PFM Operation

Connecting the SYNC/MODE pin to SGND sets the LM2612

to hysteretic PFM operation. While in PFM (Pulse Frequency

Modulation) mode, the output voltage is regulated by switch-

ing with a discrete energy per cycle and then modulating the

cycle rate, or frequency, to control power to the load. This is

done by using an error comparator to sense the output

voltage and control the PFET switch. The device waits as the

load discharges the output filter capacitor, until the output

voltage drops below the lower threshold of the PFM error-

comparator. Then the error comparator initiates a cycle by

turning on the PFET switch. This allows current to flow from

the input, through the inductor to the output, charging the

output filter capacitor. The PFET switch is turned off when

the output voltage rises above the regulation threshold of the

PFM error comparator. After the PFET switch turns off, the

output voltage rises a little higher as the inductor transfers

stored energy to the output capacitor by pushing current into

the output capacitor. Thus, the output voltage ripple in PFM

mode is proportional to the hysteresis of the error compara-

tor and the inductor current.

In PFM mode, the device only switches as needed to service

the load. This lowers current consumption by reducing power

consumed during the switching action in the circuit due to

transition losses in the internal MOSFETs, gate drive cur-

rents, eddy current losses in the inductor, etc. It also im-

proves light-load voltage regulation. During the second part

of the cycle, the intrinsic body diode of the NFET synchro-

nous rectifier conducts until the inductor current ramps to

zero. The LM2612 does not turn on the synchronous rectifier

while in PFM mode.

Operating Mode Selection

(SYNC/MODE Pin)

The SYNC/MODE digital input pin is used to select between

PWM or PFM operating modes. Set SYNC/MODE high

(above 1.3V) for 600kHz PWM operation when the system is

active and the load is above 50mA. Set SYNC/MODE low

(below 0.4V) to select PFM mode when the load is less than

50mA for precise regulation and reduced current consump-

tion when the system is in standby. The LM2612 has an

over-voltage protection feature that may activate if the de-

to prevent the output voltage from rising too high. See Ov-

ervoltage Protection, for more information.

Select modes with the SYNC/MODE pin using a signal with

a slew rate faster than 5V/100µs. Use a comparator Schmitt

trigger or logic gate to drive the SYNC/MODE pin. Do not

leave the pin floating or allow it to linger between logic levels.

These measures will prevent output voltage errors that could

otherwise occur in response to an indeterminate logic state.

Ensure a minimum load to keep the output voltage in regu-

lation when switching modes frequently. The minimum load

requirement varies depending on the mode change fre-

quency. A typical load of 8µA is required when modes are

changed at 100 ms intervals, 85µA for 10 ms and 800µA for

1 ms.

Frequency Synchronization

(SYNC/MODE Pin)

The SYNC/MODE input can also be used for frequency

synchronization. To synchronize the LM2612 to an external

clock, supply a digital signal to the SYNC/MODE pin with a

voltage swing exceeding 0.4V to 1.3V. During synchroniza-

tion, the LM2612 initiates cycles on the rising edge of the

clock. When synchronized to an external clock, it operates in

PWM mode. The device can synchronize to an external

clock over frequencies from 500kHz to 1MHz.

Use the following waveform and duty-cycle guidelines when

applying an external clock to the SYNC/MODE pin. Each

duty cycle between 30% and 70% and the total clock period

should be 2µs or less. Clock under/overshoot should be less

than 100mV below GND or above V

clock signals, especially sharp edged signals from a long

cable during evaluation, terminate the cable at its character-

istic impedance; add an RC filter to the SYNC pin, if neces-

sary, to soften the slew rate and over/undershoot. Note that

PWM Mode Switching Waveform

PFM Mode Switching Waveform

20041843

A: INDUCTOR CURRENT, 500mA/div

B: SW PIN, 2V/div

20041842

A: INDUCTOR CURRENT, 500mA/div

B: SW PIN, 2V/div

FIGURE 3. Typical Circuit Waveforms in (a) PWM Mode and (b) PFM Mode

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