LTC3704EMS Linear Technology, LTC3704EMS Datasheet - Page 16

• Manufacturer Part Number: LTC3704EMS
• Description: IC INV SYNC 5.2V 50MA 10MSOP
• Manufacturer: Linear Technology
• Type: Invertingr

Specifications of LTC3704EMS

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: -5.2V
• Current - Output: 50mA
• Frequency - Switching: 50kHz ~ 1MHz
• Voltage - Input: 2.5 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

LTC3704
APPLICATIO S I FOR ATIO
Another method of choosing which power MOSFET to use
is to check the maximum output current for a given
R
available in discrete values.
For the case where a conventional sense resistor is used,
Sense resistors are generally low TC and are available with
different ranges of tolerance depending on price. The
power dissipated in the sense resistor is:
Calculating Power MOSFET Switching and Conduction
Losses and Junction Temperatures
In order to calculate the junction temperature of the power
MOSFET, the power dissipated by the device must be
known. This power dissipation is a function of the duty
cycle, the load current and the junction temperature itself
(due to the positive temperature coefficient of its R
16
Figure 11. Maximum SENSE Threshold Voltage vs Duty Cycle
DS(ON)
I
R
P
O MAX
SENSE
SENSE
(
, since MOSFET on-resistances are generally
)
200
100
150
=
50
=
=
0
I
0
V
SW PEAK
–
SENSE MAX
V
SENSE MAX
(
0.2
U
(
)
(
2
•
DUTY CYCLE
0.4
)
R
U
•
)
SENSE
•
⎛
⎜
⎝
1
⎛
⎜
⎝
+
0.5
1
D
+
χ
2
•
MAX
χ
D
2
⎞
⎟
⎠
W
1
⎞
⎟
⎠
MAX
•
0.8
–
I
•
O MAX
–
R
D
(
3704 F11
1
DS ON
MAX
1.0
(
)
)
U
•
ρ
DS(ON)
Τ
).
As a result, some iterative calculation is normally required
to determine a reasonably accurate value. Since the
controller is using the MOSFET as both a switching and a
sensing element, care should be taken to ensure that the
converter is capable of delivering the required load current
over all operating conditions (line voltage and tempera-
ture), and for the worst-case specifications for V
and the R
data sheet.
The power dissipated by the MOSFET in a positive-to-
negative converter is:
where I
The first term in the equation above represents the
I
ing losses. The constant, k = 1.7, is an empirical factor
inversely related to the gate drive current and has the
dimension of 1/current.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
2
R losses in the device, and the second term, the switch-
T
P
J
FET
= T
O(MAX)
+
Figure 12. Normalized R
=
A
DS(ON)
k V
2.0
1.5
1.0
0.5
⎛
⎜
⎝
+ P
0
• (
– 50
1
–
–
I
FET
O MAX
IN
and V
D
(
of the MOSFET listed in the manufacturer’s
MAX
–
• R
JUNCTION TEMPERATURE (°C)
V
0
)
O
O
TH(JA)
⎞
⎟
⎠
)
are negative numbers.
2
1 85
.
•
R
50
DS ON
•
1
I
DS(ON)
(
O MAX
–
(
D
)
MAX
100
•
vs Temperature
D
)
MAX
•
3704 F12
C
RSS
150
•
ρ
T
•
SENSE(MAX)
f
3704fb