LM2735XSDEVAL National Semiconductor, LM2735XSDEVAL Datasheet - Page 18

• Manufacturer Part Number: LM2735XSDEVAL
• Description: BOARD EVAL LM2735 1.6MHZ 6LLP
• Manufacturer: National Semiconductor

Specifications of LM2735XSDEVAL

• Main Purpose: DC/DC, Step Up
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 12V
• Current - Output: 500mA
• Voltage - Input: 3 ~ 5.5V
• Regulator Topology: Boost
• Frequency - Switching: 1.6MHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM2735
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

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The value for should be equal to the resistance at the junction
temperature you wish to analyze. As an example, at 125°C
and V
Switching losses are also associated with the internal NMOS
switch. They occur during the switch on and off transition pe-
riods, where voltages and currents overlap resulting in power
loss.
The simplest means to determine this loss is to empirically
measuring the rise and fall times (10% to 90%) of the switch
at the switch node:
Quiescent Power Losses
I
4mA.
Example Efficiency Calculation:
Quiescent Power Losses
Switching Power Losses
Internal NFET Power Losses
Q
is the quiescent operating current, and is typically around
P
P
SWR
SWF
IN
V
3V
5V
3V
5V
IN
= 5V, R
ΣP
Typical Switch-Node Rise and Fall Times
= 1/2(V
= 1/2(V
COND
P
P
R
T
T
R
V
I
F
TABLE 1. Operating Conditions
V
SWR
SWF
OUT
V
FALL
DSon
RISE
I
OUT
I
DCR
D
SW
IN
Q
P
IN
D
DSON
+ P
SW
OUT
OUT
= 1/2(V
= 1/2(V
P
= P
SW
V
P
= 250 mΩ (See typical graphs for value).
Q
12V
12V
18V
x I
x I
5V
SW
OUT
R
= I
+ P
SWR
IN
IN
DSON
P
Q
= P
OUT
Q
OUT
x F
x F
DIODE
x V
= I
+ P
SWR
SW
= 250 mΩ
SW
x I
x I
Q
IN
SWF
IN
x V
IN
x T
x T
+ P
= 20 mW
+ P
x F
x F
IN
FALL
T
RISE
= 150 mW
6nS
6nS
7nS
7nS
IND
SWF
RISE
SW
SW
)
)
+ P
x T
≊
≊
x T
1.60MHz
250mΩ
500mA
50mΩ
6 ns
5 ns
Q
0.4V
4mA
0.64
1.4A
12V
6nS
5nS
RISE
FALL
5V
= P
≊
≊
)
)
LOSS
70 mW
80 mW
T
4nS
5nS
5nS
5nS
FALL
18
Diode Losses
V
Inductor Power Losses
R
Total Power Losses are:
Calculating
We now know the internal power dissipation, and we are try-
ing to keep the junction temperature at or below 125°C. The
next step is to calculate the value for
actually very simple to accomplish, and necessary if you think
you may be marginal with regards to thermals or determining
what package option is correct.
The LM2735 has a thermal shutdown comparator. When the
silicon reaches a temperature of 160°C, the device shuts
down until the temperature reduces to 150°C. Knowing this,
one can calculate the
Because the junction to top case thermal impedance is much
lower than the thermal impedance of junction to ambient air,
the error in calculating
you will need to attach a small thermocouple onto the top case
of the LM2735 to obtain the
Knowing the temperature of the silicon when the device shuts
down allows us to know three of the four variables. Once we
calculate the thermal impedance, we then can work back-
wards with the junction temperature set to 125°C to see what
D
DCR
= 0.45V
R
T
T
V
R
I
F
= 75 mΩ
V
OUT
V
FALL
RISE
DSon
OUT
I
DCR
D
SW
η
Q
IN
D
P
CONDUCTION
P
TABLE 2. Power Loss Tabulation
INTERNAL
P
DIODE
P
IND
1.6MHz
250mΩ
500mA
75mΩ
0.623
0.4V
4mA
86%
12V
6nS
5nS
= I
= V
5V
= P
= I
IN
D
IN
2
COND
or the
x I
2
x R
and
x D x R
is lower than for
IN
DCR
(1-D) = 236 mW
+ P
value.
PDIODE
PCOND
PLOSS
= 145 mW
PSWR
PSWF
POUT
SW
PIND
DSON
PQ
of a specific application.
= 475 mW
x 305 mW
and/or
236mW
305mW
145mW
856mW
. However,
80mW
70mW
20mW
6W
. This is