LT1676CS8#TRPBF Linear Technology, LT1676CS8#TRPBF Datasheet - Page 11

• Manufacturer Part Number: LT1676CS8#TRPBF
• Description: IC SW REG STEP-DOWN HI-EFF 8SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT1676CS8#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.24 ~ 51 V
• Current - Output: 700mA
• Frequency - Switching: 100kHz
• Voltage - Input: 7.4 ~ 60 V
• Operating Temperature: 0°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIONS
Minimum Component Count Application
Figure 4a shows a basic “minimum component count”
application. The circuit produces 5V at up to 500mA I
with input voltages in the range of 12V to 48V. The typical
P
skipping is observed down to zero external load. As
shown, the SHDN and SYNC pins are unused, however
either (or both) can be optionally driven by external signals
as desired.
frequency resonance problems, proper layout of the com-
ponents connected to the IC is essential, especially the
power path. B field (magnetic) radiation is minimized by
keeping output diode, switch pin and intput bypass
capacitor leads as short as possible. E field radiation is
kept low by minimizing the length and area of all traces
connected to the switch pin (V
always be used under the switcher circuitry to prevent
interplane coupling.
The high speed switching current path is shown schemati-
cally in Figure 3. Minimum lead length in these paths is
essential to ensure clean switching and minimal EMI. The
paths containing the input capacitor, output switch and
output diode are the only ones containing nanosecond rise
and fall times. Keep these paths as short as possible.
Additionally, it is possible for the LT1676 to cause EMI
problems by “coupling to itself”. Specifically, this can
occur if the V
uncontrolled manner to the part’s high impedance nodes,
TYPICAL APPLICATIONS
OUT
/P
V
IN
IN
Figure 3. High Speed Current Switching Paths
+
efficiency is shown in Figure 4b. No pulse
SW
C1
pin is allowed to capacitively couple in an
LT1676
U
V
INFORMATION
IN
U
SW
V
SW
). A ground plane should
U
W
D1
L1
+
U
C2
1676 F03
V
OUT
OUT
User Programmable Undervoltage Lockout
Figure 5 adds a resistor divider to the basic application.
This is a simple, cost-effective way to add a user-program-
mable undervoltage lockout (UVLO) function. Resistor R5
is chosen to have approximately 200 A through it at the
nominal SHDN pin lockout threshold of roughly 1.25V.
The somewhat arbitrary value of 200 A was chosen to be
significantly above the SHDN pin input current to minimize
its error contribution, but significantly below the typical
3.2mA the LT1676 draws in lockout mode. Resistor R4 is
then chosen to yield this same 200 A, less 2.5 A, with the
i.e., SHDN, SYNC, V
operation such as odd/even cycle behavior, pulse width
“nervousness”, improper output voltage and/or prema-
ture current limit action.
As an example, assume that the capacitance between the
V
further assume that the high impedance node in question
exhibits a capacitance of 1pF to ground. Due to the high
dV/dt, large excursion behavior of the V
couple a nearly 5V transient to the high impedance pin,
causing abnormal operation. (This assumes the “typical”
48V
added to the node will reduce the amplitude of the distur-
bance to more like 50mV (although settling time will
increase).
Specific pin recommendations are as follows:
SW
SHDN: If unused, add a 100pF capacitor to ground.
SYNC: Ground if unused.
V
explicit compensation network. A value of one-tenth of
the main compensation capacitor is recommended, up
to a maximum of 100pF.
FB: Assuming the V
usually requires no explicit capacitor of its own, but
keep this node physically small to minimize stray ca-
pacitance.
IN
C
node and a high impedance pin node is 0.1pF, and
: Add a capacitor directly to ground in addition to the
to 5V
OUT
application.) An explicit 100pF capacitor
C
C
and FB. This can cause erratic
pin is handled properly, this pin
SW
node, this will
LT1676
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