LT1977 Linear Technology, LT1977 Datasheet - Page 21

• Manufacturer Part Number: LT1977
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
t
t
t
f = switch frequency
Example: with V
Total power dissipation is:
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:
With the FE16 package (Q
temperature of 70°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
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.
r
f
IR
= (V
= (V
P
T
T
P
P
P
0 04
= t
BOOST
J
J
SW
Q
TOT
.
= T
= 70 + 45(0.40) = 98°C
IF
IN
IN
=
= (I
=
/1.1)ns
/1.8)ns
= 0.297 + 0.065 + 0.033 = 0.40W
+
12 0 0015
A
( )( ) ( )
TBD
+ Q
(
=
0 3 1 5
OUT
.
( ) (
.
JA
5
/0.05)ns
12
=
IN
2
(P
12
0 125 0 172 0 297
2
1 32
= 12V, V
.
TOT
)
U
/
+
)
5 0 003
)
+
+
(
=
(
U
57 6
.
0 002
.
OUT
JA
.
.
= 45°C/W) at an ambient
e
)
= 5V and I
−
=
=
W
9
0 033
)
W
( ) ( )( )(
.
.
1 2 1 12 500 3
/
IN
W
W
OUT
achievable in a
= 1A:
U
e
)
For example, while the combination of V
5V at 1A and f
multaneously raising V
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
requires a correspondingly narrow minimum switch on
time. An approximate expression for this (assuming con-
tinuous mode operation) is given as follows:
where:
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
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
FREQUENCY COMPENSATION
Before starting on the theoretical analysis of frequency
response the following should be remembered—the worse
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.
high f
switch on time. Cycle skipping and/or Burst Mode be-
havior will result causing an increase in output voltage
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
OSC
= V
can result in an unacceptably short minimum
OUT
OSC
OUT
and high f
= 500kHz may be easily achievable, si-
+ V
F
IN
/V
to 60V and f
IN
(f
OSC
OSC
may not be achievable in
)
OSC
IN
IN
to 700kHz is not
to V
IN
= 40V, V
, low V
LT1977
OUT
, as this
OUT
21
OUT
and
1977f
=