually specified by the MOSFET manufac-
V
V
k V
OUT
V
OUT
•
IN
TH(JA)
OUT
–
V
•
IN
V V
⎛
⎜
⎝
IN
2
3
V
V
OUT
•
)
IN
I
OUT MAX
OUT
•
V
(
I
IN
OUT MAX
•
I
OUT MAX
(
)
•
(
C
RSS
)
⎞
⎟
⎠
2
)
2
•
•
•
ρ
f
ρ
T
T
•
•
R
R
DS ON
DS ON
(
3780f
(
)
)