LTC1629 LINER [Linear Technology], LTC1629 Datasheet - Page 22

• Manufacturer Part Number: LTC1629
• Description: PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulators
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC1629/LTC1629-PG
Although the LTC1629 has a maximum input voltage of
36V, most applications will be limited to 30V by the
MOSFET BV
Design Example (Using Two Phases)
As a design example, assume V
(max), V
The inductance value is chosen first based on a 30% ripple
current assumption. The highest value of ripple current
occurs at the maximum input voltage. Tie the FREQSET pin
to the INTV
inductance for 30% ripple current is:
A 2 H inductor will produce 20% ripple current. The peak
inductor current will be the maximum DC value plus one
half the ripple current, or 11.5A. The minimum on-time
occurs at maximum V
The R
maximum current sense voltage specification with some
accomodation for tolerances:
22
L
R
t
ON MIN
SENSE
SENSE
1 35
V
f I
300
.
OUT
OUT
Figure 8. Automotive Application Protection
12V
GENERAL INSTRUMENT
CC
kHz
resistors value can be calculated by using the
DSS
= 1.8V, I
TRANSIENT VOLTAGE
H
60
11 5
1
V
pin for 300kHz operation. The minimum
V f
.
1 8
OUT
mV
.
IN
SUPPRESSOR
.
30
V
50A I
A
V
OUT
V
U
1.5KA24A
IN
%
MAX
PK
IN
RATING
0 005
5 5
.
:
10
= 20A, T
.
U
A
V
1 8
V
IN
.
IN
300
1
= 5V (nominal), V
V
A
= 70 C and f = 300kHz.
kHz
1 8
5 5
W
.
.
LTC1629
V
V
1 1
1629 F08
.
s
U
IN
= 5.5V
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 input current for
both channels.
Choosing 1% resistors: R1=13.2k and R2=16.5k yields an
output voltage of 1.80V and satisfies the above condition.
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:
DS(ON)
I
R
P
P
SC
MAIN
SYNC
1
MIN
= 0.013 , C
0 005
25
.
0 013
1 8
1 29
5 5
310
5 5
mV
.
.
.
10
.
.
20
j
(estimated) = 110 C at an elevated ambient
2
V
V
V
5 5
k
kHz
k
W
.
10
2 4
V
2 4
1 8
.
2
1
.
.
1 7 5 5
2
V
RSS
1 8
V V
V
.
0 61
V
200
.
1
.
OUT
V
10
1 8
= 300pF. At maximum input
ns
.
.
W
2
0 005 110
OUT
A
.
V
V
H
5 5
2
2
.
1 48 0 013
10
V
.
30
A
k
300
5 28
C
.
.
pF
25
A
C