LTC3775 View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC3775 Datasheet PDF : 34 Pages
| |||
LTC3775
APPLICATIONS INFORMATION
Design Example
As a design example, take a supply with the following
specifications: VIN = 5V to 26V (12V nominal), VOUT =
1.2V ±5%, IOUT(MAX) = 15A, f = 500kHz.
First, verify the minimum on-time which occurs at maxi-
mum VIN:
tON(MIN)
=
(26V
1.2V
)(500kHz)
=
92.3ns
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
RDS(ON) sensing at high VIN.
Next, verify the maximum duty cycle which occurs at
minimum VIN:
Maximum
Duty
Cycle =
1.2V
5V
= 24%
This is below the LTC3775 maximum duty cycle of 90%.
Next, calculate RSET to give the 500kHz operating
frequency:
RSET
=
19500
500
=
39k
Next, choose the inductor value for about 40% ripple
current at maximum VIN:
L
=
1.2V
(500kHz ) (0.4)
(15A)
1–
1.2
26
=
0.38μH
Select 0.36μH which is the nearest standard value.
The resulting maximum ripple current is:
IL
=
1.2V
(500kHz ) (0.36μH)
1–
1.2V
26V
=
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 (RDS(ON) =
13mΩ (max), CMILLER = QGD/10V = 150pF, VGS(TH) = 2.5V,
θJA = 40°C/W) is sufficient. Check its power dissipation
at current limit with = ρ100°C = 1.4:
( ) PTOP
=
1.2V
26V
(15A)2 • 1.4 • 13m
+
(26V)2
15A
2
(2.5)
(150pF)
5.2
1
–
2.5
+
1
2.5
500kHz
= 0.19W + 0.73W = 0.92W
And double check the assumed TJ in the MOSFET:
TJ = 70°C + (0.92W)(40°C/W) = 107°C
The junction temperatures will be significantly less at
nominal current, but this analysis shows that careful at-
tention to heat sinking will be necessary.
A RENESAS RJK0301DPB (RDS(ON) = 4mΩ (max),
θJA = 40°C/W) is chosen for the synchronous MOSFET.
( ) PBOT
=
26V – 1.2V
26V
(15A)2 • 1.4 • 4m
= 1.26W
And double check the assumed TJ in the MOSFET:
TJ = 70°C + (1.26W)(40°C/W) = 120°C
Next, the INTVCC LDO current is calculated:
IINTVCC = (500kHz)(8nC + 32nC) + 3.5mA = 23.5mA
And double check the TJ in the LTC3775:
TJ = 70°C + (23.5mA)(26V)(68°C/W) = 112°C
Next, set the current limit resistors with a sense resistor
of 3mΩ.
RILIMT
=
1.2
•
3m
•
15A
+ 0.5 •
90μA
6.4A
=
728
RILIMB
=
5
•
1.4
•
4m
•
15A
+ 0.5 •
9μA
6.4A
=
56.62k
Use the next higher standard values of 732Ω and 57.6k.
3775fa
26
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