Datasheet

LM26400Y
www.ti.com SNVS457C –FEBRUARY 2007–REVISED APRIL 2013
APPLICATION HINTS
GENERAL
The LM26400Y is a dual PWM peak-current mode buck regulator 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 reference 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.
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.
START-UP AND SHUTDOWN
During a soft-start, the ramp of the output voltage is proportional to the ramp of the SS pin. When the EN pin is
pulled high, an internal 16µA current source starts to charge the corresponding 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 (V
REF
). 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 continue to ramp all the way up to the internal 2.7V
supply voltage before leveling off.
To calculate the needed SS capacitance for a given soft-start duration, use the following equation.
(1)
I
SS
is SS pin charging current, typically 16µA. V
REF
is the internal reference voltage, typically 0.6V. t
SS
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 V
FB
entry in the Electrical Characteristic table for the
V
REF
tolerance.
Inductor current during soft-start can be calculated by the following equation.
(2)
V
OUT
is the target output voltage, I
OUT
is the load current during start-up, and C
OUT
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.
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 discharged 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 reference and the level of the soft-start reference will be the lower of SS voltage and 0.6V.
When the output is pre-biased, the LM26400Y can usually start up successfully if there is at least a 2-Volt
difference between 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.
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).
Due to the frequency foldback mechanism, the switching frequency during start-up will be lower than the normal
value before V
FB
reaches 0.35V or so. See Frequency Foldback plot in the Typical Performance Characteristics
section.
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