LM26003MH NSC [National Semiconductor], LM26003MH Datasheet - Page 9

• Manufacturer Part Number: LM26003MH
• Description: 3A Switching Regulator with High Efficiency Sleep Mode
• Manufacturer: NSC [National Semiconductor]

the inductor starts at zero and ramps-up to its peak, then
ramps-down to zero again. Until the next cycle, the inductor
current remains at zero. At nominal load currents, in FPWM
mode, the device operates in continuous conduction mode,
where positive current always flows in the inductor. Typical
discontinuous operation waveforms are shown below.
At very light load, in FPWM mode, the LM26003 may enter
sleep mode. This is to prevent an over-voltage condition from
occurring. However, the FPWM sleep threshold is much lower
than in normal operation.
ENABLE
The LM26003 provides a shutdown function via the EN pin to
disable the device when the output voltage does not need to
be maintained. EN is an analog level input with typically 180
mV of hysteresis. The device is active when the EN pin is
above 1.18V (typical) and in shutdown mode when EN is be-
low this threshold. When EN goes high, the internal VDD
regulator turns on and charges the VDD capacitor. When
VDD reaches 3.9V (typical), the soft-start pin begins to source
current. In shutdown mode, the VDD regulator shuts down
and total quiescent current is reduced to 10.8 µA (typical).
Because the EN pin sources 4.85 µA (typical) of pull-up cur-
rent, this pin can be left open for always-on operation. When
open, EN will be pulled up to VIN.
If EN is connected to VIN, it must be connected through a 10
kΩ resistor to limit noise spikes. EN can also be driven exter-
nally with a maximum voltage of 38V or VIN + 15V, whichever
is lower.
SOFT-START
The soft-start feature provides a controlled output voltage
ramp-up at startup. This reduces inrush current and elimi-
nates output overshoot at turn-on. The soft-start pin, SS, must
be connected to GND through a capacitor. At power-on, en-
able, or UVLO recovery, an internal 2.5 µA (typical) current
charges the soft-start capacitor. During soft-start, the error
amplifier output voltage is controlled by both the soft-start
voltage and the feedback loop. As the SS pin voltage ramps-
up, the duty-cycle increases proportional to the soft-start
ramp, causing the output voltage to ramp-up. The rate at
which the duty-cycle increases depends on the capacitance
of the soft-start capacitor. The higher the capacitance, the
slower the output voltage ramps-up. The soft-start capacitor
value can be calculated with the following equation:
FIGURE 4. Discontinuous Mode Waveforms
75 mA Load, Vin = 12V
30067608
9
Where tss is the desired soft-start time and Iss is the soft-start
source current. During soft-start, current limit and synchro-
nization remain in effect, while sleep mode and frequency
foldback are disabled. Soft-start mode ends when the SS pin
voltage reaches 1.23V typical. At this point, output voltage
control is transferred to the FB pin and the SS pin is dis-
charged.
CURRENT LIMIT
The peak current limit is set internally by directly measuring
peak inductor current through the internal switch. To ensure
accurate current sensing, AVIN should be bypassed with a
minimum 100 nF ceramic capacitor as close as possible to
AVIN and GND pins. Also the PVIN pin should be bypassed
with at least 2.2 µF to ensure low jitter operation.
When the inductor current reaches the current limit threshold,
the internal FET turns off immediately allowing inductor cur-
rent to ramp-down until the next cycle. This reduction in duty-
cycle corresponds to a reduction in output voltage.
The current limit comparator is disabled for less than 150 ns
at the leading edge for increased immunity to switching noise.
Because the current limit monitors peak inductor current, the
DC load current limit threshold varies with inductance and
frequency. Assuming a minimum current limit of 3.15A, max-
imum load current can be calculated as follows:
Where Iripple is the peak-to-peak inductor ripple current, cal-
culated as shown below:
To find the worst case (lowest) current limit threshold, use the
maximum input voltage and minimum current limit specifica-
tion.
During high over-current conditions, such as output short cir-
cuit, the LM26003 employs frequency foldback as a second
level of protection. If the feedback voltage falls below the short
circuit threshold of 0.9V, operating frequency is reduced,
thereby reducing average switch current. This is especially
helpful in short circuit conditions, when inductor current can
rise very high during the minimum on-time. Frequency reduc-
tion begins at 20% below the nominal frequency setting. The
minimum operating frequency in foldback mode is 45 kHz
typical.
If the FB voltage falls below the frequency foldback threshold
during frequency synchronized operation, the SYNC function
is disabled. Operating frequency versus FB voltage in short
circuit conditions is shown in the typical performance charac-
teristics section.
In conditions where the on-time is close to minimum (less than
200 ns typically), such as high input voltage and high switch-
ing frequency, the current limit may not function properly. This
is because the current limit circuit cannot reduce the on-time
below minimum which prevents entry into frequency foldback
mode. There are two ways to ensure proper current limit and
foldback operation under high input voltage conditions. First,
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