Datasheet

LTC3775
26
3775fa
APPLICATIONS INFORMATION
Design Example
As a design example, take a supply with the following
specifi cations: V
IN
= 5V to 26V (12V nominal), V
OUT
=
1.2V ±5%, I
OUT(MAX)
= 15A, f = 500kHz.
First, verify the minimum on-time which occurs at maxi-
mum V
IN
:
t
V
VkHz
ns
ON MIN()
.
.=
()( )
=
12
26 500
92 3
The minimum on-time is lower than the top current limit
comparator blanking time of 100ns with sense resistor
sensing. The controller will rely on the bottom MOSFET
R
DS(ON)
sensing at high V
IN
.
Next, verify the maximum duty cycle which occurs at
minimum V
IN
:
Maximum Duty Cycle
V
V
.
%==
12
5
24
This is below the LTC3775 maximum duty cycle of 90%.
Next, calculate R
SET
to give the 500kHz operating
frequency:
Rk
SET
==
19500
500
39
Next, choose the inductor value for about 40% ripple
current at maximum V
IN
:
L =
1.2V
500kHz
()
0.4
()
15A
( )
1–
1.2
26






= 0.38μH
Select 0.36H which is the nearest standard value.
The resulting maximum ripple current is:
I
L
=
1.2V
500kHz
()
0.36μH
( )
1–
1.2V
26
V






= 6.4A
Next, choose the top and bottom MOSFET switch. Since
the drain of each MOSFET will see the full supply voltage
26V (max) plus any ringing, choose a 30V MOSFET to
provide a margin of safety. Because the top MOSFET is
on for a short time, a RENESAS RJK0305DPB (R
DS(ON)
=
13m (max), C
MILLER
= Q
GD
/10V = 150pF, V
GS(TH)
= 2.5V,
θ
JA
= 40°C/W) is suffi cient. Check its power dissipation
at current limit with = ρ
100°C
= 1.4:
P
TO P
=
1.2V
26
V
15A
()
2
• 1.4 •13m
()
+ 26V
()
2
15A
2






2.5
()
150pF
()
1
5.2 –
2.5






+
1
2.5






500kHz
= 0.19W + 0.73W = 0.92W
And double check the assumed T
J
in the MOSFET:
T
J
= 70°C + (0.92W)(40°C/W) = 107°C
The junction temperatures will be signifi cantly less at
nominal current, but this analysis shows that careful at-
tention to heat sinking will be necessary.
A RENESAS RJK0301DPB (R
DS(ON)
= 4m (max),
θ
JA
= 40°C/W) is chosen for the synchronous MOSFET.
P
BOT
=
26V – 1.2V
26
V
15A
()
2
• 1.4 • 4m
()
=1.26W
And double check the assumed T
J
in the MOSFET:
T
J
= 70°C + (1.26W)(40°C/W) = 120°C
Next, the INTV
CC
LDO current is calculated:
I
INTVCC
= (500kHz)(8nC + 32nC) + 3.5mA = 23.5mA
And double check the T
J
in the LTC3775:
T
J
= 70°C + (23.5mA)(26V)(68°C/W) = 112°C
Next, set the current limit resistors with a sense resistor
of 3m.
R
ILIMT
=1.2 • 3m •
15A + 0.5 • 6.4A
90
μA
= 728
R
ILIMB
= 5 • 1.4 • 4m •
15A + 0.5 • 6.4A
9
μA
= 56.62k
Use the next higher standard values of 732 and 57.6k.