lm22680mrx-adj National Semiconductor Corporation, lm22680mrx-adj Datasheet - Page 9

• Manufacturer Part Number: lm22680mrx-adj
• Description: 2a Simple Switcher ?step-down Voltage Regulator With Features
• Manufacturer: National Semiconductor Corporation

Soft-Start
The soft-start feature allows the regulator to gradually reach
the initial steady state operating point, thus reducing start-up
stresses and surges. The soft-start can be adjusted by se-
lecting an external soft-start capacitor. An internal 50 µA
current source charges up the external soft-start capacitor.
The generated voltage is the voltage the internal reference
limits. If no external soft-start capacitor is used, there is an
internal soft-start feature with 500 µs (typical) start-up time.
Recommended soft-start capacitor values are between 100
nF to 1 µF.
Switching Frequency Setting and
Synchronization
There are three different modes for the RT/SYNC pin. It can
be left floating for a 500 kHz switching frequency.
A resistor from the RT/SYNC pin to ground can be used to
adjust the switching frequency between 200 kHz and 1 MHz.
An external synchronization pulse can be applied to the RT/
SYNC pin for switching frequencies up to 1 MHz. The
LM22680 internally sets the RT/SYNC mode during start up.
Many applications are limited to lower switching frequencies
due to minimum on-time and minimum off-time requirements
as well as increased switching losses at switching frequen-
cies higher than 500 kHz. In case of synchronizing to an
external sync pulse, the clock pulse must be in a valid low or
high state when the LM22680 starts up and can be applied
after device regulation. The internal oscillator will synchronize
to the external sync pulse rising edge. If the external sync
pulse is disconnected the LM22680 switching frequency will
return to default 500 kHz (typ.).
When operating at frequencies above 500 kHz, external com-
ponents such as the inductor and the output capacitors can-
not be reduced dramatically. This is due to the fixed internal
compensation and stability requirements. Running the
LM22680 at frequencies higher than 500 kHz is intended for
applications with EMI or beat frequency concerns. The flexi-
bility of increasing the switching frequency above 500 kHz can
be used to operate outside a critical signal frequency band of
an application. This can also be used to set multiple switchers
in an application to the same frequency to reduce beat fre-
quencies and simplify filtering.
See the current limit section of this datasheet for information
about the safe operating area. When synchronizing to an ex-
ternal synchronization pulse, the LM22680 will not fold back
the switching frequency in an over current condition.
The typical curve below shows adjusted switching frequen-
cies with different frequency set resistors from the RT/SYNC
pin to ground.
9
Self Synchronize
It is also possible to self-synchronize multiple LM22680 reg-
ulators to share the same switching frequency. This can be
done by attaching the RT/SYNC pins together and putting a
1 kΩ resistor to ground. The diagram in Figure 3 illustrates
this setup. The two regulators will be clocked at the same fre-
quency but slightly phase shifted according to the minimum
off-time of the regulator with the fastest running oscillator. The
slight phase shift helps to reduce the stress on the input ca-
pacitors of the power supply.
Boot Pin
The LM22680 integrates an N-Channel FET switch and as-
sociated floating high voltage level shift / gate driver. This gate
driver circuit works in conjunction with an internal diode and
an external bootstrap capacitor. A 0.01 µF ceramic capacitor
connected with short traces between the BOOT pin and the
SW pin is recommended to effectively drive the internal FET
switch. During the off-time of the switch, the SW voltage is
approximately -0.5V and the external bootstrap capacitor is
charged from the internal supply through the internal boot-
strap diode. When operating with a high PWM duty-cycle, the
buck switch will be forced off each cycle to ensure that the
bootstrap capacitor is recharged. See the maximum duty-cy-
cle section for more details.
FIGURE 2. Switching Frequency vs RT/SYNC Resistor
FIGURE 3. Self Synchronizing Setup
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