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LM1770TMF Datasheet(PDF) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
Part # LM1770TMF
Description  SOT-23 Synchronous Buck Controller
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Manufacturer  NSC [National Semiconductor (TI)]
Direct Link  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM1770TMF Datasheet(HTML) 9 Page - National Semiconductor (TI)

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Application Information (Continued)
on-time and expands the minimum off-time. In addition the
part will only activate the PMOS allowing a discontinuous
mode of operation enabling a pre-biased startup. The time
spent in soft-start will depend on the load applied to the
output, but is usually close to a set time that is dependent on
the timing option. The approximate soft-start time can be
seen below for each timing option.
Product ID
Timing
T
SS
LM1770S
0.5µs
1ms
LM1770T
1.0µs
1.2ms
LM1770U
2.0µs
1.8ms
It should be noted that as soon as soft-start terminates the
short-circuit protection is enabled. This means that if the
output voltage does not reach at least 68% of its final value
the part will latch off. Therefore, if the input supply is ex-
tremely slow rising such that at the end of soft-start the input
voltage is still near the UVLO threshold, a timing option
should be chosen to ensure that maximum duty-cycle per-
mits the output to meet the minimum condition. As a general
recommendation it is advisable to use the 2000ns option
(LM1770U) in conditions where the output voltage is 2.5V or
greater to avoid false latch offs when there is concern re-
garding the input supply slew rate.
JITTER
The LM1770 utilizes a constant on-time control scheme that
relies on the output voltage ripple to provide a consistent
switching frequency. Under certain conditions, excessive
noise can couple onto the feedback pin causing the switch
node to appear to have a slight amount of jitter. This is not
indicative of an unstable design. The output voltage will still
regulate to the exact same value. Careful component selec-
tion and layout should minimize any external influence.
In addition to any external noise that can add to the jitter
seen on the switch node, the LM1770 will always have a
slight amount of switch jitter. This is because the LM1770
makes a small alteration in the reference voltage every 128
cycles to improve its accuracy and long term performance.
This has the effect of causing a change in the switching
frequency at that instant. When viewed on an oscilloscope
this can be seen as a jitter in the switch node. The change in
feedback voltage or output voltage, however, is almost indis-
tinguishable.
Design Guide
The following section walks the designer through the steps
necessary to select the external components to build a fully
functional power supply. As with any DC-DC converter nu-
merous trade-offs are possible to optimize the design for
efficiency, size or performance. These will be taken into
account and highlighted throughout this discussion.
The first equation to calculate for any buck converter is
duty-cycle. Ignoring conduction losses associated with the
FETs and parasitic resistances it can be approximated by:
A more accurate calculation for duty-cycle can be used that
takes into account the voltage drops across the FETs. This
equation can be used to determine the slight load depen-
dency on switch frequency if needed. Otherwise the simpli-
fied equation works well for component calculation.
FREQUENCY SELECTION
The LM1770 is available with three preset timing options that
select the on-time and hence determine the switching fre-
quency of the application. Increasing the switching fre-
quency has the effect of reducing the inductor size needed
for the application while requiring a slight trade-off in effi-
ciency. The table below shows the same frequency table as
shown earlier, with the exception that the recommended
timing option for each V
OUT is highlighted. It is not recom-
mended to use a high switching frequency with V
OUT equal
to or greater than 2.5V due to the maximum duty-cycle
limitations of the device coupled with the internal startup.
V
OUT
Timing Options
500ns
1000ns
2000ns
0.8
485
242
-
1
606
303
-
1.2
727
364
-
1.5
909
455
227
1.8
-
545
273
2.5
-
-
379
3.3
-
-
500
Recommended switching frequency (kHz) based on output voltage and
timing option.
INDUCTOR SELECTION
The inductor selection is an iterative process likely requiring
several passes before settling on a final value. The reason
for this is because it influences the amount of ripple seen at
the output, a critical component to ensure general stability of
an adaptive on-time circuit. For the first pass at inductor
selection the value can be obtained by targeting a maximum
peak-to-peak ripple current equal to 30% of the maximum
load current. The inductor current ripple (
∆I
L) can be calcu-
lated by:
Therefore, L can be initially set to the following by applying
the 30% rule:
The other features of the inductor that can be selected
besides inductance value are saturation current and core
material. Because the LM1770 does not have a current limit,
it is recommended to have a saturation current higher than
the maximum output current to handle any ripple or momen-
tary over-current events. The core material also influences
the saturation characteristics as ferrite materials have a hard
saturation curve and care should be taken such that they
never saturate during normal use. A shielded inductor or low
www.national.com
9


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