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LM21212-1

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SNVS671E – FEBRUARY 2011 – REVISED MARCH 2013

OPERATION DESCRIPTION

GENERAL

The LM21212-1 switching regulator features all of the functions necessary to implement an efficient low voltage

buck regulator using a minimum number of external components. This easy to use regulator features two

integrated switches and is capable of supplying up to 12A of continuous output current. The regulator utilizes

voltage mode control with trailing edge modulation to optimize stability and transient response over the entire

output voltage range. The device can operate at high switching frequency allowing use of a small inductor while

still achieving high efficiency. The precision internal voltage reference allows the output to be set as low as 0.6V.

Fault protection features include: current limiting, thermal shutdown, over voltage protection, and shutdown

capability. The device is available in the HTSSOP-20 package featuring an exposed pad to aid thermal

dissipation. The LM21212-1 can be used in numerous applications to efficiently step-down from a 5V or 3.3V

bus.

FREQUENCY SYNCHRONIZATION

The sync (SYNC) pin allows the LM21212-1 to be switched at an external clock frequency. When a clock signal

is present on the SYNC pin within the allowable frequency range, 300 kHz to 1.5 MHz, the device will

synchronize the turn-on of the high side FET (switch rising) to the negative edge of the clock signal, as seen in

Figure 25 . If no clock signal is present, the LM21212-1 will default to a switching frequency of 1 MHz. The clock

signal can be present on the SYNC pin before the device is powered on with no loading on the clock signal.

Alternatively, if no clock is present while the device is powered up, it will begin switching at the default frequency

of 1 MHz. Once the clock signal is present, the device will begin synchronizing to the clock frequency. The length

of time necessary for the synchronization depends on the clock frequency.

Figure 25. Frequency synchronization

PRECISION ENABLE

The enable (EN) pin allows the output of the device to be enabled or disabled with an external control signal.

This pin is a precision analog input that enables the device when the voltage exceeds 1.35V (typical). The EN pin

has 110 mV of hysteresis and will disable the output when the enable voltage falls below 1.24V (typical). If the

EN pin is not used, it can be left open, and will be pulled high by an internal 2 µA current source. Since the

enable pin has a precise turn-on threshold it can be used along with an external resistor divider network from VIN

to configure the device to turn-on at a precise input voltage.

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