LM3687TL-1813 NSC [National Semiconductor], LM3687TL-1813 Datasheet - Page 15

• Manufacturer Part Number: LM3687TL-1813
• Description: Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode
• Manufacturer: NSC [National Semiconductor]

Soft Start
The DC-DC converter has a soft-start circuit that limits in-rush
current during start-up. During start-up the switch current limit
is increased in steps. Soft start is activated only if EN_DCDC
goes from logic low to logic high after V
Soft start is implemented by increasing switch current limit in
steps of 85mA, 170mA, 340mA and 1120mA (typical switch
current limit). The start-up time thereby depends on the output
capacitor and load current demanded at start-up. Typical
start-up times with a 10µF output capacitor and 750mA load
is 455 µs and with 1mA load is 180µs.
LINEAR REGULATOR OPERATION
In the typical post regulation application the power input volt-
age V
converter. Using a buck converter to reduce the battery volt-
age to a lower input voltage for the linear regulator translates
to higher efficiency and lower power dissipation.
It's also possible to operate the linear regulator independent
of the DC-DC converter output voltage either from V
different source. In this case it's important that V
not exceed V
regulator as well, it supplies internal circuitry.
An input capacitor of 1µF at V
other filter or bypass capacitor is present in the V
Startup Mode
If the linear regulator is enabled (logic high at EN_LIN), the
power input voltage V
nominal output voltage of the linear regulator V
If V
offering a rated output current of 350mA and supplied by
V
If V
providing a reduced rated output current of 50mA typical,
supplied by V
100mV is established. This feature is intended to enable the
supply of loads at the output of the linear regulator while the
output of the DC-DC converter is still ramping up.
In the typical post regulation application with both enable pins
connected to V
IN_LIN
IN_LIN
IN_LIN
IN_LIN
.
> V
< V
for the linear regulator is generated by the DC-DC
OUT_LIN(NOM)
OUT_LIN(NOM)
BATT
BATT
BATT
. Between these two levels a hystersis of
at any time. V
and V
IN_LIN
+ 200mV the main regulator is active,
+ 100mV the startup LDO is active,
IN_LIN
is continuously compared to the
IN_LIN
supplied by V
BATT
needs to be added if no
is needed for the linear
BATT
OUT_DCDC
reaches 2.7V.
OUT_LIN
IN_LIN
IN_LIN
BATT
.
path.
as an
does
or a
15
example, the linear regulator turns on in startup mode (I
50mA) supplied out of V
converter turns on, but V
internal signal 'Mode Switch' monitors the voltage level of
V
ulator changes to normal mode (I
of V
linear regulator switches back to startup mode.
Current Limiting
The LM3687 incorporates also a current limit feature for the
linear regulator to protect itself and external components dur-
ing overload conditions at V
over-current condition at V
the NFET pass device will be limited.
FIGURE 6. Startup Sequence, V
IN_LIN
IN_LIN
. Once V
. If V
IN_LIN
IN_LIN
drops below V
> V
OUT_LIN(NOM)
OUT_DCDC
BATT
OUT_LIN
OUT_LIN
. At the same time the DC-DC
MAX
EN_DCDC
the output current through
startup time is longer. The
OUT_LIN(NOM)
. In the event of a peak
+ 200mV, the linear reg-
= 350mA) supplied out
= V
EN_LIN
+ 100mV the
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= V
20210132
MAX
BATT
=