LTC3878 Linear Technology Corporation, LTC3878 Datasheet - Page 17

• Manufacturer Part Number: LTC3878
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

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As a rule of thumb the gain crossover frequency should be
less than 20% of the switching frequency. For a detailed
explanation of switching control loop theory see Applica-
tion Note 76.
High Switching Frequency Operation
Special care should be taken when operating at switching
frequencies greater than 800kHz. At high switching frequen-
cies there may be an increased sensitivity to PCB noise
which may result in off-time variation greater than normal.
This off-time instability can be prevented in several ways.
First, carefully follow the recommended layout techniques.
Second, use 2μF or more of X5R or X7R ceramic input
capacitance per Amps of load current. Third, if necessary,
increase the bottom MOSFET ripple voltage to 30mV
or greater. This ripple voltage is equal to R
at 25°C • I
Design Example
Figure 7 is a power supply design example with the fol-
lowing specifi cations: V
V
by calculating the timing resistor, R
APPLICATIONS INFORMATION
OUT
R
ON
= 1.2V ±5%, I
=
220pF
0 7
10.0k
C
10.0k
R
P-P
R1
C1
FB1
.
0.1μF
C
V
.
SS
•
R
5.11k
80.6k
12.1k
FB2
R2
R
400
C
1 2
OUT(MAX)
.
kHz
432k
R
V
ON
C
33pF
100k
R
C2
PG
IN
•
10
= 4.5V to 28V (12V nominal),
1
2
3
4
5
6
7
8
pF
RUN/SS
PGOOD
V
MODE
I
SGND
I
V
= 15A and f = 400kHz. Start
TH
ON
RNG
FB
=
LTC3878
429
ON
BOOST
PGND
INV
k
:
Figure 7. Design Example: 1.2V/15A at 400kHz
SW
V
BG
NC
TG
CC
IN
16
15
14
13
12
11
10
9
DS(ON)
4.7μF
C
VCC
C
0.22μF
B
typical
CMDSH-3
D
P-P
B
Select the nearest standard resistor value of 432k for a
nominal operating frequency of 396kHz. Set the inductor
value to give 35% ripple current at maximum V
the adjusted operating frequency:
Select 0.56μH which is the nearest value.
The resulting maximum ripple current is:
Choose the synchronous bottom MOSFET switch and
calculate the V
V
at 25°C) is required to account for variation in MOSFET
on-resistance with temperature. Choosing an RJK0330
(R
4.5V, θ
RNG
DS(ON)
L
ΔI
V
DS
=
L
and V
396
JA
=
=
396
= 2.8mΩ (nominal) 3.9mΩ (maximum), V
⎛
⎝ ⎜
= 40°C/W) yields a drain source voltage of:
M1
RJK0305DPB
M2
RJK0330DPB
C
C
C
L1: VISHAY IHLP4040DZ-11 0.56μH
I
kHz
LIMIT
IN1
OUT1
OUT2
DS
: UMK325BJ106MM 3
kHz
: SANYO 2R5TPE330M9 2
: MURATA GRM31CR60J476M 2
1 2
, the ρτ term normalization factor (unity
• .
1 2
RNG
.
0 35 15
.
–
• .
V
V
0 56
2
1
0.56μH
C
470μF
current limit set-point. To calculate
(
10μF
OUT1
2.5V
C
L1
50V
I
•
IN1
RIPPLE
3
2
μH
+
A
⎛
⎝ ⎜
⎛
⎝ ⎜
1
)
1
+
⎞
⎠ ⎟
–
–
3 9 Ω ρτ
1 2
28
1 2
.
28
C
100μF
6.3V
C
100μF
50V
.
.
OUT2
IN2
m
V
V
⎞
⎠ ⎟
⎞
⎠ ⎟
=
3878 F07
=
( )
0 55
LTC3878
5 1
.
V
4.5V TO 28V
V
1.2V
15A
.
IN
OUT
A
μH
IN
17
using
GS
3878f
=