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| LTC3878_15 Datasheet, PDF (17/26 Pages) Linear Technology – Fast, Wide Operating Range No RSENSE Step-Down DC/DC Controller | |||
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LTC3878
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 30mVP-P
or greater. This ripple voltage is equal to RDS(ON) typical
at 25°C ⢠IP-Pâ.
Design Example
Figure 7 is a power supply design example with the fol-
lowing specifications: VIN = 4.5V to 28V (12V nominal),
VOUT = 1.2V ±5%, IOUT(MAX) = 15A and f = 400kHz. Start
by calculating the timing resistor, RON:
RON
=
0.7V
â¢
1.2V
400kHz
â¢
10pF
=
429k
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 VIN using
the adjusted operating frequency:
L
=
1.2V
396kHz ⢠0.35
â¢
15A

ï£ï£¬
1â
1.2 
28 
=
0.55µH
Select 0.56µH which is the nearest value.
The resulting maximum ripple current is:
âIL
=
1.2V
396kHz ⢠0.56µH

ï£ï£¬
1â
1.2V
28V


=
5.1A
Choose the synchronous bottom MOSFET switch and
calculate the VRNG current limit set-point. To calculate
VRNG and VDS, the ÏÏ term normalization factor (unity
at 25°C) is required to account for variation in MOSFET
on-resistance with temperature. Choosing an RJK0330
(RDS(ON) = 2.8mΩ (nominal) 3.9mΩ (maximum), VGS =
4.5V, θJA = 40°C/W) yields a drain source voltage of:
( ) VDS
=

ï£ï£¬
ILIMIT
â1
2
IRIPPLE


3.9mâ¦
(ÏÏ)
CSS
0.1µF
R1 R2
10.0k 80.6k
1 RUN/SS BOOST 16
RPG
100k
2
LTC3878
PGOOD
TG 15
DB
CMDSH-3
CB
0.22µF
CC1
220pF
RC
12.1k
CC2
33pF
3 VRNG
4 FCB
5 ITH
6 SGND
SW 14
PGND 13
BG 12
INVCC 11
CVCC
4.7µF
RFB1
10.0k
RFB2 RON
5.11k 432k
7 ION
8 VFB
VIN 10
NC 9
CIN1
10µF
50V
s3
M1
RJK0305DPB
L1
0.56µH
+ CIN2
100µF
50V
M2
RJK0330DPB
COUT1 +
330µF
2.5V
s2
COUT2
47µF
6.3V
s2
VIN
4.5V TO 28V
VOUT
1.2V
15A
CIN1: UMK325BJ106MM s3
COUT1: SANYO 2R5TPE330M9 s2
COUT2: MURATA GRM31CR60J476M s2
L1: VISHAY IHLP4040DZ-11 0.56µH
3878 F07
Figure 7. Design Example: 1.2V/15A at 400kHz
3878fa
17
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