Datasheet

LTC3878
17
3878fa
applicaTions inForMaTion
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
P-P
or greater. This ripple voltage is equal to R
DS(ON)
typical
at 25°C • I
P-P
.
Design Example
Figure 7 is a power supply design example with the fol
-
lowing specifications:
V
IN
= 4.5V to 28V (12V nominal),
V
OUT
= 1.2V ±5%, I
OUT(MAX)
= 15A and f = 400kHz. Start
by calculating the timing resistor, R
ON
:
R
V
V kHz pF
k
ON
= =
1 2
0 7 400 10
429
.
. • •
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
IN
using
the adjusted operating frequency:
L
V
kHz A
µH=






=
1 2
396 0 35 15
1
1 2
28
0 55
.
• . •
–
.
.
Select 0.56µH which is the nearest value.
The resulting maximum ripple current is:
∆I
V
kHz µH
V
V
A
L
=






=
1 2
396 0 56
1
1 2
28
5 1
.
• .
–
.
.
Choose the synchronous bottom MOSFET switch and
calculate the V
RNG
current limit set-point. To calculate
V
RNG
and V
DS
, the ρτ term normalization factor (unity
at 25°C) is required to account for variation in MOSFET
on-resistance with
temperature. Choosing an RJK0330
(R
DS(ON)
= 2.8mΩ (nominal) 3.9mΩ (maximum), V
GS
=
4.5V, θ
JA
= 40°C/W) yields a drain source voltage of:
V I I m
DS LIMIT RIPPLE
=
( )






( )
– .
1
2
3 9 Ω ρτ
Figure 7. Design Example: 1.2V/15A at 400kHz
+
RUN/SS
LTC3878
BOOST
16
C
B
0.22µF
M1
RJK0305DPB
C
VCC
4.7µF
C
C1
220pF
C
C2
33pF
D
B
CMDSH-3
L1
0.56µH
C
OUT1
330µF
2.5V
s2
C
OUT2
47µF
6.3V
s2
+
C
IN1
10µF
50V
s3
C
IN2
100µF
50V
V
OUT
1.2V
15A
3878 F07
V
IN
4.5V TO 28V
1
PGOOD
R
PG
100k
R2
80.6k
R
C
12.1k
R
FB1
10.0k
R1
10.0k
TG
152
V
RNG
SW
143
FCB PGND
134
I
TH
BG
125
SGND INV
CC
116
I
ON
V
IN
107
V
FB
NC
98
R
ON
432k
R
FB2
5.11k
M2
RJK0330DPB
C
IN1
: UMK325BJ106MM s3
C
OUT1
: SANYO 2R5TPE330M9 s2
C
OUT2
: MURATA GRM31CR60J476M s2
L1: VISHAY IHLP4040DZ-11 0.56µH
C
SS
0.1µF