LT1767EMS8-5#TRPBF Linear Technology, LT1767EMS8-5#TRPBF Datasheet - Page 11

• Manufacturer Part Number: LT1767EMS8-5#TRPBF
• Description: IC SW REG STEP-DOWN 5V 8-MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1767EMS8-5#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 1.5A
• Frequency - Switching: 1.25MHz
• Voltage - Input: 3 ~ 25 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS
prevent subharmonic switching is reduced. This type of
subharmonic switching only occurs at input voltages less
than twice output voltage. Higher inductor values will tend
to eliminate this problem. See Frequency Compensation
section for a discussion of an entirely different cause of
subharmonic switching before assuming that the cause is
insufficient slope compensation. Application Note 19 has
more details on the theory of slope compensation.
LAYOUT CONSIDERATIONS
As with all high frequency switchers, when considering
layout, care must be taken in order to achieve optimal
electrical, thermal and noise performance. For maximum
efficiency, switch rise and fall times are typically in the
nanosecond range. To prevent noise both radiated and
conducted, the high speed switching current path, shown
in Figure 5, must be kept as short as possible. This is
implemented in the suggested layout of Figure 6. Shorten-
ing this path will also reduce the parasitic trace inductance
of approximately 25nH/inch. At switch off, this parasitic
inductance produces a flyback spike across the LT1767
switch. When operating at higher currents and input
voltages, with poor layout, this spike can generate volt-
ages across the LT1767 that may exceed its absolute
maximum rating. A ground plane should always be used
under the switcher circuitry to prevent interplane coupling
and overall noise.
The V
possible from the switch and boost nodes. The LT1767
pinout has been designed to aid in this. The ground for
these components should be separated from the switch
current path. Failure to do so will result in poor stability or
subharmonic like oscillation.
C
V
IN
and FB components should be kept as far away as
C3
Figure 5. High Speed Switching Path
V
IN
U
CIRCULATING
FREQUENCY
LT1767
PATH
HIGH
INFORMATION
U
SW
W
D1 C1
L1
LOAD
U
1767 F05
5V
LT1767-2.5/LT1767-3.3/LT1767-5
Board layout also has a significant effect on thermal
resistance. Soldering the exposed pad to as large a copper
area as possible and placing feedthroughs under the pad
to a ground plane, will reduce die temperature and in-
crease the power capacity of the LT1767. For the
nonexposed package, Pin 4 is connected directly to the
pad inside the package. Similar treatment of this pin will
result in lower die temperatures.
THERMAL CALCULATIONS
Power dissipation in the LT1767 chip comes from four
sources: switch DC loss, switch AC loss, boost circuit
current, and input quiescent current. The following
formulas show how to calculate each of these losses.
These formulas assume continuous mode operation, so
they should not be used for calculating efficiency at light
load currents.
Switch loss:
Boost current loss for V
Quiescent current loss:
R
17ns = Equivalent switch current/voltage overlap time
f = Switch frequency
Example: with V
SW
P
P
P
P
P
P
SW
Q
SW
Q
BOOST
BOOST
= Switch resistance ( 0.27 when hot)
V
10 0 001 0 01
IN
0 135 0 21 0 34
R
0 27 1 5
.
SW OUT
0 001
.
.
V
.
5
OUT
10
I
IN
2
10
V
2
1 50
= 10V, V
2
.
IN
/
V
I
2
LT1767/LT1767-1.8/
OUT
IN
.
V
BOOST
OUT
W
/
.
50
17 10
OUT
0 05
.
W
•
= V
= 5V and I
17
W
OUT
ns I
9
:
1 10 1 25 10
OUT
OUT
V
= 1A:
IN
.
sn1767 1767fas
11
f
•
6