LT1676CS8#TRPBF Linear Technology, LT1676CS8#TRPBF Datasheet - Page 7

• 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: -

OPERATIO
Please refer to the High dV/dt Mode Timing Diagram. A
typical oscillator cycle is as follows: The logic section first
generates an SWDR signal that powers up the current
comparator and allows it time to settle. About 1 s later, the
SWON signal is asserted and the BOOST signal is pulsed
for a few hundred nanoseconds. After a short delay, the
V
current indicated by the control voltage V
reached (current mode control), the SWON and SWDR
signals are turned off, and SWOFF is pulsed for several
hundred nanoseconds. The use of an explicit turn-off
device, i.e., Q5, improves turn-off response time and thus
aids both controllability and efficiency.
The system as previously described handles heavy loads
(continuous mode) at good efficiency, but it is actually
counterproductive for light loads. The method of jam-
ming charge into the PNP bases makes it difficult to turn
them off rapidly and achieve the very short switch ON
times required by light loads in discontinuous mode.
Furthermore, the high leading edge dV/dt rate similarly
adversely affects light load controllability.
The solution is to employ a “boost comparator” whose
inputs are the V
APPLICATIONS
Selecting a Power Inductor
There are several parameters to consider when selecting
a power inductor. These include inductance value, peak
current rating (to avoid core saturation), DC resistance,
construction type, physical size, and of course, cost.
In a typical application, proper inductance value is dictated
by matching the discontinuous/continuous crossover point
with the LT1676 internal low-to-high dV/dt threshold. This
is the best compromise between maintaining control with
light loads while maintaining good efficiency with heavy
loads. The fixed internal dV/dt threshold has a nominal
value of 1.4V, which referred to the V
control voltage to switch transconductance, corresponds
to a peak current of about 200mA. Standard Buck con-
verter theory yields the following expression for induc-
tance at the discontinuous/continuous crossover:
SW
pin slews rapidly to V
C
U
control voltage and a fixed internal
U
INFORMATION
U
IN
. Later, after the peak switch
W
C
pin threshold and
C
U
has been
threshold reference, V
mode switching topology, the V
peak switch current.) When the V
previously described “high dV/dt” action is performed.
When the V
absent, as can be seen in the Low dV/dt Mode Timing
Diagram. Now the DC current, activated by the SWON
signal alone, drives Q4 and this transistor drives Q1 by
itself. The absence of a boost pulse, plus the lack of a
second NPN driver, result in a much lower slew rate which
aids light load controllability.
A further aid to overall efficiency is provided by the
specialized bias regulator circuit, which has a pair of
inputs, V
the switching supply output. During start-up conditions,
the LT1676 powers itself directly from V
the switching supply output voltage reaches about 2.9V,
the bias regulator uses this supply as its input. Previous
generation Buck controller ICs without this provision
typically required hundreds of milliwatts of quiescent
power when operating at high input voltage. This both
degraded efficiency and limited available output current
due to internal heating.
For example, substituting 48V, 5V, 200mA and 100kHz
respectively for V
220 H. Note that the left half of this expression is indepen-
dent of input voltage while the right half is only a weak
function of V
means that a single inductor value will work well over a
range of “high” input voltage. And although a progres-
sively smaller inductor is suggested as V
approach V
under these conditions are much more forgiving with
respect to controllability and efficiency issues. Therefore
when a wide input voltage range must be accommodated,
say 10V to 50V for 5V
inductance value based on the maximum input voltage.
L
f I
IN
V
OUT
•
and V
OUT
PK
C
IN
signal is below V
, note that the much higher ON duty cycles
when V
CC
IN
V
IN
, V
. The V
–
V
OUT
IN
IN
TH
V
OUT
OUT
, I
is much greater than V
. (Remember that in a current
CC
PK
, the user should choose an
pin is normally connected to
and f yields a value of about
TH
C
C
voltage determines the
signal is above V
, the boost pulses are
IN
. However, after
IN
LT1676
begins to
OUT
TH
. This
, the
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