LTC3879 Linear Technology Corporation, LTC3879 Datasheet - Page 20

• Manufacturer Part Number: LTC3879
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

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at 25°C) is required to account for variation in MOSFET
on-resistance with temperature. Choosing an RJK0330
(R
4.5V, θ
V
voltage on the bottom MOSFET switch. As a result, the
average DC current limit includes signifi cant temperature
and component variability. Design to guarantee that the
average DC current limit will always exceed the rated oper-
ating output current by assuming worst-case component
tolerance and temperature.
The worst-case minimum INTV
MOSFET worst-case R
temperature is 80°C above a 70°C ambient with ρ
1.5. Set T
low and the inductor 15% high.
By setting I
voltage which corresponds to a V
LTC3879
APPLICATIONS INFORMATION
20
RNG
DS(ON)
V
V
V
DS
DS
RNG
sets current limit by fi xing the maximum peak V
JA
=
=
=
220pF
⎛
⎝ ⎜
⎛
⎝ ⎜
= 2.8mΩ (nominal) 3.9mΩ (maximum), V
= 40°C/W) yields a drain source voltage of:
ON
10.0k
C
10.0k
R
I
R1
C1
15
7 5
FB1
LIMIT
LIMIT
. •
0.1μF
equal to the minimum specifi cation of 15%
C
A
SS
R
10.0k
– • .
80.6k
V
27k
FB2
R2
R
–
equal to 15A we get 79mV for peak V
DS
C
2
1
2
1
(
5 1
432k
R
I
RIPPLE
ON
C
33pF
100k
DS(ON)
R
C2
A
PG
•
1
2
3
4
5
6
7
8
0 85
1 15
)
.
.
TRACK/SS
PGOOD
V
MODE
I
SGND
I
V
TH
ON
⎞
⎠ ⎟
RNG
FB
is 3.9mΩ and the junction
3 9 Ω ρτ
⎞
⎠ ⎟
LTC3879
CC
.
RNG
3 9
m
5 15
5 3
.
is 5.15V. The bottom
.
.
BOOST
m
PGND
INV
RUN
equal to 592mV:
V V
Figure 10. Design Example: 1.2V/15A at 400kHz
SW
V
BG
( )
Ω
V
TG
CC
IN
• .
16
15
14
13
12
11
10
9
1 5
4.7μF
C
VCC
C
0.22μF
B
150°C
CMDSH-3
GS
D
B
DS
DS
=
=
Verify that the calculated nominal T
assumed worst-case T
Because the top MOSFET is on for a short time, an
RJK0305DPB (R
mum), C
4.5V, V
power dissipation at current limit with = ρ
The junction temperatures will be signifi cantly less at
nominal current, but this analysis shows that careful
attention to heat sinking will be necessary.
Select C
at 85°C. The output capacitor C
ESR of 4.5mΩ to minimize output voltage changes due to
P
P
T
T
BOT
J
TOP
J
=
=
MILLER
70
70
=
IN
=
=
MILLER
(
0 18
M1
RJK0305DPB
M2
RJK0330DPB
C
C
C
L1: VISHAY IHLP4040DZ-11 0.56μH
28
° ° +
° +
1 2
to give an RMS current rating greater than 4A
28
150
IN1
OUT1
OUT2
.
C
C
.
: UMK325BJ106MM 3
V
= 3V, θ
V
V
28
: SANYO 2R5TPE330M9 2
: MURATA GRM31CR60J476M 2
pF
W
1 25
0 76
– .
(
.
= Q
.
V
15
1 2
DS(ON)
+ +
)
⎛
⎝ ⎜
0 58
0.56μH
C
330μF
JA
A
W
GD
W
V
5
10μF
OUT1
2.5V
C
.
L1
50V
)
IN1
2 5
V
= 40°C/W) is suffi cient. Checking its
(
2
•
/10V = 150pF , V
3
2
•
J
.
15
40
= 10mΩ (nominal) 13mΩ (maxi-
–
+
• . •
40
W
in the bottom MOSFET:
1 4 13
Ω
3
A
°
°
V
=
C W
)
C W
2
+
0 76
/
+
/
.
• . • .
1 2
1 5 3 9
OUT1
3
C
47μF
6.3V
=
m
.
C
100μF
50V
=
OUT2
2
IN2
V
W
120
Ω
100
Ω
⎞
⎠ ⎟
+
is chosen for a low
3879 F10
J
400
°
°
BOOST
(
C
C
m
28
is less than the
V
4.5V TO 28V
V
1.2V
15A
IN
OUT
Ω
kHz
V
100°C
=
)
2
= 5V, V
1 25
⎛
⎝ ⎜
.
15
= 1.4:
2
W
A
GS
⎞
⎟ ⎟
⎠
3879f
=