Datasheet

LM2651
www.ti.com SNVS032D –FEBRUARY 2000–REVISED APRIL 2013
BLOCK DIAGRAM
Operation
The LM2651 operates in a constant frequency (300 kHz), current-mode PWM for moderate to heavy loads; and it
automatically switches to hysteretic mode for light loads. In hysteretic mode, the switching frequency is reduced
to keep the efficiency high.
MAIN OPERATION
When the load current is higher than the sleep mode threshold, the part is always operating in PWM mode. At
the beginning of each switching cycle, the high-side switch is turned on, the current from the high-side switch is
sensed and compared with the output of the error amplifier (COMP pin). When the sensed current reaches the
COMP pin voltage level, the high-side switch is turned off; after 40 ns (deadtime), the low-side switch is turned
on. At the end of the switching cycle, the low-side switch is turned off; and the same cycle repeats.
The current of the top switch is sensed by a patented internal circuitry. This unique technique gets rid of the
external sense resistor, saves cost and size, and improves noise immunity of the sensed current. A feedforward
from the input voltage is added to reduce the variation of the current limit over the input voltage range.
When the load current decreases below the sleep mode threshold, the output voltage will rise slightly, this rise is
sensed by the hysteretic mode comparator which makes the part go into the hysteretic mode with both the high
and low side switches off. The output voltage starts to drop until it hits the low threshold of the hysteretic
comparator, and the part immediately goes back to the PWM operation. The output voltage keeps increasing
until it reaches the top hysteretic threshold, then both the high and low side switches turn off again, and the
same cycle repeats.
PROTECTIONS
The cycle-by-cycle current limit circuitry turns off the high-side MOSFET whenever the current in MOSFET
reaches 2A.
Design Procedure
This section presents guidelines for selecting external components.
INPUT CAPACITOR
A low ESR aluminum, tantalum, or ceramic capacitor is needed betwen the input pin and power ground. This
capacitor prevents large voltage transients from appearing at the input. The capacitor is selected based on the
RMS current and voltage requirements. The RMS current is given by:
(1)
The RMS current reaches its maximum (I
OUT
/2) when V
IN
equals 2V
OUT
. For an aluminum or ceramic capacitor,
the voltage rating should be at least 25% higher than the maximum input voltage. If a tantalum capacitor is used,
the voltage rating required is about twice the maximum input voltage. The tantalum capacitor should be surge
current tested by the manufacturer to prevent being shorted by the inrush current. It is also recommended to put
a small ceramic capacitor (0.1 μF) between the input pin and ground pin to reduce high frequency spikes.
INDUCTOR
The most critical parameters for the inductor are the inductance, peak current and the DC resistance. The
inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages:
(2)
Copyright © 2000–2013, Texas Instruments Incorporated Submit Documentation Feedback 9
Product Folder Links: LM2651