Datasheet

LM26400Y
SNVS457C –FEBRUARY 2007–REVISED APRIL 2013 www.ti.com
It is generally okay to connect the EN pin to V
IN
to simplify the system design. However, if the V
IN
ramp is slow
and the load current is relatively high during soft-start, the V
OUT
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
V
IN
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 V
IN
is above 5V or so. Make sure the logic
signal is never higher than V
IN
by 0.3V. The other is to use an external 5V bootstrap bias if it is ready before V
IN
hits 2.7V or 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 inductor 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 decrease. The lower
the FB voltage the lower the PWM frequency. See Frequency Foldback plot in the Typical Performance
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 input 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 versa. If
the amount of such overshoot exceeds the allowed limit for a system, add a C
FF
capacitor in parallel with the
upper feedback resistor to eliminate the overshoot. See the section LOAD STEP RESPONSE for more details on
C
FF
.
When one channel gets into over-current protection mode, the operation of the other channel will not be affected.
LOOP STABILITY
To the first order approximation, the LM26400Y has a V
FB
-to-Inductor Current transfer admittance (i.e. ratio of
inductor current to FB pin voltage, in frequency domain) close to the plot in Figure 30. The transfer admittance
has a DC value of 104dBS (dBS stands for decibel Siemens. The equivelant of 0dBS is 1 Siemens.). There is a
pole at 1Hz and a zero at approximately 8kHz. The plateau after the 8kHz zero is about 27dBS. There are also
high frequency poles that are not shown in the figure. They include a double pole at 1.2MHz or so, and another
double pole at half the switching frequency. Depending on factors such as inductor ripple size and duty cycle, the
double pole at half the switching frequency may become two separate poles near half the switching frequency.
Figure 30. V
FB
-to-Inductor Current Transfer Admittance
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