reference and the level of the soft-start reference will be the
lower of SS voltage and 0.6V.
Application Hints
When the output is pre-biased, the LM26400Y can usually
start up successfully if there is at least a 2-Volt difference be-
tween the input voltage and the pre-bias. An output pre-bias
condition refers to the case when the output is sitting at a non-
zero voltage at the beginning of a start-up. The key to a
successful start-up under such a situation is enough initial
voltage across the bootstrap capacitor. When an output pre-
bias condition is anticipated, the power supply designer
should check the start-up behavior under the highest potential
pre-bias.
GENERAL
The LM26400Y is a dual PWM peak-current mode buck reg-
ulator with two integrated power MOSFET switches. The part
is designed to be easy to use. The two regulators are mostly
identical and share the same input voltage and the same ref-
erence voltage (0.6V). The two PWM clocks are of the same
frequency but 180° out of phase. The two channels can have
different soft-start ramp slopes and can be turned on and off
independently.
Loop compensation is built in. The feedback loop design is
optimized for ceramic output capacitors.
A pre-bias condition caused by a glitch in the enable signal
after start-up or by an input brown-out condition normally is
not an issue because the bootstrap capacitor holds its charge
much longer than the output capacitor(s).
Since the power switches are built in, the achievable output
current level also has to do with thermal environment of the
specific application. The LM26400Y enters thermal shutdown
when the junction temperature exceeds 165°C or so.
Due to the frequency foldback mechanism, the switching fre-
quency during start-up will be lower than the normal value
before VFB reaches 0.35V or so. See Frequency Foldback plot
in the Typical Performance Characteristics section.
START-UP AND SHUTDOWN
During a soft-start, the ramp of the output voltage is propor-
tional to the ramp of the SS pin. When the EN pin is pulled
high, an internal 16µA current source starts to charge the cor-
responding SS pin. The capacitance between the SS pin and
ground determines how fast the SS voltage ramps up. The
non-inverting input of the transconductance error amplifier,
i.e. the moving reference during soft-start, will be the lower of
SS voltage and the 0.6V reference (VREF). So before SS
reaches 0.6V, the reference to the error amplifier will be the
SS voltage. When SS exceeds 0.6V, the non-inverting input
of the transconductance amplifier will be a constant 0.6V and
that will be the time soft-start ends. The SS voltage will con-
tinue to ramp all the way up to the internal 2.7V supply voltage
before leveling off.
It is generally okay to connect the EN pin to VIN to simplify the
system design. However, if the VIN ramp is slow and the load
current is relatively high during soft-start, the VOUT ramp may
have a notch in it and a slight overshoot at the end of startup.
This is due to the reduced load current handling capability of
the LM26400Y for VIN lower than 5V. If this kind of behavior
is a problem for the system designer, there are two solutions.
One is to control the EN pin with a logic signal and do not pull
the EN high until VIN is above 5V or so. Make sure the logic
signal is never higher than VIN by 0.3V. The other is to use an
external 5V bootstrap bias if it is ready before VIN hits 2.7V or
so. See LOW INPUT VOLTAGE CONSIDERATIONS section
for more information.
To calculate the needed SS capacitance for a given soft-start
duration, use the following equation.
OVER-CURRENT PROTECTION
The instantaneous switch current is limited to a typical of 3
Amperes. Any time the switch current reaches that value, the
switch will be turned off immediately. This will result in a
smaller duty cycle than normal, which will cause the output
voltage to dip. The output voltage will continue drooping until
the load draws a current that is equal to the peak-limited in-
ductor current. As the output voltage droops, the FB pin
voltage will also droop proportionally. When the FB voltage
dips below 0.35V or so, the PWM frequency will start to de-
crease. The lower the FB voltage the lower the PWM fre-
quency. See Frequency Foldback plot in the Typical Perfor-
mance Characteristics section.
ISS is SS pin charging current, typically 16µA. VREF is the in-
ternal reference voltage, typically 0.6V. tSS is the desired soft-
start duration. For example, if 1ms is the desired soft-start
time, then the nominal SS capacitance should be 25nF. Apply
tolerances if necessary. Use the VFB entry in the Electrical
Characteristic table for the VREF tolerance.
Inductor current during soft-start can be calculated by the fol-
lowing equation.
The frequency foldback helps two things. One is to prevent
the switch current from running away as a result of the finite
minimum ON time (40 ns or so for the LM26400Y) and the
small duty cycle caused by lowered output voltage due to the
current limit. The other is it also helps reduce thermal stress
both in the IC and the external diode.
The current limit threshold of the LM26400Y remains constant
over all duty cycles.
VOUT is the target output voltage, IOUT is the load current dur-
ing start-up, and COUT is the output capacitance. For example,
if the output capacitor is 10µF, output voltage is 2.5V, soft-
start capacitor is 10nF and there is no load, then the average
inductor current during soft-start will be 62.5mA.
One thing to pay attention to is that recovery from an over-
current condition does not go through a soft-start process.
This is because the reference voltage at the non-inverting in-
put of the error amplifier always sits at 0.6V during the over-
current protection. So if the over-current condition is suddenly
removed, the regulator will bring the FB voltage back to 0.6V
as quickly as possible. This may cause an overshoot in the
output voltage. Generally, the larger the inductor or the lower
the output capacitance the more the overshoot, and vice ver-
sa. If the amount of such overshoot exceeds the allowed limit
for a system, add a CFF capacitor in parallel with the upper
When EN pin is pulled below 0.4V or so, the 16µA current
source will stop charging the SS pin. The SS pin will be dis-
charged through a 330Ω internal FET to ground. During this
time, the internal power switch will remain turned off while the
output is discharged by the load.
If EN is again pulled high before SS and output voltage are
completely discharged, soft-start will begin with a non-zero
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