LM26400YMH NSC [National Semiconductor], LM26400YMH Datasheet - Page 12

• Manufacturer Part Number: LM26400YMH
• Description: Dual 2A, 500kHz Wide Input Range Buck Regulator
• Manufacturer: NSC [National Semiconductor]

www.national.com
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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.
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 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 (V
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.
To calculate the needed SS capacitance for a given soft-start
duration, use the following equation.
I
ternal reference voltage, typically 0.6V. t
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
Characteristic table for the V
Inductor current during soft-start can be calculated by the fol-
lowing equation.
V
ing start-up, and C
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 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
SS
OUT
is SS pin charging current, typically 16µA. V
is the target output voltage, I
OUT
is the output capacitance. For example,
REF
tolerance.
OUT
FB
is the load current dur-
entry in the Electrical
SS
REF
is the desired soft-
). So before SS
REF
is the in-
12
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 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.
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 fre-
quency during start-up will be lower than the normal value
before V
in the Typical Performance Characteristics section.
It is generally okay to connect the EN pin to V
system design. However, if the V
current is relatively high during soft-start, the V
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 V
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 V
signal is never higher than V
external 5V bootstrap bias if it is ready before V
so. See LOW INPUT VOLTAGE CONSIDERATIONS section
for more information.
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.
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.
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 C
FB
reaches 0.35V or so. See Frequency Foldback plot
IN
IN
is above 5V or so. Make sure the logic
FF
lower than 5V. If this kind of behavior
capacitor in parallel with the upper
IN
by 0.3V. The other is to use an
IN
ramp is slow and the load
IN
OUT
to simplify the
IN
hits 2.7V or
ramp may