LT3437 LINER [Linear Technology], LT3437 Datasheet - Page 19

• Manufacturer Part Number: LT3437
• Description: High Voltage 500mA, 200kHz Step-Down Switching Regulator with 100uA Quiescent Current
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Example: with V
Total power dissipation is:
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:
With the DD package (Q
temperature of 70°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
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
(t
t
t
t
f = switch frequency
P
T
T
P
P
P
0 008 0 092 0 1
r
f
IR
SW
BOOST
Q
J
J
TOT
r
.
= (V
= (V
+ t
= T
= 70 + 45(0.1) = 74.5°C
= t
=
=
= 0.1 + 0.065 + 0.024 = 0.13W
f
IF
40 0 0005
A
IN
IN
+ t
+
( )(
+ Q
/2)ns
= (I
/0.6)ns
1 0 25
(
=
IR
.
.
( ) (
JA
OUT
5
+ t
.
40
IN
2
(P
IF
/0.05)ns
=
) ( )
)
= 40V, V
0 25 30
40
TOT
2
)
U
. /
+
5
.
)
5 0 0008
W
(
+
U
.
( ) ( ) (
)
JA
92 12 0 25 40 200 3
OUT
=
= 45°C/W) at an ambient
0 005
= 5V and I
.
)
/
=
W
0 024
W
.
.
)( )(
IN
OUT
W
achievable in
= 250mA:
U
e
)
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
5V at 700mA and f
simultaneously raising V
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
as this requires a correspondingly narrow minimum switch
on time. An approximate expression for this (assuming
continuous mode operation) is given as follows:
where:
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
skipping or Burst Mode behavior is exhibited.
In summary:
1. Be aware that the simultaneous requirements of high
2. The simultaneous requirements of high V
C
t
V
V
V
f
V
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.
and high f
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.
control voltage to the point where some sort of cycle-
ON(MIN)
OSC
IN
OUT
F
IN
= Schottky diode forward drop
, high I
= input voltage
= switching frequency
= output voltage
= (V
OSC
OUT
OUT
can result in an unacceptably short mini-
and high f
OSC
+ V
= 200kHz may be easily achievable,
F
IN
)/V
to 80V and f
IN
OSC
(f
OSC
may not be achievable in
)
OSC
IN
to 700kHz is not
= 40V, V
LT3437
IN
IN
, low V
to V
19
OUT
OUT
OUT
3437f
=
,