LM2621
Detailed Description
(Continued)
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.
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.
10093415
FIGURE 2. Typical Step-Up Regulator Waveforms
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
DD
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
0.15V
DD
, the device goes into shutdown mode. The logic
input to this pin should be above 0.7V
DD
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, 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
J
) exceeds about 160˚C. The switch is
re-enabled when T
J
drops below approximately 135˚C.
7
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