參數(shù)資料
型號(hào)： LTC3856EUH#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PQCC38
封裝： 5 X 5 MM, LEAD FREE, PLASTIC, MO-220WHHD-X, QFN-32
文件頁數(shù)： 27/40頁
文件大?。?/td> 2222K
代理商： LTC3856EUH#PBF
LTC3856
3856f
applicaTions inForMaTion
A second, more severe transient is caused by switching
in loads with large (>1F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with COUT, causing a rapid drop in VOUT. No regulator can
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
CLOAD to COUT is greater than 1:50, the switch rise time
should be controlled so that the load rise time is limited
to approximately 25 CLOAD. Thus a 10F capacitor would
require a 250s rise time, limiting the charging current
to about 200mA.
Design Example (Using Two Phases)
As a design example, assume:
VIN = 5V (nominal)
VIN = 5.5V (max),
VOUT = 1.8V,
IMAX = 20A
TA = 70°C
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. 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:
L
V
f I
V
kHz
OUT
IN
≥
( )
≥ (
)(
1
1 8
300
30
.
%
))( )
≥
10
1
1 8
5 5
1 35
A
V
H
.
A 2H 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 VIN:
t
V
V f
V
kHz
s
ON MIN
OUT
IN
( ) =
= ( )(
)
=
1 8
5 5
300
1 1
.
With the ILIM pin tied to ground, the RSENSE resistors
value can be calculated by using the minimum current
sense voltage specification with some accommodation
for tolerances:
R
mV
A
SENSE =
≈
25
11
0 002
.
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;
RDS(ON) = 0.013Ω, CRSS = 300pF. At maximum input
voltage with TJ(estimated)=110°Catanelevatedambient
temperature:
P
V
C
MAIN =
( ) +(
) ° °
(
)
1 8
5 5
10
1 0 005 110
25
2
.
+
( )
(
)(
)
.
.
0 013
5 5
10
2
300
1
5
2
V
A
pF
V
..
.
.
6
1
2 6
300
0 606 0 022 0 628
V
kHz
+
(
)
=
+
=
W
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:
P
V
A
SYNC =
( ) ( )
(
)
=
5 5
1 8
5 5
10
1 25 0 013
1 0
2
.
. 99W
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