LM2650MX-ADJ datasheet(8/11 Pages) NSC

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Operation (Continued)

The diode D2 is the body diode of Q2. The hysteretic circuit

uses D2 as a rectifier instead of switching Q2 as a synchro-

nous rectifier.

When the load current drops below the prescribed sleep-in

threshold, the LM2650 shuts down the PWM loop and starts

up the hysteretic loop. The hysteretic loop supports light

loads more efficiently because it uses less power to support

its own operation; it uses less bias power because it’s a

simpler loop having less circuit blocks to bias, and it switches

slower, so it incurs lower switching losses.

The hysteretic control loop does not switch at a constant

frequency. Instead, it monitors V

OUT and switches only when

V

OUT reaches either side of a narrow window centered on

the desired output voltage. C2 directs the switching based

on its reading of the feedback voltage. Switching in this

manner yields a regulated voltage consisting of the desired

output voltage and an AC ripple voltage. The magnitude of

the AC component can be approximated using

(4)

For example, with V

OUT set to 5V, VOUT_PP is approximately

120mV,

(5)

When it starts up, the hysteretic loop turns Q1 on. While Q1

is on, the input power supply charges C

OUT and supplies

current to the load. Current from the supply reaches C and

the load via the series path provided by Q1 and L1. As the

feedback voltage just surpasses the upper hysteretic thresh-

old of C2, the output of C2 changes from high to low, and HD

responds by pulling the gate of Q1 down turning Q1 off. As

Q1 turns off, L1 generates a negative-going voltage transient

that D2 clamps at just below ground. D2 remains on only

briefly as the current in L1 runs out. While both Q1 and D2

are off, C

OUT alone supplies current to the load. As the

feedback voltage just surpasses the lower hysteretic thresh-

old of C2, the output of C2 changes states from low to high,

and DH responds by pulling the gate of Q1 up turning Q1 on

and starting the hysteretic cycle over.

Note that as the load current decreases, it takes increasingly

longer periods for the load current to discharge C

OUT

through the hysteretic window, and as the load current in-

creases, the periods become even shorter. It can be seen

from the above observation that the switching frequency of

the hysteretic loop varies as the load varies. The switching

frequency can be approximated using

(6)

Here f is the switching frequency in hertz, I is the load current

in amperes, C

OUT is the value of the capacitor in farads, and

V

OUT_PP is the magnitude of the AC ripple voltage in volts.

Typical switching frequencies range anywhere from a few

hertz for very light loads to a few thousand hertz for light

loads bordering on the moderate level.

Application Circuits

Figure 4 is a schematic of the typical application circuit. use

the component values shown in the figure and those con-

tained in

Table 1 to build a 5V, 3A, or 3.3V, 3A step-down

DC/DC converter. As with the design of any DC/DC con-

verter, the design of these circuits involved tradeoffs be-

tween efficiency, size, and cost. Here more weight was given

to efficiency than to size as evidenced by the low switching

frequency which keeps switching losses low but pushes the

value and size of the inductor up.

From a smaller circuit, use the component values shown in

Figure 4 and those contained in Table 3. These circuits trade

slightly higher switching losses for a much smaller inductor.

Note,

Figure 4 does not show R

FA, the resistor required to

adjust the switching frequency from 90 kHz up to 200 kHz.

Connect R

FA between the FREQ ADJ pin and ground.

DS012848-19

FIGURE 4. The Typical 90 kHz Application Circuit

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