Datasheet

LT3740
14
3740fc
APPLICATIONS INFORMATION
Selecting a standard value of 2.0µH results in a ripple
current of:
ΔI
L
=
2.5V
(300kHz) •(2μH)
•1–
2.5V
15V
⎛
⎝
⎜
⎞
⎠
⎟
= 3.47A
Set Range = V
IN
. At minimum input voltage V
IN
= 7V,
the maximum current sensing voltage is 145mV. Use an
external sensing resistor of 12mΩ. The inductor current
valley will be clamped to 12A. The ripple current at V
IN
= 7V
is 2.68A, there is about 33% of margin for the 10A load
current to cover the maximum current sensing voltage
variation.
For the case of using MOSFET on-resistance for current
sensing, choosing a Si4840 (R
DS(ON)
= 0.008Ω (NOM)
0.0095Ω (MAX) for V
GS
= 7V, θ
JA
= 40°C/W) yields a
nominal sense voltage of:
V
SN(NOM)
= (10A)(1.3)(0.0095Ω) =123mV
To check if the current limit is acceptable, assume a junc-
tion temperature of about 55°C above a 70°C ambient
with ρ
125°C
= 1.5:
I
LIMIT
=
145mV
1.5 • 0.0095Ω
+
2.68A
2
=11.5A
Double check the assumed T
J
in the MOSFET at V
IN
= 7V
with maximum load current:
P
BOT
= D
BOT
• IL
2
• R
DS(ON)
,
BOT
P
BOT
= 1–
2.5V
7V
⎛
⎝
⎜
⎞
⎠
⎟
•10A
()
2
•1.5 • 0.0095 = 0.92W
Double check the assumed T
J
in the MOSFET:
T
J
= 70°C + (0.92W)(40°C/W) = 107°C
The power dissipation in the bottom MOSFET increases
with input voltage. For V
IN
= 20V,
P
BOT
= 1–
2.5V
20V
⎛
⎝
⎜
⎞
⎠
⎟
•10A
()
2
•1.5 • 0.0095Ω=1.25W
Double check the assumed T
J
in the MOSFET:
T
J
= 70°C + (1.25W)(40°C/W) = 120°C
Choose a Si4840 (C
RSS
= 200pF) for the top MOSFET and
check its power dissipation at maximum load current with
ρ
100°C
= 1.3:
P
TOP
=
2.5V
20V
•10A
()
2
•1.3 • 0.0095Ω+17 •(20V)
2
•
10A •200pF • 300kHz = 0.15W +0.41W = 0.56W
T
J
= 70°C + (0.56W)(40°C/W) = 92°C
This analysis shows that careful attention to heat sinking
will be necessary in this circuit.
Check the reverse current comparator margin. The maxi-
mum ripple current happens at maximum input voltage:
ΔI
L(MAX)
=
2.5V
300kHz • 2μH
•1–
2.5V
20V
⎛
⎝
⎜
⎞
⎠
⎟
= 3.65A
At no load, the maximum reverse current voltage is:
R
S
•
ΔI
L(MAX)
2
=12mΩ •
3.65A
2
= 22mV