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參數(shù)資料
| 型號： | LT1956-5EGN |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 570 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封裝： | 0.150 INCH, PLASTIC, SSOP-16 |
| 文件頁數(shù)： | 10/24頁 |
| 文件大?。?/td> | 437K |
| 代理商： | LT1956-5EGN |
18
LT1956/LT1956-5
Switch loss:
P
RI
V
tI
V
f
SW
SW OUT
OUT
IN
EFF
OUT
IN
=
() ( )
+
()( )( )
2
12
(/ )
Boost current loss:
P
VI
V
BOOST
OUT
IN
=
()
2
36
/
Quiescent current loss:
PV
V
Q
IN
OUT
=
()+
()
0 0015
0 003
..
RSW = Switch resistance (≈ 0.3) hot
tEFF = Effective switch current/voltage overlap time
= (tr + tf + tIr + tIf)
tr = (VIN/1.2)ns
tf = (VIN/1.7)ns
tIr = tIf = (IOUT/0.05)ns
f = Switch frequency
Example: with VIN = 12V, VOUT = 5V and IOUT = 1A:
APPLICATIO S I FOR ATIO
WU
UU
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the LT1956
is specified at 60V. This is based solely on internal semi-
conductor junction breakdown effects. Due to internal
power dissipation, the actual maximum VIN achievable in
a particular application may be less than this.
A detailed theoretical basis for estimating internal power
loss is given in the Thermal Calculations section. Note that
AC switching loss is proportional to both operating fre-
quency and output current. The majority of AC switching
loss is also proportional to the
square of input voltage. For
example, while the combination of VIN = 25V, VOUT = 5V at
1A and fOSC = 500kHz may be easily achievable, simulta-
neously raising VIN to 60V and fOSC to 700kHz is not
possible. Nevertheless, input voltage
transients up to 60V
can usually be accommodated, assuming the resulting
increase in internal dissipation is of insufficient time
duration to raise die temperature significantly.
A second consideration is controllability. A potential limi-
tation occurs with a high step-down ratio of VIN to VOUT,
as this requires a correspondingly narrow minimum switch
on time. An approximate expression for this (assuming
continuous mode operation) is given as follows:
Min t
VV
Vf
ON
OUT
F
IN
=
+
()
where:
VIN = input voltage
VOUT = output voltage
VF = Schottky diode forward drop
fOSC = switching frequency
A potential controllability problem arises if the LT1956 is
called upon to produce an on time shorter than it is able to
produce. Feedback loop action will lower then reduce the
VC control voltage to the point where some sort of cycle-
skipping or odd/even cycle behavior is exhibited.
In summary:
1. Be aware that the simultaneous requirements of high
VIN, high IOUT and high fOSC may not be achievable in
practice due to internal dissipation. The Thermal Calcu-
lations section offers a basis to estimate internal power.
P
W
PW
SW
BOOST
Q
= ()( ) ( ) +
()()()( )()
=+
=
= () () =
=
()+ () =
03 1
5
12
57 10
1 2 1 12 500 10
0 125 0 171 0 296
51 36
12
0 058
12 0 0015
5 0 003
0 033
2
93
2
.
/
..
.
/
.
..
.
Total power dissipation is 0.296 + 0.058 + 0.033 = 0.39W.
Thermal resistance for the LT1956 package is influenced
by the presence of internal or backside planes. With a full
plane under the GN16 package, thermal resistance will be
about 85
°C/W. No plane will increase resistance to about
95
°C/W. To calculate die temperature, use the proper
thermal resistance number and add in worst-case ambient
temperature:
TJ = TA + θJA (PTOT)
With the GN16 package (
θJA = 85°C/W), at an ambient
temperature of 85
°C,
TJ = 85 + 85(0.39) = 118°C
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