GATED OSCILLATOR CONTROL SCHEME

A unique gated oscillator control scheme enables the

LM2621 to have an ultra-low quiescent current and provides

a high efficiency over a wide load range. The switching

frequency of the internal oscillator is programmable using an

external resistor and can be set between 300 kHz and 2

MHz.

This control scheme uses a hysteresis window to regulate

the output voltage. When the output voltage is below the

upper threshold of the window, the LM2621 switches con-

tinuously with a fixed duty cycle of 70% at the switching

frequency selected by the user. During the first part of each

switching cycle, the internal N-channel MOSFET switch is

turned on. This causes the current to ramp up in the inductor

and store energy. During the second part of each switching

cycle, the MOSFET is turned off. The voltage across the

inductor reverses and forces current through the diode to the

output filter capacitor and the load. Thus when the LM2621

switches continuously, the output voltage starts to ramp up.

When the output voltage hits the upper threshold of the

window, the LM2621 stops switching completely. This

causes the output voltage to droop because the energy

stored in the output capacitor is depleted by the load. When

the output voltage hits the lower threshold of the hysteresis

window, the LM2621 starts switching continuously again

causing the output voltage to ramp up towards the upper

threshold. Figure 2 shows the switch voltage and output

voltage waveforms.

Because of this type of control scheme, the quiescent cur-

rent is inherently very low. At light loads the gated oscillator

control scheme offers a much higher efficiency compared to

the conventional PWM control scheme.

LOW VOLTAGE START-UP

The LM2621 can start-up from input voltages as low as 1.1V.

On start-up, the control circuitry switches the N-channel

MOSFET continuously at 70% duty cycle until the output

voltage reaches 2.5V. After this output voltage is reached,

the normal step-up regulator feedback and gated oscillator

control scheme take over. Once the device is in regulation it

can operate down to a 0.65V input, since the internal power

for the IC can be boot-strapped from the output using the

V

SHUTDOWN

The LM2621 features a shutdown mode that reduces the

quiescent current to less than a guaranteed 2.5µA over

temperature. This extends the life of the battery in battery

powered applications. During shutdown, all feedback and

control circuitry is turned off. The regulator’s output voltage

drops to one diode drop below the input voltage. Entry into

the shutdown mode is controlled by the active-low logic input

pin EN (Pin 2). When the logic input to this pin pulled below

0.15V

input to this pin should be above 0.7V

work in normal step-up mode.

OUTPUT VOLTAGE RIPPLE FREQUENCY

A major component of the output voltage ripple is due to the

hysteresis used in the gated oscillator control scheme. The

frequency of this voltage ripple is proportional to the load

current. The frequency of this ripple does not necessitate the

use of larger inductors and capacitors however, since the

size of these components is determined by the switching

frequency of the oscillator which can be set upto 2MHz using

an external resistor.

INTERNAL CURRENT LIMIT AND THERMAL

PROTECTION

An internal cycle-by-cycle current limit serves as a protection

feature. This is set high enough (2.85A typical, approxi-

mately 4A maximum) so as not to come into effect during

normal operating conditions. An internal thermal protection

circuitry disables the MOSFET power switch when the junc-

tion temperature (T

re-enabled when T

10093415

FIGURE 2. Typical Step-Up Regulator Waveforms

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