LM2621 datasheet(7/16 Pages) TI1

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LM2621

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SNVS033C – MAY 2004 – REVISED MARCH 2005

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 VDD pin.

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 0.15VDD, 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.

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,

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.

Design Procedure

SETTING THE OUTPUT VOLTAGE

The output voltage of the step-up regulator can be set between 1.24V and 14V by connecting a feedback

resistive divider made of RF1 and RF2. The resistor values are selected as follows:

RF2 = RF1 /[(VOUT/ 1.24) −1]

(1)

A value of 150k

Ω is suggested for RF1. Then, RF2 can be selected using the above equation. A 39pF capacitor

(CF1) connected across RF1 helps in feeding back most of the AC ripple at VOUT to the FB pin. This helps reduce

the peak-to-peak output voltage ripple as well as improve the efficiency of the step-up regulator, because a set

hysteresis of 30mV at the FB pin is used for the gated oscillator control scheme.

BOOTSTRAPPING

When the output voltage (VOUT) is between 2.5V and 5.0V a bootstrapped operation is suggested. This is

achieved by connecting the VDD pin (Pin 6) to VOUT. However if the VOUT is outside this range, the VDD pin should

be connected to a voltage source whose range is between 2.5V and 5V. This can be the input voltage (VIN), VOUT

stepped down using a linear regulator, or a different voltage source available in the system. This is referred to as

non-bootstrapped operation. The maximum acceptable voltage at the BOOT pin (Pin 7) is 10V.

SETTING THE SWITCHING FREQUENCY

The switching frequency of the oscillator is selected by choosing an external resistor (RFQ) connected between

FREQ and VDD pins. See the graph titled „Switching Frequency vs RFQ” in the Typical Operating Characteristics

section of the datasheet for choosing the RFQ value to achieve the desired switching frequency. A high switching

frequency allows the use of very small surface mount inductors and capacitors and results in a very small

solution size. A switching frequency between 300kHz and 2MHz is recommended.

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