LTC1628IG-PG#PBF Linear Technology, LTC1628IG-PG#PBF Datasheet - Page 26

• Manufacturer Part Number: LTC1628IG-PG#PBF
• Description: IC SW REG STEP-DOWN 28-SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1628IG-PG#PBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: Adj to 0.8V
• Current - Output: 3A
• Frequency - Switching: 220kHz
• Voltage - Input: 3.5 ~ 30 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
LTC1628/LTC1628-PG
Design Example
As a design example for one channel, assume V
12V(nominal), V
and f = 300kHz.
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 4.7 H inductor will produce 23% ripple current and a
3.3 H will result in 33%. The peak inductor current will be
the maximum DC value plus one half the ripple current, or
5.84A, for the 3.3 H value. Increasing the ripple current
will also help ensure that the minimum on-time of 200ns
is not violated. The minimum on-time occurs at maximum
V
The R
maximum current sense voltage specification with some
accommodation for tolerances:
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 pins specified input
current.
26
IN
R
R
t
:
ON MIN
1
SENSE
I
L
(
SENSE
MAX
(
( )( )
V
)
f L
)
OUT
CC
resistor value can be calculated by using the
5 84
50
24
24
V
.
pin for 300kHz operation. The minimum
IN MAX
mV
V
k
K
IN
– 1
(
OUT
A
= 22V(max), V
2 4
U
2 4
V
.
.
V
OUT
)
0 01
IN
V V
f
V
0 8
0 8
.
.
.
–
– .
U
V
V
22 300
1 8
OUT
V
V
(
1 8
.
OUT
V
W
kHz
32
= 1.8V, I
k
)
273
U
MAX
ns
= 5A,
IN
=
Choosing 1% resistors; R1 = 25.5k and R2 = 32.4k yields
an output voltage of 1.816V.
The power dissipation on the top side MOSFET can be
easily estimated. Choosing a Siliconix Si4412DY results
in; R
voltage with T(estimated) = 50 C:
A short-circuit to ground will result in a folded back current
of:
with a typical value of R
0.1. The resulting power dissipated in the bottom MOSFET
is:
which is less than under full-load conditions.
C
temperature assuming only this channel is on. C
chosen with an ESR of 0.02 for low output ripple. The
output ripple in continuous mode will be highest at the
maximum input voltage. The output voltage ripple due to
ESR is approximately:
IN
V
I
P
P
SC
is chosen for an RMS current rating of at least 3A at
MAIN
SYNC
ORIPPLE
DS(ON)
0 01
25
.
0 042
mV
= 0.042 , C
220
434
1 8
22
= R
22
.
.
V
V
V
ESR( IL)
mW
22
mW
– .
2
5 1 0 005 50
1
V
1 8
2
200
V
1 7 22
.
= 0.02 (1.67A) = 33mV
RSS
3 3
DS(ON)
3 2
ns
.
( .
.
(
= 100pF. At maximum input
A
22
H
V
2
V
and = (0.005/ C)(20) =
2
)(
)
1 1 0 042
5
.
A
3 2
C
100
.
.
–
A
25
pF
C
)
300
P–P
OUT
kHz
1628fb
is