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LM2621 Datasheet(PDF) 7 Page - Texas Instruments

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Part No. LM2621
Description  LM2621 Low Input Voltage, Step-Up DC-DC Converter
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Manufacturer  TI [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI - Texas Instruments

LM2621 Datasheet(HTML) 7 Page - Texas Instruments

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SNVS033C – MAY 2004 – REVISED MARCH 2005
Because of this type of control scheme, the quiescent current is inherently very low. At light loads the gated
oscillator control scheme offers a much higher efficiency compared to the conventional PWM control scheme.
Figure 14. Typical Step-Up Regulator Waveforms
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 VDD pin.
The LM2621 features a shutdown mode that reduces the quiescent current to less than a ensured 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 0.15VDD, the device goes into shutdown mode. The logic input to this pin should be above
0.7VDD for the device to work in normal step-up mode.
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.
An internal cycle-by-cycle current limit serves as a protection feature. This is set high enough (2.85A typical,
approximately 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 junction temperature (TJ) exceeds about 160°C.
The switch is re-enabled when TJ drops below approximately 135°C.
Copyright © 2004–2005, Texas Instruments Incorporated
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