LM2621

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SNVS033D – MAY 2004 – REVISED NOVEMBER 2015

Feature Description (continued)

Figure 12. Typical Step-Up Regulator Waveforms

7.3.2 Low Voltage Start-Up

The LM2621 can start-up from input voltages as low as 1.1 V. On start-up, the control circuitry switches the N-

channel MOSFET continuously at 70% duty cycle until the output voltage reaches 2.5 V. 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 0.65-V input, since the internal power for the IC can be boot-

strapped from the output using the VDD pin.

7.3.3 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 up to 2 MHz using an external resistor.

7.3.4 Internal Current Limit and Thermal Protection

An internal cycle-by-cycle current limit serves as a protection feature. This is set high enough (2.85 A typical,

approximately 4 A maximum) so as not to come into effect during normal operating conditions. An internal

thermal protection circuitry disables the MOSFET power switch when the junction temperature (TJ) exceeds

about 160°C. The switch is re-enabled when TJ drops below approximately 135°C.

7.4 Device Functional Modes

7.4.1 Shutdown

The LM2621 features a shutdown mode that reduces the quiescent current to less than a specified 2.5-µA

overtemperature. 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 0.15 VDD, the device goes into shutdown mode. The logic input to this pin

should be above 0.7 VDD for the device to work in normal step-up mode.

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