LTC3856IFE#TRPBF Linear Technology, LTC3856IFE#TRPBF Datasheet - Page 33

• Manufacturer Part Number: LTC3856IFE#TRPBF
• Description: IC REG CTLR 2CHAN SYNC 38TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC3856IFE#TRPBF

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.6 ~ 5 V
• Frequency - Switching: 250kHz ~ 770kHz
• Voltage - Input: 4.5 ~ 38 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 38-TSSOP Exposed Pad, 38-eTSSOP, 38-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -

applicaTions inForMaTion
A second, more severe transient is caused by switching
in loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If the ratio of
C
should be controlled so that the load rise time is limited
to approximately 25 • C
require a 250µs rise time, limiting the charging current
to about 200mA.
Design Example (Using Two Phases)
As a design example, assume:
V
V
V
I
T
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. Use a 71.5k resistor
from FREQ to ground to set the switching frequency at
about 300kHz. The minimum inductance for 30% ripple
current is:
MAX
f = 300kHz
A
LOAD
IN
IN
OUT
= 70°C
L
= 5V (nominal)
= 5.5V (max),
≥
≥
≥
= 20A
= 1.8V,
OUT
1 35
to C
(
V
f
300
( )
.
OUT
∆
, causing a rapid drop in V
I
OUT
µ
kHz

 
H
1
is greater than 1:50, the switch rise time
−
)(
1 8
.
V
30
V
OUT
V
IN
%
) ) (
LOAD

 
10
A
. Thus a 10µF capacitor would
)

 
1
−
1 8
5 5
OUT
.
.
V
V
. No regulator can

 
A 2µH inductor will produce 20% ripple current. The peak
inductor current will be the maximum DC value plus one
half the ripple current, or 11A. The minimum on-time oc-
curs at maximum V
With the ILIM pin tied to ground, the R
value can be calculated by using the minimum current
sense voltage specification with some accommodation
for tolerances:
Choosing 1% resistors: R1 = 10k and R2 = 20k yields an
output voltage of 1.80V.
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 dissipated by the synchronous
MOSFET under normal operating conditions at elevated
ambient temperature and estimated 50°C junction
temperature rise is:
DS(ON)
R
P
P
t
ON MIN
MAIN
SYNC
SENSE
( )
= 0.013Ω, C
=
=
=

 
=
•
=
=
1 8
5 5
1 0
5 5
5
0 606 0 022 0 628
J
0 013
.
.
. 9 9 W
25
V
.
V
(estimated) = 110°C at an elevated ambient
.
V f
11
.
OUT
V
V
IN
−
V
5 5
mV
1
A
( )
.
2
−
10
IN
. .
=
Ω
V
1 8
6
+
≈
:
.
V
(
RSS
+
2
5 5
0 002
.
V
+
.
 
(
.
1
5 5
(
V
2 6
+
10
= 300pF . At maximum input
.
1 8
.
)(
1
(
.
V
=
A
Ω
0 005 110
300
V
)
.
V
) (
2

 
2
.

 
•
kHz
1 25 0 013
10
(
.
)
2
300
(
A
)
W W
LTC3856
=

 
)
(
1 1
SENSE
( )(
kHz
2
.
° −
Ω
.
C
µs
)
300
resistors
25
Ω

)
°
C
pF
3856f
)

 
)