參數(shù)資料
型號(hào)： LT3437EFE
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS
中文描述： 0.9 A SWITCHING REGULATOR, 240 kHz SWITCHING FREQ-MAX, PDSO16
封裝： 4.40 MM, PLASTIC, TSSOP-16
文件頁(yè)數(shù)： 19/24頁(yè)
文件大?。?/td> 523K
代理商： LT3437EFE
LT3437
19
3437f
APPLICATIOU
(t
r
+ t
f
+ t
IR
+ t
IF
)
t
r
= (V
IN
/0.6)ns
t
f
= (V
IN
/2)ns
t
IR
= t
IF
= (I
OUT
/0.05)ns
f = switch frequency
Example: with V
IN
= 40V, V
OUT
= 5V and I
OUT
= 250mA:
W
U
U
P
e
W
P
W
P
W
SW
BOOST
Q
=
( )(
) ( )
+
( )
( )
(
=
=
( )
(
40 0 0005
.
)( )(
)
+
)
=
=
)
+
)
=
5
40
/
.
0 008
.
0 092
.
0 1
.
5
0 2530
40
0 005
.
5 0 0008
.
0 024
.
2
2
.
.
/
Total power dissipation is:
P
TOT
= 0.1 + 0.065 + 0.024 = 0.13W
Thermal resistance for the LT3437 package is influenced
by the presence of internal or backside planes. With a full
plane under the package, thermal resistance will be about
45
°
C for the FE and DD packages. No plane will increase
resistance to about 150
°
C/W. To calculate die temperature,
use the proper thermal resistance number for the desired
package and add in worst-case ambient temperature:
T
J
= T
A
+ Q
JA
(P
TOT
)
With the DD package (Q
JA
= 45
°
C/W) at an ambient
temperature of 70
°
C:
T
J
= 70 + 45(0.1) = 74.5
°
C
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the LT3437
is specified at 80V. This is based solely on internal semi-
conductor junction breakdown effects. Due to internal
power dissipation, the actual maximum V
IN
achievable in
a particular application may be less than this.
A detailed theoretical basis for estimating internal power
loss is given in the section Thermal Considerations. Note
that AC switching loss is proportional to both operating
frequency and output current. The majority of AC switch-
ing loss is also proportional to the square of input voltage.
For example, while the combination of V
IN
= 40V, V
OUT
=
5V at 700mA and f
OSC
= 200kHz may be easily achievable,
simultaneously raising V
IN
to 80V and f
OSC
to 700kHz is not
possible. Nevertheless, input voltage transients up to 80V
can usually be accommodated, assuming the resulting
increase in internal dissipation is of insufficient time dura-
tion to raise die temperature significantly.
A second consideration is controllability. A potential limi-
tation occurs with a high step-down ratio of V
IN
to V
OUT
,
as this requires a correspondingly narrow minimum switch
on time. An approximate expression for this (assuming
continuous mode operation) is given as follows:
t
ON(MIN)
= (V
OUT
+ V
F
)/V
IN
(f
OSC
)
where:
V
IN
= input voltage
V
OUT
= output voltage
V
F
= Schottky diode forward drop
f
OSC
= switching frequency
A potential controllability problem arises if the LT3437 is
called upon to produce an on time shorter than it is able to
produce. Feedback loop action will lower, then reduce, the
V
C
control voltage to the point where some sort of cycle-
skipping or Burst Mode behavior is exhibited.
In summary:
1. Be aware that the simultaneous requirements of high
V
IN
, high I
OUT
and high f
OSC
may not be achievable in
practice due to internal dissipation. The Thermal Con-
siderations section offers a basis to estimate internal
power. In questionable cases, a prototype supply should
be built and exercised to verify acceptable operation.
2. The simultaneous requirements of high V
IN
, low V
OUT
and high f
OSC
can result in an unacceptably short mini-
mum switch on time. Cycle skipping and/or Burst Mode
behavior will result causing an increase in output volt-
age ripple while maintaining the correct output voltage.
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