Datasheet

LM22680 LM22680
RT/SYNC RT/SYNC
V
out
1 k:
1 k:
2N7002
EN EN
ENABLE
LM22680
www.ti.com SNVS595K –SEPTEMBER 2008–REVISED APRIL 2013
Self Synchronization
It is possible to synchronize multiple LM22680 regulators together to share the same switching frequency. This
can be done by tieing the RT/SYNC pins together through a MOSFET and connecting a 1 KΩ resistor to ground
at each pin. Figure 19 shows this connection. The gate of the MOSFET should be connected to the regulator
with the highest output voltage. Also, the EN pins of both regulators should be tied to the common system
enable, in order to properly initialize both regulators. The operation is as follows: When the regulators are
enabled, the outputs are low and the MOSFET is off. The 1 kΩ resistors pull the RT/SYNC pins low, thus
enabling the synchronization mode. These resistors are small enough to pull the RT/SYNC pin low, rather than
activate the frequency adjust mode. Once the output voltage of one of the regulators is sufficient to turn on the
MOSFET, the two RT/SYNC pins are tied together and the regulators will run in synchronized mode. The two
regulators will be clocked at the same frequency but slightly phase shifted according to the minimum off-time of
the regulator with the fastest internal oscillator. The slight phase shift helps to reduce stress on the input
capacitors of the regulator. It is important to choose a MOSFET with a low gate threshold voltage so that the
MOSFET will be fully enhanced. Also, a MOSFET with low inter-electrode capacitance is required. The 2N7002
is a good choice.
Figure 19. Self Synchronizing Setup
Boot-Strap Supply
The LM22680 incorporates a floating high-side gate driver to control the power MOSFET. The supply for this
driver is the external boot-strap capacitor connected between the BOOT pin and SW. A good quality 10 nF
ceramic capacitor must be connected to these pins with short, wide PCB traces. One reason the regulator
imposes a minimum off-time is to ensure that this capacitor recharges every switching cycle. A minimum load of
about 5 mA is required to fully recharge the boot-strap capacitor in the minimum off-time. Some of this load can
be provided by the output voltage divider, if used.
Thermal Protection
Internal thermal shutdown circuitry protects the LM22680 should the maximum junction temperature be
exceeded. This protection is activated at about 150°C, with the result that the regulator will shutdown until the
temperature drops below about 135°C.
Internal Loop Compensation
The LM22680 has internal loop compensation designed to provide a stable regulator over a wide range of
external power stage components.
Ensuring stability of a design with a specific power stage (inductor and output capacitor) can be tricky. The
LM22680 stability can be verified using the WEBENCH® Designer online circuit simulation tool at www.ti.com. A
quick start spreadsheet can also be downloaded from the online product folder.
The complete transfer function for the regulator loop is found by combining the compensation and power stage
transfer functions. The LM22680 has internal type III loop compensation, as detailed in Figure 20. This is the
approximate "straight line" function from the FB pin to the input of the PWM modulator. The power stage transfer
function consists of a D.C. gain and a second order pole created by the inductor and output capacitor(s). Due to
the input voltage feedforward employed in the LM22680, the power stage D.C. gain is fixed at 20dB. The second
order pole is characterized by its resonant frequency and its quality factor (Q). For a first pass design, the
product of inductance and output capacitance should conform to the following equation:
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