Frequency Synchronization

sharp edged signals from a pulse or function generator can

develop under/overshoot as high as 10V at the end of an

improperly terminated cable.

Overvoltage Protection

The LM2612 has an over-voltage comparator that prevents

the output voltage from rising too high when the device is left

in PWM mode under low-load conditions. Otherwise, the

output voltage could rise out of regulation from the minimum

energy transferred per cycle due to the 200ns minimum

on-time of the PFET switch while in PWM mode. When the

output voltage rises by 50mV over its regulation threshold,

the OVP comparator inhibits PWM operation to skip pulses

until the output voltage returns to the regulation threshold. In

over voltage protection, output voltage and ripple increase

slightly.

Shutdown Mode

Setting the EN input low to SGND places the LM2612 in a

0.02uA (typ) shutdown mode. During shutdown, the PFET

switch, NFET synchronous rectifier, reference, control and

bias of the LM2612 are turned off. Setting EN high to VDD

enables normal operation. While turning on, soft start is

activated.

EN must be set low to turn off the LM2612 during undervolt-

age conditions when the supply is less than the 2.8V mini-

mum operating voltage. The LM2612 is designed for mobile

phones and similar applications where power sequencing is

determined by the system controller and internal UVLO (Un-

der Voltage LockOut) circuitry is unnecessary. The LM2612

has no UVLO circuitry. Although the LM2612 exhibits safe

behavior while enabled at low input voltages, this is not

guaranteed.

Internal Synchronous Rectification

While in PWM mode, the LM2612 uses an internal NFET as

a synchronous rectifier to improve efficiency by reducing

rectifier forward voltage drop and associated power loss. In

general, synchronous rectification provides a significant im-

provement in efficiency whenever the output voltage is rela-

tively low compared to the voltage drop across an ordinary

rectifier diode.

Under moderate and heavy loads, the internal NFET syn-

chronous rectifier is turned on during the inductor current

down-slope in the second part of each cycle. The synchro-

nous rectifier is turned off prior to the next cycle, or when the

inductor current ramps near zero at light loads. The NFET is

designed to conduct through its intrinsic body diode during

transient intervals before it turns on, eliminating the need for

an external diode.

Synchronous rectification is disabled and the NFET con-

ducts through its body diode during the second part of each

cycle while in PFM mode to reduce quiescent current asso-

ciated with the synchronous rectifier’s control circuitry. To

increase efficiency in PFM or PWM conditions, place an

external Schottky diode from PGND to SW.

Current Limiting

A current limit feature allows the LM2612 to protect itself and

external components during overload conditions. Current

limiting is implemented using an independent internal com-

parator that trips at current limit of the device. In PWM mode,

cycle-by-cycle current limiting is normally used. If an exces-

sive load pulls the output voltage down to approximately

0.7V, then the device switches to a timed current limit mode.

In timed current limit mode the internal P-FET switch is

turned off after the current comparator trips and the begin-

ning of the next cycle is inhibited for 2.5µs to force the

instantaneous inductor current to ramp down to a safe value.

PFM mode also uses timed current limit operation. The

synchronous rectifier is off in timed current limit mode. Timed

current limit prevents the loss of current control seen in some

products when the output voltage is pulled low in serious

overload conditions.

Current Limiting and PWM Mode

Transient Response

Considerations

The LM2612 was designed for fast response to moderate

load steps. Harsh transient conditions during loads above

300mA can cause the inductor current to swing up to the

maximum current limit, resulting in PWM mode jitter or insta-

bility from activation of the current limit comparator. To avoid

this jitter or instability, do not power-up or start the LM2612

into a full load (loads near or above 400mA). Do not change

operating modes or output voltages when operating at a full

load. Avoid extremely sharp and wide-ranging load steps to

Pin Selectable Output Voltage

The LM2612 features pin-selectable output voltage to elimi-

nate the need for external feedback resistors. The output

can be set to 1.05V, 1.3V, 1.5V or 1.8V by configuring the

VID0 and VID1 pins. See Setting the Output Voltage in the

Application Information section for further details.

Soft-Start

The LM2612 has soft start to reduce current inrush during

power-up and startup. This reduces stress on the LM2612

and external components. It also reduces startup transients

on the power source.

Soft start is implemented by ramping up the internal refer-

ence in the LM2612 to gradually increase the output voltage.

The reference ramps up in about 400µs. When powering up

in PWM mode, soft start may take an additional 200us to

allow time for the error amplifier compensation network to

charge.

Thermal Shutdown Protection

The LM2612 has thermal shutdown protection that operates

to protect from short-term misuse and overload conditions.

When the junction temperature exceeds about 150˚C, the

device shuts down, re-starting in soft start after the tempera-

ture drops below 130˚C. Prolonged operation in thermal

overload conditions may damage the device and is consid-

ered bad practice.

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