Datasheet

LT1977
21
1977fa
APPLICATIO S I FOR ATIO
WUUU
Boost current loss:
P
VI
V
BOOST
OUT OUT
IN
=
()
()
2
32/
Quiescent current loss:
P
Q
= V
IN
(0.0015) + V
OUT
(0.003)
R
SW
= switch resistance (≈0.3 when hot )
t
EFF
= effective switch current/voltage overlap time
(t
r
+ t
f
+ t
IR
+ t
IF
)
t
r
= (V
IN
/1.1)ns
t
f
= (V
IN
/1.8)ns
t
IR
= t
IF
= (I
OUT
/0.05)ns
f = switch frequency
Example: with V
IN
= 12V, V
OUT
= 5V and I
OUT
= 1A:
Pee
TBD W
PW
PW
SW
BOOST
Q
=
()()()
+
()
()
()( )( )
+= + =
=
()
()
=
=
()
+
()
=
−
03 1 5
12
57 6 1 2 1 12 500 3
0 04 0 125 0 172 0 297
5132
12
0 002
12 0 0015 5 0 003 0 033
2
9
2
.
./
....
/
.
...
Total power dissipation is:
P
TOT
= 0.297 + 0.065 + 0.033 = 0.40W
Thermal resistance for the LT1977 package is influenced
by the presence of internal or backside planes. With a full
plane under the FE16 package, thermal resistance will be
about 45°C/W. 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 FE16 package (Q
JA
= 45°C/W) at an ambient
temperature of 70°C:
T
J
= 70 + 45(0.40) = 98°C
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the LT1977
is specified at 60V. 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 1A and f
OSC
= 500kHz may be easily achievable, si-
multaneously raising V
IN
to 60V and f
OSC
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 dura-
tion to raise die temperature significantly.
A second consideration is control. A potential limitation
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 con-
tinuous 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 control problem arises if the LT1977 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