LTC1929 LINER [Linear Technology], LTC1929 Datasheet - Page 18

• Manufacturer Part Number: LTC1929
• Description: 2-Phase, High Efficiency, Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1929
Design Example (Using Two Phases)
As a design example, assume V
(max), V
R
If L1 = L2 = 2 H the actual value of the ripple current for
each channel, the following equation is used:
The highest value of the ripple current occurs at the
maximum input voltage:
The ripple current for each inductor is 20% at maximum
output current which is conservative.
Next verify the minimum on-time of 200ns is not violated.
The minimum on-time occurs at maximum V
Since the output voltage is below 2.4V the output resistive
divider will need to be sized to not only set the output
voltage but also to absorb the sense pin current for both
channels.
Choosing 1% resistors; R1=13.2k and R2=16.5k yields an
output voltage of 1.80V.
18
SENSE1
R
R
t
ON MIN
SENSE1
1
I
I
L
L
MIN
OUT
and R
V
310
OUT
fL
= R
= 1.8V, I
10
20
kHz
SENSE2
V
2
SENSE2
1 8
IN MAX
k
k
1
V
.
OUT
2 4
U
2 4
.
2
V
MAX
.
V
OUT
can immediately be calculated:
V
1 8
= 50mV/10A = 0.005
IN
V V
H
V
f
.
OUT
= 20A, T
V
U
1 8
1
.
OUT
5 5 310
IN
V
.
1 8
5 5
= 5V (nominal), V
V
.
.
A
1 8
= 70 C and f = 310kHz,
.
30
W
V
k
1 95
kHz
.
A
IN
U
1
:
IN
s
= 5.5V
The power dissipation on the topside MOSFET can be
easily estimated. Using a Siliconix Si4420DY for example;
R
voltage with T
temperature:
The worst-case power disipated by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50 C junction tem-
perature rise is:
A short-circuit to ground will result in a folded back current
of:
The worst-case power disipated by the synchronous
MOSFET under short-circuit conditions at elevated ambi-
ent temperature and estimated 50 C junction temperature
rise is:
which is much less than normal, full-load conditions.
Incidentally, since the load no longer dissipates power in
the shorted condition, total system power dissipation is
decreased by over 99%.
DS(ON)
P
P
I
P
SC
MAIN
SYNC
SYNC
= 0.013 , C
0 005
25
.
0 013
1 8
1 29
5 5
360
310
5 5
5 5
.
mV
.
.
.
.
.
j
(estimated) = 110 C at an elevated ambient
V
V
V
V
5 5
5 5
mW
kHz
W
.
.
10
V
V
1 8
1 8
2
1
.
.
2
1 7 5 5
RSS
.
0 65
V
V
200
1
.
10
5 28
= 300pF. At maximum input
ns
.
.
2
W
0 005 110
A
.
V
H
5 5
A
2
2
.
1 48 0 013
2
10
V
.
1 48 0 013
.
A
300
5 28
C
.
.
.
pF
25
A
C